Astronomy & Astrophysics manuscript no. xdcpgrondpierini˙rev3
January 11, 2012
c ESO 2012
First simultaneous optical/near-infrared imaging
of an X-ray selected, high-redshift cluster of galaxies with GROND
The galaxy population of XMMU J0338.7+0030 at z = 1.1
arXiv:1201.1925v1 [astro-ph.CO] 9 Jan 2012
D. Pierini1,⋆ , R. S̆uhada2,1 , R. Fassbender1 , A. Nastasi1 , H. Böhringer1 , M. Salvato3,4,1 , G. W. Pratt5 , M. Lerchster1,∗ ,
P. Rosati6 , J. S. Santos7 , A. de Hoon8 , J. Kohnert8 , G. Lamer8 , J. J. Mohr9,1,10 , M. Mühlegger1,∗ , H. Quintana11 , A.
Schwope8 , V. Biffi1 , G. Chon1 , S. Giodini12 , J. Koppenhoefer1,2 , M. Verdugo1 , F. Ziparo1 , P. M. J. Afonso13,1 , C.
Clemens1,∗ , J. Greiner1 , T. Krühler14,10,1 , A. Küpcü Yoldaş15 , F. Olivares E.1 , A. Rossi16 , and A. Yoldaş15
(Affiliations can be found after the references)
Received ... ..., ...; accepted ... ..., ...
ABSTRACT
Context. The XMM-Newton Distant Cluster Project is a serendipitous survey for clusters of galaxies at redshifts z ≥ 0.8 based on deep archival
XMM-Newton observations. X-ray sources identified as extended are screened against existing optical all-sky surveys for galaxies, in case of
candidate high-z clusters followed up with imaging at 4m-class telescopes and, ultimately, multi-object spectroscopy at 8m-class telescopes. Lowsignificance candidate high-z clusters are followed up with the seven-channel imager GROND (Gamma-Ray Burst Optical and Near-Infrared
Detector) that is mounted at a 2m-class telescope. Its unique capability of simultaneous imaging in the g′ , r′ , i′ , z′ , J, H, Ks bands enables the use
of the photometric redshift technique.
Aims. Observing strategy, data reduction and analysis, depth and accuracy of the simultaneous multi-wavelength photometry are discussed with
the goal of establishing GROND as a useful instrument to confirm X-ray selected (high-z) clusters.
Methods. The test case is XMMU J0338.7+0030, suggested to be at z ∼ 1.45 ± 0.15 (1σ) from the analysis of the z − H vs H colour–magnitude
diagram obtained from the follow-up imaging. Later VLT-FORS2 spectroscopy enabled us to identify four members, which set this cluster at
z = 1.097 ± 0.002 (1σ). To reach a better knowledge of its galaxy population, we observed XMMU J0338.7+0030 with GROND for about 6 hr.
The publicly available photo-z code le Phare was used.
Results. The Ks-band number counts of the non-stellar sources out of the 832 detected down to z′ ∼ 26 AB mag (1σ) in the 3.9 × 4.3 arcmin2
region of XMMU J0338.7+0030 imaged at all GROND bands clearly exceed those computed in deep fields/survey areas at ∼ 20.5–22.5 AB mag.
The photo-z’s of the three imaged spectroscopic members yield z = 1.12 ± 0.09 (1σ). The spatial distribution and the properties of the GROND
sources with a photo-z in the range 1.01–1.23 confirm the correspondence of the X-ray source with a galaxy over-density at a significance of at least
4.3 σ. Candidate members that are spectro-photometrically classified as elliptical galaxies define a red locus in the i′ − z′ vs z′ colour–magnitude
diagram that is consistent with the red sequence of the cluster RDCS J0910+5422 at z = 1.106. XMMU J0338.7+0030 hosts also a population of
bluer late-type spirals and irregulars. The starbursts among the photometric members populate both loci, consistently with previous results.
Conclusions. The analysis of the available data set indicates that XMMU J0338.7+0030 is a low-mass cluster (M200 ∼ 1014 M⊙ ) at z = 1.1.
With the photometric accuracy yielded by the present unoptimized multi-band observations with GROND, we not only confirm the spectroscopic
redshift of this cluster but also show that it hosts a galaxy population that can still undergo significant bursts of star-formation activity.
Key words. X-rays: galaxies: clusters – galaxies: clusters: individual: XMMU J0338.7+0030 – galaxies: evolution – galaxies: high-redshift
1. Introduction
A process of self-similar gravitational clustering in an expanding universe leads to the build-up of structures in a hierarchical
fashion across time (White & Rees 1978; White et al. 1987).
Ensembles of galaxies and hot (T ≥ 106 K) plasma within gravitationally bound haloes dominated by dark matter define groups
and clusters, with total masses from ∼ 1013 to ∼ 1015 M⊙ , respectively. These objects offer a biased view of the evolution of
the large-scale structure (Kaiser 1984) that is easy to model theoretically from ab-initio principles.
The identification of a group/cluster requires observations
at rest-frame optical/near-infrared (IR) wavelengths (where the
photospheric stellar emission dominates), in X-rays (where the
⋆
Currently visiting astronomer at the MPE
hot plasma1 emits), or at sub-millimeter wavelengths (owing to
the Sunyaev-Zel’dovich - SZ - effect, Sunyaev & Zel’dovich
1972). In particular, the X-ray or SZ-effect detection ensures that
a galaxy system is truly bound and associated with a deep gravitational potential well. X-ray observations of groups/clusters
can now be pushed towards the distant universe (i.e., z ≥ 0.8)
thanks to the Chandra and XMM-Newton space-borne observatories (e.g., Stanford et al. 2001, 2002; Rosati et al. 2004; Mullis
et al. 2005; Finoguenov et al. 2007; S̆uhada et al. 2010; however,
see Rosati et al. 1998). With the SZ-effect this has become possible very recently but only for massive clusters (Staniszewski et
al. 2009; Vanderlinde et al. 2010; Marriage et al. 2011).
Compiling a sample of distant galaxy clusters has a twofold
importance. Firstly, it offers a way to trace structure growth in
the far universe that enables the study of the effects of cold dark
1
Whether the hot X-ray emitting plasma belongs to a group or a
cluster, we will refer to it as the intracluster medium (ICM).
1
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
matter (CDM) and dark energy (DE) on the cosmic structure
evolution (e.g., Albrecht et al. 2006; Vikhlinin et al. 2009; Mantz
et al. 2010a). Secondly, it fosters the investigation of the evolution of the galaxy population, the baryon mass component of
groups/clusters, the thermodynamics and chemical abundance of
the ICM (e.g., Postman et al. 2005; Mei et al. 2006a,b; Balestra
et al. 2007; Gobat et al. 2008; Maughan et al. 2008; Giodini et
al. 2009; Mantz et al. 2010b; Santos et al. 2010; Strazzullo et al.
2010). About twenty clusters at z = 0.8–2.1 have been discovered and studied in some detail (e.g., Stanford et al. 2001, 2002,
2006; Blakeslee et al. 2003; Mullis et al. 2005; Demarco et al.
2007; Hilton et al. 2007, 2009; Eisenhardt et al. 2008; Lamer et
al. 2008; Menci et al. 2008; Wilson et al. 2009; Kurk et al. 2009;
Tanaka, Finoguenov & Ueda 2010; Papovich et al. 2010; Henry
et al. 2010; Gobat et al. 2011; Fassbender et al. 2011a). However,
a better sampling in redshift and total mass is necessary to build
a representative sample of high-z groups and clusters. Existing
X-ray surveys allow clusters in the redshift range 0.8–1.5 to be
detected, with about 1.5 such clusters per square degree down to
a flux level of ∼ 6 × 10−15 erg s−1 cm−2 (S̆uhada et al. 2012).
The XMM-Newton Distant Cluster Project (XDCP) is a
serendipitous survey for galaxy clusters at z ≥ 0.8 based on deep
archival XMM-Newton observations: it now covers an area of
80 deg2 (Böhringer et al. 2005; Fassbender 2007; Fassbender et
al. 2011c). XDCP is concerned with the build-up of a representative and complete sample of distant clusters and the accurate
determination of the total mass of each system. Both aspects are
crucial for a test of the current understanding of structure formation and evolution in ΛCDM universes (e.g., Hoyle, Jimenez
& Verde 2011). In XDCP, X-ray sources identified as extended
are screened against existing optical all-sky surveys and eventually followed up with imaging in the z, H or I, J bands at
4m-class telescopes (occasionally in the r, Z bands at 8m-class
telescopes). If the distribution of the galaxies associated with
an X-ray source exhibits evidence of a so-called “red sequence”
(e.g. Bower, Lucey & Ellis 1992; Kodama et al. 1998; Gladders
& Yee 2000; see Arimoto & Yoshii 1987 for its physical interpretation) in the ensuing colour–magnitude diagram, a candidate high-z cluster is robustly selected and proposed for spectroscopic confirmation at 8m-class telescopes. This strategy has
already yielded more than 30 spectroscopically confirmed distant clusters since the discovery of XMMU J2235.3−2557 at z =
1.393 (Mullis et al. 2005). They include XMMU J0338.8+0021
at z = 1.49 (Nastasi et al. 2011), XMMU J1007.4+1237 at
z = 1.555 (Fassbender et al. 2011b) and XMMU J0044.0−2033
at z = 1.579 (Santos et al. 2011).
Among the main results from XDCP we quote the multiwavelength studies of XMMU J1229+0151 at z = 0.975 (Santos
et al. 2009) and XMMU J2235.3−2557 (Rosati et al. 2009;
Strazzullo et al. 2010); the weak-lensing estimate of the total
mass of XMMU J2235.3−2557 (Jee et al. 2009); the total mass
estimate of XMMU J100750.5+125818 (z = 1.082) from strong
lensing, optical spectroscopy and X-ray imaging (Schwope et
al. 2010); the pan-chromatic study of XMMU J1230.3+1339 at
z = 0.975 (Fassbender et al. 2011a) and its total mass estimate
from weak-lensing analysis of ground-based imaging (Lerchster
et al. 2011); the evidence of starburst activity in the core of
XMMU J1007.4+1237 (Fassbender et al. 2011b).
In order to maximize the numerical throughput from XDCP,
low-significance candidate high-z clusters are followed up
with the seven-channel imager GROND (Gamma-Ray Burst
Optical and Near-Infrared Detector; Greiner et al. 2008), that
is mounted at a 2m-class telescope, before being proposed for
multi-object spectroscopy (MOS) at 8m-class telescopes. The
2
optical confirmation of these distant clusters lies in the discovery of a significant feature in the distribution of the photometric
redshifts of galaxies selected in a region corresponding to the
extended X-ray emission. Otherwise, it relies upon the fact that
colour- or photo-z-selected galaxies within a given projected distance from the centroid of the extended X-ray emission define an
over-density with respect to the surrounding region and/or a red
sequence in a given colour–magnitude diagram.
Here we report on the feasibility of this approach by illustrating results obtained from the multi-wavelength observations of XMMU J0338.7+0030. Follow-up z, H photometry
was originally obtained with the near-IR, wide-field camera
OMEGA2000 mounted at the prime focus of the 3.5m telescope at Calar Alto, Spain. A redshift z ∼ 1.45 ± 0.15 was
inferred from comparison between the average z − H colour
of the reddest galaxies within 45′′ from the X-ray position
of XMMU J0338.7+0030 and the synthetic z − H colours expected for a large suite of simple stellar population (SSP) models (see Fassbender et al. 2011c). The synthetic photometry was
computed using the spectro-photometric evolutionary synthesis
model PÉGASE v2 (Fioc & Rocca-Volmerange 1997) in analogy
with Pierini et al. (2005) and Wilman et al. (2008). A calibration
of the best-performing SSP model was originally based on observed red-sequence galaxies in 10 X-ray selected clusters with
spectroscopic redshifts up to 1.5 (Fassbender 2007). This fiducial model corresponds to a Salpeter (1955) stellar initial mass
function (IMF) between 0.1 and 120 M⊙ (as all the others), a
solar metallicity and a formation redshift equal to 5.
In addition to its being potentially one of the few clusters at
z ≥ 1.4 known so far, XMMU J0338.7+0030 represented a test
case for XDCP sources at the detection limit of the survey. It was
an ideal target for GROND imaging because its region is covered
by the Sloan Digital Sky Survey (SDSS; York et al. 2000) and the
2 Micron All Sky Survey (2MASS; Skrutskie et al. 2006), which
enables a robust photometric calibration for all seven bands. For
the same region, sparse redshift information was later available
from observations with the spectrograph FORS2 mounted at the
ESO Very Large Telescope (VLT) at Paranal, Chile.
The XMM-Newton, OMEGA2000, GROND and FORS2
observations of XMMU J0338.7+0030, plus the reduction and
analysis of the ensuing data, are discussed in Sect. 2. Estimates
of the X-ray bolometric luminosity, temperature and total mass
of this cluster, plus the spectroscopic determination of its redshift, are given there. Section 3 contains results obtained from
the GROND data, i.e. galaxy number counts, the photo-z distribution of the detected sources, the two-dimensional (2-D) spatial
distribution of photo-z-selected galaxies and their distribution
in specific colour–magnitude diagrams. Discussion and conclusions appear in Sect. 4 and 5, respectively.
Throughout we adopt a ΛCDM cosmological model (Ωm =
0.3, ΩΛ = 0.7) with H0 = 70 km s−1 Mpc−1 , that is broadly
consistent with the main results from the 5 yr operations of the
Wilkinson Microwave Anisotropy Probe (WMAP5, Dunkley et
al. 2009; Komatsu et al. 2009). Accordingly, at the spectroscopic
redshift of XMMU J0338.7+0030 (z = 1.097; Sect. 2.4.2), the
age of the Universe is 5.4 Gyr, the cosmic evolution factor
E(z) = H(z)/H0 is equal to 1.86, the luminosity distance is
7409.2 Mpc, the angular scale is 8.168 kpc/′′ . Hence, an angular
distance of 1′ corresponds to ∼ 490 kpc at the cluster distance.
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
Fig. 1. A z+H colour composite image (4.5′ on the side) of the environment of XMMU J0338.7+0030 with an overlaid contour map
of the X-ray emission detected by XMM-Newton (in yellow). These log-spaced X-ray contours correspond to significance levels of
2–16σ; they were derived from the adaptively smoothed, combined images of the cluster environment. Red contours highlight the
over-density of galaxies with very red z − H colours (3.0 ≤ z − H ≤ 4.8 Vega mag), as obtained from the OMEGA2000 imaging (see
Sect. 2.2.2). White dashed and solid lines encircle regions within 30′′ and 60′′ from the X-ray centroid of the cluster (R.A.(J2000.0):
03h38m44.2s, Dec(J2000.0): +00d30m01.8s), respectively; they guide the eye. The red contour closest to the X-ray position of the
cluster corresponds to a significance of 4.6σ above the mean background.
2. Observations, data reduction and analysis
2.1. X-ray imaging with XMM-Newton
2.1.1. Initial X-ray selection
A total of 470 XMM-Newton (Jansen et al. 2001) archival
fields with nominal exposure times longer than 10 ks have
so far been processed and analyzed in XDCP as extensively described in Fassbender (2007) and Fassbender et al.
(2011c). The serendipitous, weak X-ray source identified as
XMMU J0338.7+0030 was detected in the field with observation identification number (OBSID) 0036540101. The target
was SDSS 033829.31+002156.3 (R.A.(J2000.0): 03h38m38.5s,
Dec(J2000.0): +00d39m28s), a quasar at z = 5.07091 (Péroux et
al. 2001) imaged on February 22, 2002 with XMM-Newton for
a nominal exposure time of 22.9 ks.
This serendipitous X-ray source was found at an offaxis angle of 9.6′ (R.A.(J2000.0): 03h38m44.2s, Dec(J2000.0):
+00d30m01.8s) and a significance of about 5σ during the early
XDCP source detection run performed with SAS v6.1. The tasks
eboxdetect was used for a sliding box detection, followed by a
subsequent maximum likelihood analysis with emldetect. This
yielded an extent of 3.48 pixels (i.e., 13.92 arcsec), a source extent likelyhood of about 2σ and a flux of ∼ 8×10−15 erg s−1 cm−2
for ∼ 32 PN counts, close to the detection threshold. Hence,
XMMU J0338.7+0030 appeared as a marginally extended X-ray
source at the XMM-Newton detection threshold for extended
sources. An X-ray contour map is overlaid on the z+H colour
composite image obtained from the medium–deep imaging with
OMEGA2000 (Sect. 2.2.1) in Fig. 1.
The circular area centred on the X-ray emission centroid of
XMMU J0338.7+0030 and delimited by a radius of 1′ is assumed as the “bona fide” cluster region throughout.
2.1.2. The upgraded X-ray picture of XMMU J0338.7+0030
The XMM-Newton field containing XMMU J0338.7+0030 was
reprocessed with SAS v10.0.0 following S̆uhada et al. (2011).
These observations are strongly flared and an automatic flare removal is impossible; hence, the contaminated periods were excised manually and the automatic two-step flare cleaning process
was run on the remaining good part of the data for the removal
of high background periods. The loss of information is severe:
clean net exposure times on axis amount to 9 ks for either MOS
camera and 7.8 ks for the PN instrument, whereas the effective
(vignetted) clean net exposure times on source amount to 4.5–
5.5 ks for all detectors. Even after cleaning a slight residual quiescent contamination (about 10% above normal) remains; large
part of it should be captured by the background model, so this
anomalous contamination likely has a negligible effect.
With SAS v10.0.0 XMMU J0338.7+0030 is detected as a
very weak source without a significant extent, located about 10′′
to the east with respect to the X-ray position originally estimated
(Sect. 2.1.1). A Kolmogorov–Smirnov (KS) test with the null hypothesis that the extracted X-ray profile corresponds to that of a
point-like source gives −log PKS = 2.08 and −log PKS = 1.57
for the PN and MOS data, respectively. In other words, there is
a chance of 0.8% or 2.7% that XMMU J0338.7+0030 is a pointlike source. The result based on the PN instrument is likely unrealistic, because PN has a chip gap very close to the centre.
Furthermore, the performed test uses the on-axis point-spread3
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
2.2. Near-infrared follow-up imaging with OMEGA2000
2.2.1. Observations and data reduction
XMMU J0338.7+0030 was observed with the near-IR camera
OMEGA2000 (Bailer-Jones, Bizenberger & Storz 2000) that
is mounted at the 3.5m telescope at the Centro Astronómico
Hispano Alemán (CAHA) at Calar Alto, Spain. OMEGA2000
provides a field-of-view (FoV) of 15.4′ × 15.4′ . In particular,
medium–deep imaging of the XMMU J0338.7+0030 region in
the H-band (50 min) and z-band (53 min) was obtained on
January 5, 2006 under good but non-photometric conditions
of the sky and with a seeing (or PSF) that varied between
1.0′′ and 1.5′′ (z-band) or 1.6′′ (H-band) in full-width-at-halfmaximum (FWHM). The same region was reobserved in z-band
(5 min) on October 30, 2006 in photometric conditions of the
2
R∆ (∆ = 500) is the radius within which the total mass density of a
group/cluster is equal to ∆ times the critical density of the universe (ρc ).
Correspondingly, M∆ = ∆ ρc (z) (4π/3) R3∆ is the total mass within R∆ .
4
1.5
fX(<r) [10−14 erg s−1cm−2]
function (PSF) of XMM-Newton, so the output probabilities
are slightly underestimated for the off-axis source identified as
XMMU J0338.7+0030. We conclude that this source is weak
and marginally extended.
This is supported by the regular behaviour of the growth
curves, where the growth curve analysis (GCA) method of
Böhringer et al. (2000) was applied to measure the flux in the
soft 0.5–2 keV band. The two growth curves obtained with the
MOS cameras or the PN instrument agree (see Fig. 2). However,
they are not independent: the latter was slightly corrected to
obtain a rough agreement, because PN has a chip gap very
close to the centre as already mentioned. The double component background model was used in the first instance; the alternative use of a spline fitting led to the same results. As a result, we measured a flux fX,500 ≃ 7.1(±2.3) × 10−15 erg s−1 cm−2
within a circular aperture of radius equal to R500 2 (corresponding to 51.2′′ ). At the cluster redshift, this translates into an Xray luminosity of LX,500 ≃ 4.5(±1.4) × 1043 erg s−1 in the soft
44
band or Lbol
erg s−1 for the bolometX,500 ≃ 1.1(±0.3) × 10
ric luminosity. The latter quantity was inferred as described in
S̆uhada et al. (2010). In brief, the growth curve was used to iteratively obtain a self-consistent set of parameters using the X-ray
luminosity–temperature and X-ray luminosity–total mass relations from Pratt et al. (2009).
Owing to the faintness of the source with less than 50 net
counts in the soft band, the determination of additional structural or spectral parameters is currently not feasible. The potential flux contribution of point sources can presently not be
accounted for either, which implies that the stated luminosities
have to be taken as upper limits. However, based on the luminosity scaling relation in Pratt et al. (2009), first rough estimates
on the expected temperature of T X ∼ 2.4 keV and the total mass
of M500 ∼ 7 × 1013 M⊙ (i.e., M200 ∼ 1014 M⊙ ) can be obtained
from the previous values. Hence, XMMU J0338.7+0030 appears
to be a system at the boundary between X-ray selected groups
and clusters, as XMMU J1532.2−0836 at z = 1.358 (S̆uhada et
al. 2011) and XMMU J0338.8+0021 at z = 1.49 (Nastasi et al.
2011).
An exposure-corrected count image of the innermost region
of the cluster is produced after masking point-like sources (cf.
Fig. 1) and is then adaptively smoothed with a circular top hat
filter. The ensuing X-ray contours are reproduced in the right
panel of Fig. 4 (Sect. 2.3.1) using a squared scale.
PN
MOS1+MOS2
r500
rplat
Fplat
1.0
0.5
0.0
0
20
40
60
r [arcsec]
80
100
Fig. 2. Results from the application of the GCA method of
Böhringer et al. (2000) in the soft 0.5–2 keV band to the MOS
and PN observations of XMMU J0338.7+0030. The two growth
curves are well behaved and agree with each other.
sky. Observations of a designated SDSS standard star (Smith et
al. 2002) were performed then to enable a photometric calibration of the science frames.
These data were reduced with the OMEGA2000 near-IR
pipeline (Fassbender 2007), which performs the basic frame calibrations (flat-fielding, bad pixel correction), a two-pass background subtraction with masked objects, frame alignment and
optimally weighted co-addition of the individual 40 s (60 s) exposures in H-band (z-band). For each band, the individual frames
were stacked without PSF-matching, which gave a measured
seeing of 1.3′′ (z-band) or 1.4′′ (H-band) in the final image.
For the CAHA data, source detection, extraction of photometry and star/galaxy separation are described in detail in
Fassbdender et al. (2007). The SExtractor (Bertin & Arnouts
1996) photometry was calibrated to the Vega magnitude system
using 2MASS point sources (Cutri et al. 2003) in H-band and
designated SDSS standard star observations in z-band, crosschecked with SDSS photometry in the science frame. In particular, total magnitudes were measured in a standard Kron (1980)
aperture of factor 2.5 and minimum radius of 3.5. The limiting (Vega) magnitudes3 were determined to be Hlim ∼ 21.2 and
zlim ∼ 23.1. They correspond to the expected apparent magnitudes of an old, passively evolving galaxy at the estimated redshift of z ∼ 1.45 (see Sect. 1) for an absolute magnitude equal
to ∼ M ⋆ + 1.3 in H-band and ∼ M ⋆ in z-band, where M ⋆ is the
characteristic magnitude of the corresponding galaxy luminosity
function at z = 1.45 (see Fassbender 2007).
2.2.2. An over-density of very red galaxy at the location of
XMMU J0338.7+0030
In Fig. 1 (Sect. 2.1.1), the projected distribution of galaxies with
very red colours (3.0 ≤ z − H ≤ 4.8 Vega mag) exhibits a significant over-density: 4.6σ above the mean background. The peak
of this over-density is offest to the East by about 20′′ with respect
to the X-ray position of XMMU J0338.7+0030. When considering the degradation of the angular resolution of XMM-Newton
(∼ 14′′ half-energy-width - HEW) at the off-axis angle of this
3
These limiting magnitudes correspond to a 50% completeness, as
estimated from the number counts obtained from the OMEGA2000
imaging only, as in Fassbender et al. (2011a).
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
X-ray source (Sect. 2.1.1), an association between the previous
over-density of red galaxies and the cluster is very likely.
Figure 3 shows the z − H vs H colour–magnitude diagram
for the galaxies detected in the XMMU J0338.7+0030 region
imaged with OMEGA2000. In particular, seven galaxies within
45′′ from the X-ray position of the cluster exhibit 2.8 ≤ z − H ≤
3.8 Vega mag). A redshift z ∼ 1.45 ± 0.15 was initially inferred
from comparison between the average z − H colour of these
galaxies and the synthetic z − H colours expected for the fiducial
SSP model described in Sect. 1. The assumption was that such a
template described all those galaxies.
2.3. Optical/near-infrared imaging with GROND
2.3.1. Simultaneous images of XMMU J0338.7+0030
A region of the sky centred on the XMM-Newton X-ray position
of XMMU J0338.7+0030 was imaged on four different nights
(September 24, 2008, and January 27, 28, 29, 2009), in conditions of clear sky but variable seeing, with GROND. This is
a seven-channel optical/near-IR imager primarily designed for
rapid, simultaneous multi-wavelength observations of gammaray burst afterglows and built by the high-energy group of MPE
in collaboration with the Landessternwarte Tautenburg and ESO.
GROND was commissioned at the MPI/ESO 2.2m telescope at
La Silla, Chile, in April 2007 and is operated as a PI-instrument
(for its comprehensive description see Greiner et al. 2008).
For the optical channels, the incoming telescope beam is split
using four dichroics designed in such a way that their transmission functions are identical to those of the Sloan g′ , r′ , i′ , z′ filter
system, with the exception of the i′ -band. Each of the four identical optical CCDs has a FoV of 5.4 × 5.4 arcmin2 and a scale of
0.158′′/pixel. The near-IR part of GROND is designed as a focal
reducer system yielding a FoV of 10 × 10 arcmin2 with a scale
of 0.6′′ /pixel. The GROND J, H, Ks dichroic filters are close to
other near-IR filter systems. Photometric system transformations
between the GROND g′ , r′ , i′ , z′ bands and the Sloan filter system and between the GROND J, H, Ks bands and the 2MASS
filter system are given in Greiner et al. (2008).
A total of 10 observations in the seven channels was executed
through the GROND standard observing block (OB) named
“20min4TD”. By design, this OB corresponds to four single
369 s-long exposures in the g′ , r′ channels and four 369 s-long
exposures out of four pairs of sub-exposures in the i′ , z′ channels, each long exposure being taken at one out of four different
telescope dither positions. In parallel, the three near-IR channels are operated with 10 s integrations. At each telescope dither
position, a six-position dither pattern is executed through a flip
mirror (internal dithering) only for the Ks channel; five 10 slong exposures per mirror position are executed for the J, H, Ks
channels. This produces a total of 120 single 10 s-long exposures
per OB for each near-IR channel. Given the impact of overheads
(about 50% of the total exposure time of 4 hr for each J, H, Ks
channel), the total observing time amounted to about 6 hr.
Pixel- and gain-corrected, astrometrized, stacked images for
each channel and per OB were produced using the data reduction and photometry tools of the GROND pipeline (Küpcü
Yoldaş et al. 2008; Krühler et al. 2008), based on standard
IRAF4 /PyRAF tools (Tody 1993). Astrometry was achieved
through the use of stars in common with the SDSS catalogue
4
IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for
Research in Astronomy, Inc., under cooperative agreement with the
National Science Foundation.
(Abazajian et al. 2009) for the optical bands and the 2MASS
catalogue (Skrutskie et al. 2006) for the near-IR ones. For each
image the FWHM of the average point-spread-function (PSF)
was measured from the surface brightness profiles of randomly
distributed non-saturated stars. Images corresponding to values
of the average seeing FWHM higher than 1.5′′ were not considered in the following analysis. For each band, all selected images were convolved to the worst PSF (∼ 1.5′′ FWHM) and then
stacked with a median algorithm. The total exposure times of the
stacked g′ , r′ , i′ , z′ , J, H, Ks images are equal to 11808 s, 14760 s,
14760 s, 14760 s, 12000 s, 10800 s, and 9600 s, respectively. The
stacked near-IR images were regridded onto the same scale as
the optical ones (0.158′′/pixel). All seven stacked images were
re-mapped to the same aspect with the z′ -image as reference.
Figure 4 reproduces a J, z′ , g′ colour-composite (RGB)
image of the region of XMMU J0338.7+0030 mapped with
GROND (left panel) together with the deep z′ -image (right
panel).
2.3.2. The photometric catalogue of XMMU J0338.7+0030:
source extraction and photometry
A separate stack of seven individual images in the z′ channel,
PSF-matched to the same angular resolution of 1.1′′ (FWHM),
was used as a detection image with a total exposure time equal
to 10332 s. This choice was the best compromise between image
quality, photometric depth in the red/near-IR channels, robustness of detection and penalty in the source detection and extraction of photometry associated with deblending. It also enabled a
better, independent separation between extended and point-like
sources along the accessible flux domain.
Automatic source detection and extraction of photometry
within an area of 3.9 × 4.3 arcmin2 was accomplished through
the publicly available, standard software SExtractor (Bertin &
Arnouts 1996). A source was defined by the excess over a detection threshold equal to the standard deviation (1σ) of the global
image background in at least two contiguous pixels; in addition,
a Gaussian filter with a FWHM of 1.1′′ was applied for detection. Photometry of individual sources was extracted in six circular apertures with diameters equal to 1, 2, 3, 4, 5 and 10 arcsec,
in a Kron (1980) aperture of factor 2.5 and minimum radius of
3.5 (SExtractor MAG AUTO) and in a Petrosian (1976) aperture
of factor 2.0 and minimum radius of 3.5.
In the first step of extraction of source photometry, typical
photometric zeropoints for the GROND channels5 were assumed
together with typical corrections for atmospheric absorption (or
airmass), in order to aid identification of stars in common with
the existing SDSS and 2MASS catalogues and enable the calibration of the zeropoints of the present g′ , r′ , i′ , z′ , J, H, Ks images. In particular, we made use of the seventh Sloan data release (DR7; Abazajian et al. 2009 and references therein) and
the 2MASS All Sky Catalog of point sources (Cutri et al. 2003).
After this photometric calibration, adjustments for the newly determined zeropoints were introduced and magnitudes were corrected for Galactic extinction according to the values tabulated
in Schlegel et al. (1998) in addition to the previous correction for
atmospheric absorption.
5
GROND photometric zeropoints in the AB magnitude system are
available at http://www.mpe.mpg.de/∼jcg/GROND/calib.html and in
Greiner et al. (2008) together with conversions between different filter
systems or from the AB to the Vega magnitude system. These conversions are equal to −0.008, −0.151, −0.386, −0.515, −0.910, −1.381 and
−1.795 mag for, respectively, the GROND g′ , r′ , i′ , z′ , J, H, Ks bands.
5
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
Fig. 3. z − H vs H colour–magnitude diagram for the region encompassing XMMU J0338.7+0030 imaged with OMEGA2000.
Red filled circles represent galaxies within 45′′ from the X-ray position of the cluster. The blue dashed lines reproduce the 50%
completeness of the photometric catalogue extracted from the OMEGA2000 data. The horizontal red short-dashed line represents
the z − H colour expected for an SSP model corresponding to the evolutionary phase of an instantaneous burst of star formation
observed at z = 5 with Salpeter (1955) stellar IMF and solar metallicity observed at z = 1.45. This SSP model describes the average
colour of the seven galaxies associated with XMMU J0338.7+0030 that exhibit 2.8 ≤ z − H ≤ 3.8 Vega mag. The vertical red
dotted line represents the characteristic magnitude H ⋆ at z = 1.45 (from Fassbender et al. 2007). Empty black squares mark the four
spectroscopic cluster members that were successively established to be at z = 1.1 through VLT-FORS2 observations (see Fig. 11
and Table 1 in Sect. 2.4.2); blue filled circles highlight those with ID=12 and ID=15, which are at a cluster-centric distance greater
than 45′′ . The two spectroscopic cluster members without convincing evidence of ongoing star-formation activity (with ID=2 and
ID=16) exhibit about as red z − H colours as the previous SSP model (see Appendix B, however).
For the photometric calibration of the g′ , r′ , i′ , z′ (J, H, Ks)
images, the celestial coordinates of GROND sources and SDSS
(2MASS) catalogue stars were first cross-correlated using a
search radius of 3′′ . “Machine” Petrosian magnitudes (corrected
only for atmospheric absorption) and catalogue Petrosian magnitudes (not corrected for Galactic extinction) of non-saturated
sources that are unambiguously identified as stars and have robust photometry were contrasted after taking into account eventual colour terms (see footnote 5). For a given band, the weighted
mean of the magnitude differences was taken as the correction
to the assumed zeropoint and the value of the weighted standard
deviation as the uncertainty of the photometric zeropoint. These
uncertainties are equal to ±0.009, ±0.010, ±0.011, ±0.011,
±0.041, ±0.009 and ±0.036 mag in the g′ , r′ , i′ , z′ , J, H, Ks channels of GROND, respectively. They reflect the quality of the reference photometry and the poor statistics of the matched stars
used as calibrators. The good agreement between our set of calibrated magnitudes and that of SDSS (2MASS) for all the 38 (9)
stars in common is shown in Fig. 5 (6).
Our calibrated Petrosian magnitudes are compared to the
SDSS ones (all not corrected for Galactic extinction) for the 65
sources in our photometric catalogue that are associated with
galaxies identified in DR7 using a search radius of 3′′ . In addition, twenty-four of these 65 SDSS galaxies have a photometric
redshift in DR7. The overall agreement between the two sets
of Petrosian magnitudes is satisfying (Fig. 5). Unfortunately, no
galaxy counterparts to our sources seem to exist in any 2MASS
catalogue, according to NED. This is no surprise given the shal6
low limiting magnitudes of 2MASS (see Jarrett et al. 2000;
Skrutskie et al. 2006).
A calibrated photometric catalogue with 832 sources was
built as previously described, where magnitudes are corrected
for atmospheric extinction and Galactic extinction. Limiting
magnitudes (1σ values) were computed as the median values
of magnitudes obtained in 3′′ apertures on 10 separated regions
of blank sky distributed across the individual g′ , r′ , i′ , z′ , J, H, Ks
images. They correspond to 27.30, 26.89, 26.54, 26.06, 24.40,
23.50 and 22.91 mag, respectively. We note that the corresponding 5σ values in the z′ , J, H, Ks channels are about the depths
in analogous bands that the ESO public survey VIKING (VISTA
Kilo-Degree Infrared Galaxy Survey; P.I.: W. Sutherland) is expected to reach (see e.g., Arnaboldi et al. 2007).
Corrections to the SExtractor output photometric errors for
a given band (in part caused by correlated noise in the stacked
images) were computed from the ratio of the median value of
the uncertainties of the previous 10 aperture magnitudes and the
median SExtractor uncertainty of the 3′′ aperture photometry of
sources with magnitudes within ±1σ from the limiting magnitude. These correction factors amount to 1.74, 1.67, 1.82, 1.28,
2.15, 1.36, and 1.78 for the g′ , r′ , i′ , z′ , J, H, Ks bands, respectively.
Extended and point-like sources were initially identified on
the basis of surface brightness distribution (i.e., the SExtractor
parameters stellarity index and flux radius) and apparent brightness. The photometry of point-like sources enabled us to estimate corrections from the 3′′ aperture magnitudes to magnitudes
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
Fig. 4. J, z′ , g′ colour-composite image of the 3.9 × 4.3 arcmin2 region of XMMU J0338.7+0030 mapped with GROND at all seven
bands (left panel). North is up and east to the left. The corresponding z′ -image (right panel) illustrates the 2-D distribution of the 44
sources in the GROND photometric sample with a photo-z that is consistent with the spectroscopic redshift, i.e., at 1.01 ≤ z ≤ 1.23
(empty circles). These 44 photometric members are colour-coded according to their spectro-photometric classification as E (red),
Sbc (yellow), Scd (green), Im (cyan), SB (blue) or QSO (purple). The three spectroscopic members among them are marked with
small, black empty circles. Two concentric black circles with radii of 6′′ and 60′′ mark the X-ray position and bona-fide region
of XMMU J0338.7+0030, respectively. Magenta contours represent the weak, marginally extended X-ray emission of the source
determined in Sect. 2.1.2. This emission is consistent with the projected distribution of the photometric cluster members. However,
the candidate BCG (thick red empty circle) is offset by ∼ 43′′ towards the east–south–east.
in the z′ -band Kron aperture for all bands. These aperture corrections (in the standard SExtractor format) amount to
g′ = g′ (3′′ ) − 0.1143(±0.0094) ,
r′ = r′ (3′′ ) − 0.1366(±0.0046) ,
i′ = i′ (3′′ ) − 0.1219(±0.0051) ,
z′ = z′ (3′′ ) − 0.1254(±0.0040) ,
J = J(3′′ ) − 0.1613(±0.0175) ,
H = H(3′′ ) − 0.1510(±0.0197) ,
K s = K s(3′′ ) − 0.1463(±0.0307).
We have then adopted two operational definitions of total magnitudes and investigated their performances given the
characteristics of the available imaging. As a first prescription
(method A), we adopted the Kron magnitude in the z′ band as
the reference total magnitude and computed total magnitudes
in the g′ , r′ , i′ , J, H, Ks bands from the reference total magnitude plus the corresponding colour term measured within a 3′′
aperture and corrected to the Kron aperture. As a second prescription (method B), we adopted the Kron magnitude as the
total magnitude in the z′ band but computed total magnitudes
in the g′ , r′ , i′ , J, H, Ks bands from the corresponding 3′′ aperture magnitudes, corrected to the Kron aperture. This second set
of total magnitudes yielded photometric redshifts that are better
consistent with the spectroscopic ones for the three confirmed
members of XMMU J0338.7+0030 with GROND photometry
(Sect. 3.2.1). At the same time the distribution of these three
spectroscopic members in the i′ − z′ vs z′ colour–magnitude diagram is much better consistent with the red-sequence of the
cluster RDCS J0910+5422 at z = 1.106 (Mei et al. 2006a), as
discussed in Sect. 3.2.2. For these reasons, we will make use of
the aperture-corrected magnitudes in the g′ , r′ , i′ , J, H, Ks bands
and the Kron magnitude in the z′ band for all sources (method
B), and refer to them as total magnitudes hereafter.
Figure 7 illustrates the relation between the computed total magnitude and its uncertainty in each of the GROND
g′ , r′ , i′ , z′ , J, H, Ks channels. The regular behaviour observed for
the GROND g′ , r′ , i′ , J, H, Ks broad bands is due to the fact that
two sources hardly exhibit exactly the same measured value for
their aperture fluxes. For the GROND g′ , r′ , i′ , J, H, Ks channels
the uncertainty on the total magnitude is dominated by the uncertanty on the aperture correction at bright magnitudes and by the
measurement error at faint magnitudes. The transition between
these two regimes corresponds to a total magnitude that depends
on the filter.
7
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
0.5
0
-0.5
15
20
15
20
15
20
15
20
0.5
0
-0.5
0.5
0
-0.5
0.5
0
-0.5
Fig. 5. Comparison of the Petrosian magnitudes of stars (solid circles) and galaxies (empty circles) with GROND and SDSS photometry. The photometry in the GROND g′ , r′ , i′ , z′ bands is calibrated with that of the stars in common with SDSS in the corresponding
SDSS bands (panels a, b, c, d, respectively). Saturated stars and stars with poor photometry were flagged out. The asterisks indicate
that the reference magnitudes are expressed in the GROND filter system. In each panel the red solid line represents equality.
8
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
0.5
0
-0.5
15
20
15
20
15
20
0.5
0
-0.5
0.5
0
-0.5
Fig. 6. Comparison of the Petrosian magnitudes of stars (solid circles) after their photometry in the GROND J, H, Ks bands is
calibrated with that in the corresponding 2MASS bands (panels a, b, c, respectively). Saturated stars and stars with poor photometry
were flagged out. The asterisks indicate that the reference magnitudes are expressed in the GROND filter system. In each panel the
red solid line represents equality.
2.3.3. The photometric catalogue: star/galaxy separation
As an alternative to the previous selection criteria, galaxies (extended sources) can be separated from bona-fide stars (point-like
sources) on the basis of their g′ −z′ and z′ −K s colours, in analogy
with the BzK criterion (Daddi et al. 2004; cf. e.g. Galametz et al.
2009). We adopted the formulation of the colour criterion given
by Galametz et al. (their fig. 5) and applied the empirical colour
transformation between the SDSS photometry and the Johnson–
Cousins photometric system of Jordi, Grebel & Ammon (2006)
g − B = 0.370 × (B − V) − 0.124
plus suitable corrections for the different filter transmission functions and magnitude systems. The ensuing threshold correspond-
ing to the GROND g′ z′ K s criterion (short-dashed line in Fig. 8)
was contrasted against the behaviour of our catalogue sources
classified as stars on the basis of the SExtractor stellarity index in Sect. 2.3.2. This comparison prompted the introduction
of a shift equal to +0.27 mag in z′ − K s (solid line in Fig. 8).
Therefore, the g′ z′ K s criterion hereafter adopted to single out
bona-fide stars is
z′ − K s (3′′ ) ≤ 0.3 × [g′ − z′ (3′′ )] − 0.1,
(1)
′′
where we adopted 3 -aperture measurements. Comparison with
the source classification provided by the use of the photometric
redshift technique (Fig. 13a in Sect. 3.2.1) reveals the presence
of a few extragalactic point-like sources with g′ −z′ (3′′ ) ∼ 1 and
z′ − K s (3′′ ) ∼ 0 that are misclassified as stars in Fig. 8.
9
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0
15
20
25
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0
15
20
25
15
20
25
15
20
25
0
15
20
25
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0
0
15
20
25
0.4
0.3
0.2
0.1
0
15
20
25
Fig. 7. Uncertainty of the computed total photometry in each of the g′ , r′ , i′ , z′ , J, H, Ks broad-bands of GROND as a function of the
total magnitude of a source (panels a, b, c, d, e, f, g, respectively). In each panel the short-dashed, solid and long-dashed red lines
represent the 1σ, 3σ and 5σ limits, respectively. Only sources brighter than the limiting magnitude in a given band are reproduced.
10
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
As an independent test of the overall goodness of the previous criterion, in Fig. 9, we compared the distribution of the
bona-fide stars identified through Eq. 1 in the J − K s vs z′ − K s
colour–colour diagram with the loci occupied by spectrally classified stars with SDSS and 2MASS photometry (Galametz et al.
2009, their fig. 1) after introducing suitable corrections for the
different filter transmission functions. As discussed by Galametz
et al., most of the stars in a matched 2MASS/SDSS catalogue
populate either the region of the J − K s vs z′ − K s colour–colour
diagram above the short-dashed line and tend to be redder than
z′ − K s = 0 if their individual spectral types are later than K5,
or the region below the short-dashed line and across/to the right
of the solid line if they are earlier than K5. The consistent behaviour we find confirms the validity of Eq. 1.
4
2
0
-2
2.4. Optical spectroscopy with FORS2
-2
0
2
4
2.4.1. Observations and data reduction
Fig. 8. Distribution of all sources with fluxes in the GROND
g′ , z′ , Ks channels above the respective 1σ thresholds in the
z′ − K s vs g′ − z′ colour–colour diagram. Magnitudes within
a 3′′ -wide circular aperture are used. Sources identified as pointlike on the basis of a SExtractor stellarity index larger than
0.98 are marked with red asterisks. Conversely, extended sources
within (beyond) 1′ from the position of XMMU J0338.7+0030
are marked with filled (empty) circles. The short-dashed and
solid lines represent, respectively, the computed and adopted
thresholds to select bona-fide stars (see text).
0
-0.2
-0.4
-0.6
-1
-0.5
0
0.5
1
Fig. 9. Distribution of all bona-fide stars, selected according to
Eq. 1, in the J − K s vs z′ − K s colour–colour diagram, where
3′′ -aperture photometry is used. This distribution is consistent
with the loci occupied by spectrally classified stars with SDSS
and 2MASS photometry (cf. fig. 1 in Galametz et al. 2009). As
shown by Galametz et al., stars with a spectral type later than
K5 populate the region above the short-dashed line and tend to
exhibit z′ − K s > 0. Conversely, stars with a spectral type earlier
than K5 populate the region blueward of the horizontal shortdashed line and define a linear colour–colour relation (solid line)
with extensions towards very red z′ − K s colours.
The SDSS DR7 archive contains no spectroscopy for the region of the sky encompassing XMMU J0338.7+0030. However,
sparse spectroscopic information is available from XDCP,
since the neighboring distant cluster XMMU J0338.5+0029
(Fassbender et al. 2011c) was observed on November 7, 2007
(programme ID 079.A−0634, service mode) for 3 hr with
FORS2 (Appenzeller et al. 1998). This is a multi-mode (imaging, polarimetry, long slit, MOS) instrument working at wavelengths of 3300–11000 Å, mounted on the Cassegrain focus of
VLT UT1 in Paranal, Chile.
A single mask with 32 slits of 1′′ -width each was designed
and used in the FORS2 MXU-mode (Mask eXchange Unit) with
the standard resolution collimator, which yields a 6.8′ ×6.8′ FoV
with a scale of 0.25′′ /pixel. Coupled to the 300I grism and the
OG590 order blocking filter, this provided a resolution R = 660
and a wavelength coverage of 6000–10000 Å. Six single exposures were taken during the night for the same set-up, yielding
a total net integration time of 8400 s; the seeing ranged between
1.2′′ and 1.3′′ (FWHM). Because XMMU J0338.7+0030 was
the secondary target of these FORS2 observations, only 30% of
the slits were used to probe galaxies in its region. As primary
and secondary spectroscopic targets were chosen, respectively,
very red and bluer objects in the z − H vs H colour–magnitude
diagram built out of the OMEGA2000 photometry (Fig. 3). As a
result, only five slits out of a total of 32 turned out to correspond
to objects in the region of XMMU J0338.7+0030 later imaged
with GROND, for which photometry was extracted.
A new version of the VIMOS Interactive Pipeline and
Graphical Interface (VIPGI; Scodeggio et al. 2005), modified
to make it usable on FORS2 multi-slit spectra, was used. A detailed discussion of this new multi-spectra reduction pipeline is
deferred to a forthcoming paper (Nastasi et al. in preparation). In
brief, the stacked 2-D raw data were flat-fielded, bias-subtracted
and calibrated in wavelength by means of a helium–argon lamp
used as a reference line spectrum. The final dispersion solutions had a root-mean-squares calibration uncertainty lower than
1 Å/pixel. The continuum of each spectrum was corrected for
the sensitivity function of the FORS2 instrument, and the 1-D
reduced spectrum was extracted from each slit for the redshift
determination. Each 1-D reduced spectrum was both visually inspected by means of the graphical tools available in VIPGI and
cross-correlated with template galaxy spectra using the IRAF
package RVSAO (Kurtz & Mink 1998).
11
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
200
100
0
400
300
200
100
0
400
300
200
100
0
100
50
0
6000
7000
8000
9000
Fig. 10. Histogram of the measured galaxy spectroscopic redshifts in bins of ∆z = 0.01. An overdensity at z = 1.1 is evident.
Only five of these redshifts belong to objects in the GROND
photometric catalogue.
Fig. 11. Spectra of the four cluster members smoothed with a
seven-pixel boxcar filter. Black, dashed lines indicate the expected positions of prominent spectral features at z = 1.1.
Features caused by telluric absorption (top panel) or sky emission (bottom panel) are plotted in red.
2.4.2. The spectroscopic redshift of XMMU J0338.7+0030
strength of the 4000 Å break (D4000) as shown in Table 1. In
particular, a value of D4000 ∼ 1.9 is consistent with a minimum
light-weighted age of the stellar populations of ∼ 3 Gyr, according to the set of models with different star-formation histories
and metallicities in Gallazzi et al. (2005) (A. Gallazzi private
communication). Together with the lack of detected [O II] line
emission, this confirms the spectroscopic member with ID=2
as a galaxy dominated by old, passively evolving stellar populations (i.e., an early-type galaxy). Minimum light-weighted
ages of the stellar populations equal to ∼ 1 Gyr and ∼ 1.4 Gyr
seem to characterize the spectroscopic members with ID=15 and
ID=16, respectively. These galaxy are likely evolved systems
with ongoing star-formation activity. Finally, the spectroscopic
member with ID=12 exhibits not only the strongest ongoing
star-formation activity but also, consistently, the lowest value of
D4000, which sets the minimum light-weighted age of its stellar
populations to be equal to ∼ 0.3 Gyr.
Figure 10 shows the redshift distribution of the galaxies at
z ≥ 0.5 that were observed with FORS2 in a 6.8′ × 6.8′ region of the sky that contains both XMMU J0338.5+0029 (primary target) and XMMU J0338.7+0030 and that are not members of XMMU J0338.5+0029 (see Fassbender et al. 2011c).
Four galaxies exhibit consistent redshifts and are therefore identified as spectroscopic cluster members. As a consequence,
XMMU J0338.7+0030 is at a mean spectroscopic redshift <
z >= 1.097 ± 0.002 (1σ, where the uncertainty is determined
from a boostrap analysis combined with the biweight location
estimator. These four spectroscopic members lie within 2.3′
from the X-ray position of their host cluster, which corresponds
to a projected distance of ∼ 1.1 Mpc for the assumed cosmology (Sect. 1). Their spectra are shown in Fig. 11, where the most
prominent spectral features are labelled; their main properties
are listed in Table 1.
Out of these four spectroscopic members, three (with ID=2,
ID=15 and ID=16) have counterparts in the GROND photometric catalogue and lie within 1′ from the original X-ray position
of XMMU J0338.7+0030 (see Fig. 4). The object with ID=2 can
be identified as a passively evolving galaxy (likely an early-type
galaxy) on the basis of its spectral properties. Conversely, the
spectroscopic member with ID=15 exhibits robust evidence of
on-going star-formation activity, as traced by the (unresolved)
[O II] doublet at λ = 3727 Å. This activity is not as vigorous as
in the spectroscopic member with ID=12 (modulo dust attenuation) though. The object with ID=16 has the least robust determination of photo-z and a controversial evidence of [O II] line
emission. It is located in correspondence of the X-ray centroid
of XMMU J0338.7+0030 determined in Sect. 2.1.1.
The absence/presence of the [O II] line and its equivalent
width (EW) are consistent with the measured values of the
12
3. Results
3.1. Galaxy number counts
In order to illustrate depth and completeness of the photometric
catalogue extracted from the GROND optical/near-IR imaging
(Sect. 2.3.3), the number of galaxies per 0.5 mag-bin and unit
area for each of the i′ , z′ , Ks bands is reproduced in Fig. 126 .
Galaxies are selected as in Sect. 2.3.3. Statistical uncertainties
are determined from the approximate formulae for confidence
limits based on the Poisson and binomial statistics presented in
Gehrels (1986).
6
Corrections for masked areas are introduced here; they correspond
to 0.8% of the total area, which is equal to 4.64 × 10−3 deg2 . Since these
corrections are negligible, they will not be introduced later on.
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
Table 1. Summary of the main properties of the four galaxies observed with OMEGA2000 and FORS2 identified as spectroscopic
cluster members.
ID
2
12
15
16
RA (J2000)
hh mm ss
03 38 44.136
03 38 43.026
03 38 40.912
03 38 44.299
Dec (J2000)
dd mm ss
+00 30 29.95
+00 32 18.55
+00 30 10.66
+00 30 03.71
H
Vega mag
19.39
19.03
18.96
20.29
z−H
Vega mag
2.84
1.98
2.27
3.20
The GROND i′ -, z′ - and Ks-band galaxy number counts
are compared with those determined from observations of deep
fields or survey areas like the Hawaii Hubble Deep Field North
(Hawaii HDF-N, Suprime-Cam I- and z′ -band – Capak et al.
2004), the Cosmic Evolution Survey (COSMOS, ACS WFC
F814W filter – Leauthaud et al. 2007; Suprime-Cam i+ band –
Capak et al. 2007; WIRCam Ks band – McCracken et al. 2010),
the Calar Alto Deep Imaging Survey (CADIS, OMEGA Prime
K′ band – Huang et al. 2001), the Subaru Deep Field (SDF,
CISCO K′ band – Maihara et al. 20017 ), the infrared complement of the VIMOS-VLT deep survey (VIRMOS, SOFI Ks band
– Iovino et al. 2005), the FLAMINGOS Extragalactic Survey
(FLAMEX, FLAMINGOS Ks band – Elston et al. 2006), and the
UKIRT Infrared Deep Sky Survey Ultra-deep survey (UKIDSS
UDS, UKIRT Ks band – Hartley et al. 2008).
The present GROND z′ -selected sample of galaxies has a
50% completeness limit equal to z′ compl ∼ 25.3, as determined
from the extrapolation of the galaxy number counts in Capak et
al. (2004). This completeness magnitude corresponds to a ∼ 2σ
flux detection (cf. Fig. 7d). The 50% completeness limit is equal
to ∼ 24.8 AB mag in i′ -band, which corresponds to a ∼ 5σ flux
detection. Finally, the GROND photometric sample is 50% complete down to K scompl ∼ 22.9, which corresponds to a ∼ 1σ
flux detection. This completeness magnitude also corresponds
to ∼ K s⋆ + 2.4, where K s⋆ = 20.5+0.4
−1 is the characteristic magnitude (Schechter 1976) of the Ks-band luminosity function of
cluster galaxies at z ∼ 1.2 (Strazzullo et al. 2006).
Comparison of the different galaxy number counts in Fig. 12
shows an overall agreement between our results and those in the
literature down to the completeness limit of the GROND photometric sample of galaxies in each band. This is important when
considering the differences in the sky area that is probed by different observations (leading to cosmic variance), the operational
definition of total magnitude and the adopted galaxy/star separation criteria. These differences mostly affect the bright end of
the galaxy number counts, which corresponds to magnitudes approximately brighter than 20 AB mag in Fig. 12. Furthermore,
our galaxy number counts refer to the same sample extracted in
the z′ -band, whereas the galaxy number counts in the literature
refer to individual samples extracted in the i′ -, z′ - or K-band.
In spite of the overall agreement, our number counts exhibit
an excess (by a factor of almost two) in the magnitude range between K scompl − 2 and K scompl − 0.5 (i.e., 20.8 ≤ K s ≤ 22.3)
with respect to all others. This magnitude range corresponds to
∼ K s⋆ + 0.3 ≤ K s ≤ K s⋆ + 1.8 for cluster galaxies at z ∼ 1.2
(Strazzullo et al. 2006). Consistently, a slight excess across a
broader magnitude range (i.e., approximately at magnitudes between 21.5 and 23.5 AB mag) is visible from comparison of the
galaxy number counts for the z′ band. No clear excess is apparent in the galaxy number counts for the i′ -band. This behaviour
is interpreted as an indication for the presence of an over-density
7
We plot the completeness-corrected, K′ -band galaxy number counts
provided by Maihara et al. (2001).
z
D4000
1.09437 ± 0.00090
1.09787 ± 0.00040
1.09641 ± 0.00027
1.09930 ± 0.00044
1.901 ± 0.052
1.194 ± 0.036
1.633 ± 0.028
1.690 ± 0.131
[OII] EW
Å
−32.908 ± 1.380
−7.952 ± 0.962
−9.291 ± 3.873
of galaxies associated with the weak, marginally extended X-ray
emission corresponding to XMMU J0338.7+0030.
3.2. Photometric-redshifts
The photometric redshift technique enables us to assess the
membership to XMMU J0338.7+0030 for the 832 sources in the
GROND photometric sample. Distances to individual sources
were estimated through the publicly available package for
PHotometric Analysis for Redshift Estimations, le Phare8 (S.
Arnouts & O. Ilbert), which is a set of Fortran programmes to
compute photo-z’s using the standard SED fitting technique.
We selected the large pool of observed SEDs of stars (254)
and the mixture of observed and synthetic SEDs for QSOs (28)
that are provided by le Phare, as it is standard. However, we initially adopted two sets of templates to describe observed SEDs
of galaxies and investigated their performances.
The first set was used for the COSMOS photo-z paper (Ilbert
et al. 2009). It contains 31 templates: the SEDs numbered from
1 to 7 correspond to galaxies that can be morphologically classified as elliptical (E), the SED number 8 corresponds to a lenticular (or S0) galaxy, SEDs numbered from 9 to 12 correspond to
bulge-dominated disc galaxies (i.e, Sa and Sb), SEDs numbered
from 13 to 19 correspond to bulge-less disc galaxies (i.e, Sc, Sd,
and Sdm), SEDs numbered from 20 to 31 describe starburst (SB)
galaxies. We followed the indicated prescriptions for the modeldependent attenuation9 of galaxy templates, so that a “Prevot extinction law” (Prevot et al. 1984) was applied to SEDs numbered
from 13 to 23, a so-called “Calzetti attenuation law” (Calzetti et
al. 2000) and two “modified” Calzetti attenuation laws (see Ilbert
et al. 2009) were applied to SEDs numbered from 23 to 31. This
choice accounts for the fact that high-z star-forming galaxies are
characterized by extinction laws that differ at rest-frame ultraviolet wavelengths, as it is in the Local Group (Noll et al. 2009
and references therein). In all cases, the reddening E(B − V) was
enabled to range between 0 and 0.5 mag in steps of 0.05 mag.
The second set of galaxy templates corresponds to the 66
SEDs that are used in Ilbert et al. (2006) for the analysis of the
CFHTLS Deep fields. In the CFHTLS set, the ratio between the
numbers of E and SB templates is reversed with respect to the
COSMOS set: E, Sbc, Scd, Im and SB galaxies are described
by 22, 17, 12, 11, and 4 SEDs, respectively. For Scd, Im and SB
galaxy templates, a Prevot et al. (1984) extinction law is adopted,
with values of E(B−V) ranging from 0 to 0.25 with a step of 0.05
consistently with the indications in the le Phare documentation.
8
www.oamp.fr/people/arnouts/LE PHARE.html
We note that the extinction curve describes the combined absorption and out-of-the beam scattering properties of a mixture of dust
grains of given size distribution and chemical composition in a screen
geometry as a function of wavelength; the attenuation function is the
combination of the extinction curve with the geometry of a dusty stellar
system, in which a substantial fraction of the scattered light is returned
to the line of sight (e.g. Pierini et al. 2004b).
9
13
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
4
2
20
25
i [AB mag]
4
2
20
25
z’ [AB mag]
4
2
20
25
Ks [AB mag]
Fig. 12. Comparison of the galaxy number counts determined
in the region of XMMU J0338.7+0030 imaged with GROND
for the i′ (top), z′ (centre) and Ks band (bottom) and the corresponding ones from deep fields or survey areas. Error bars are
shown only for our points. In each panel, the short-dashed, solid
and long-dashed lines represent the 1 σ, 3 σ and 5 σ thresholds of our photometry, respectively. The GROND galaxy number counts exhibit a clear excess for 20.8 ≤ K s ≤ 22.3.
14
For each library of galaxy SEDs, we considered two separate families of models, by including the contribution to the total
flux from line emission or not. We opted to do this because there
is independent evidence that the bright end of the galaxy luminosity function of a high-z cluster can be populated by [O II]
line emitters (as for XMMU J0302.2−0001 at z = 1.186, S̆uhada
et al. 2011), sometimes with vigorous starburst activity (as for
XMMU J1007.4+1237 at z = 1.555, Fassbender et al. 2011b).
Finally, for galaxy templates as well as for QSO templates,
we considered the redshift range 0–6 with a step of 0.02 in z and
the range −24 ≤ Mz′ ≤ −8 in the z′ -band absolute magnitude.
For all stellar, QSO and galaxy templates, synthetic photometry was computed for all GROND bands in the AB magnitude
system and confronted with the observed photometry. Both systematic (e.g., caused by calibration uncertainties) and statistical (i.e., caused by measurement uncertainties) errors were taken
into account, the latter being magnified by 50% (in mag).
The photometric redshift code le Phare, based on a simple χ2
fitting method, yields the best-fitting template in each class, the
best-fit value of z, the 68% range of the photo-z solutions and the
probability per redshift bin, in particular. All ouputs for individual detected sources were inspected by eye. A source was classified as a star if the best-fitting stellar template gave the lowest χ2
value. Conversely, it was classified as a QSO if the best-fitting
QSO template gave a χ2 value that was better by a factor of two
than those given by the best-fitting stellar and galaxy templates.
As a first step, we checked the reliability of the publicly
available transmission functions of the GROND dichroic filters
uploaded into le Phare by comparing the distributions of the
library stars and the stars observed by us with the most accurate photometry in a large suite of colour–colour diagrams (see
Fig. 13 for examples). Eventual problems affecting the GROND
photometry of each star in individual bands (e.g., owing to poor
S/N and deblending) were inspected: if there were compromised photometric data, they were excluded. As a result, we
found an overall good agreement between observed and synthetic stellar tracks, which required the addition of very small
shifts (−0.05 mag, +0.025 mag, +0.025 mag) to the synthetic
photometry in the r′ , i′ , H bands, respectively. Furthermore, the
colour criterion adopted to statistically select stars from g′ , z′ , Ks
photometry only (Eq. 1) was nicely confirmed (cf. Figs. 8 and
13a).
As a second step, we computed photometric redshifts for
two photometric catalogues with the same sources as part of the
test to determine the best operational definition of total magnitude for our data. This test was executed with the COSMOS
set of galaxy templates excluding the line emission contribution; in particular, it addressed the five galaxies with GROND
photometry and a spectroscopic redshift (Sect. 2.4.1). Overall
the uncertainty on the ensuing two sets of photometric redhifts
obtained for these five galaxies is similar, as well as the bestfit solutions yielded for the two foreground galaxies. However,
a mean photometric redshift equal to ∼ 1.22 ± 0.25 (1σ) or
∼ 1.02 ± 0.15(1σ) is obtained for XMMU J0338.7+0030 when
total magnitudes were computed using the operational definitions A and B in Sect. 2.3.2, respectively. Because this cluster
is spectroscopically confirmed to be at z = 1.1 (Sect. 2.4.2), the
operational definition of total magnitude B has to be favoured.
This choice is also supported by the distribution of the three
spectroscopic members in the i′ − z′ vs z′ colour–magnitude diagram (Fig. 15). Therefore, we only discuss results from the fiducial photometric catalogue where total magnitudes are computed
with method B hereafter.
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
2
0
-2
-2
0
2
4
6
6
4
2
0
-2
0
1
2
0
1
2
0
1
2
2
1
0
4
2
0
2
1
0
-0.5
0
0.5
Fig. 13. Five colour–colour diagrams for stars in the le Phare library (black filled circles) and the 15 to 23 objects detected at all
GROND bands, with the most accurate photometry in the relevant bands and robustly classified as stars by le Phare using the full
seven-channel photometry (red asterisks). These observed stars nicely fall onto the synthetic stellar sequences, after the synthetic
photometry is slighlty corrected by -0.05, +0.025 and +0.025 mag in the r′ , i′ and H bands, respectively. We recall that photometric
redshifts are particularly sensitive to the 4000 Å break in the spectrum of a galaxy, which spans the entire wavelength range of the
15
z′ broad-band filter for z = 1.1–1.6.
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
Finally, we compared the performances of the COSMOS and
CFHTLS sets, whether the emission line contribution to the templates of star-forming galaxies is included or not. We based our
judgement on the comparison of the best-fit solutions and their
uncertainties but also on the capability of the set of choice to
recover the redshifts of the five galaxies with available spectroscopy (see Sect. 2.4.1). This figure corresponds to only ∼
0.6% of the GROND photometric sample of 832 sources; the ensuing span in redshift is very narrow: z = 0.85–1.1. Nevertheless,
the results obtained for the three spectroscopic members of
XMMU J0338.7+0030 (see Sect. 2.4.2) gave us useful indications. The use of the COSMOS galaxy templates yielded a mean
photometric redshift equal to ∼ 1.02 ± 0.15 for this cluster,
whether emission lines were included or not. Conversely, the use
of the CFHTLS galaxy templates yielded values of ∼ 1.14 ± 0.11
and ∼ 1.12 ± 0.09, respectively. Given the better agreement with
the spectroscopic redshift of XMMU J0338.7+0030 (< z >=
1.097 ± 0.002) and the lower uncertainty, hereafter we discuss
results obtained from the use of the CFHTLS set of templates
without emission line contribution10.
As a consequence of this choice, out of the 832 entries in the
GROND photometric catalogue, 174 sources do not have a (reliable) photo-z estimate (owing to very poor photometry or multiple peaks in the photo-z probability distribution function), 64
sources are identified as stars, 158 sources exhibit SEDs that are
best-fit by a QSO template, 436 sources are identified as galaxies. On one hand, the past large percentage of objects identified
as QSOs reflects our arbitrary classification criterion of the stellar, galaxy and QSO templates that best-fit the SED of a given
object. On the other hand, it reflects the reduced capability of
discriminating between best-fit templates for the numerous objects with incomplete photometric information. For the majority
of the objects identified as QSOs, the photometric redshift is reassuringly consistent, if not similar, to the value associated with
the best-fit galaxy template.
The quality of the best-fit photometric redshifts obtained
with CFHTLS galaxy templates not including emission lines is
discussed in Appendix A. The distribution of these photo-z solutions is often single peaked and narrow when accurate photometry is available for all seven GROND channels.
3.3. Optical identification of the high-z cluster
3.3.1. The photometric-redshift distribution function
We computed the surface density of extragalactic sources within
or beyond a cluster-centric distance of 1′ , as well as across the
entire 3.9 × 4.3 arcmin2 region of the sky imaged with GROND,
as a function of the photometric redshift (Fig. 14a). A simple visual inspection reveals that for the redshift bins 1.0–1.1, 1.2–1.3
and 1.5–1.6, the density of sources within the bona fide cluster
region exceeds that computed in the adjacent field by a factor of
2. An equivalent over-density of i′ − K s-selected extremely red
objects (EROs)11 exists in each of the last two bins (Fig. 14b).
10
This suggests that the COSMOS galaxy templates are less adequate to represent the characteristics of the galaxy populations of
XMMU J0338.7+0030. An analogous point was made by Guennou et
al. (2010), who discussed photometric redshifts along lines of sight to
10 clusters at spectroscopic redshifts between 0.48 and 0.79 obtained
with the same COSMOS galaxy templates and photo-z code used here.
11
Colours redder than I − K = 4 Vega mag statistically select EROs
at z ∼ 1–2 (Pozzetti & Mannucci 2000 and references therein; see also
Pierini et al. 2004a, 2005). For the GROND channels, this corresponds
to i′ − K s = 2.8 in the AB magnitude system.
16
candidate cluster region
a
field
entire FoV
3
2
1
0.5
1
1.5
z
candidate cluster region EROs
b
field EROs
entire FoV EROs
3
2
1
0.5
1
1.5
z
Fig. 14. Number density as a function of the photometric redshift
for the extragalactic sources with a photo-z less than 2 in the entire 3.9 × 4.3 arcmin2 region of the sky imaged with GROND
(black solid line), for those among them inside the bona fide
cluster area (red solid line) or in the surrounding field (blue
short-dashed line) - panel a. The analogous distribution functions for the sub-sample of EROs, expected to be at z ∼ 1–2 (see
text), are also reproduced (panel b). For the redshift bins 1.0–
1.1, 1.2–1.3 and 1.5–1.6, the density of sources within a clustercentric distance of 1′ exceeds that computed in the adjacent field
by a factor of 2. In correspondence of the last two redshift bins,
there is a clear excess of EROs.
These EROs are found in the redshift range where they are expected to be, which gives additional confidence in the quality of
our photo-z’s, at least in a statistical sense.
In support of this conclusion, the spectroscopic members
with ID=2, ID=15 and ID=16 have photometric redhifts12
1.1939
1.2461
equal to 1.01101.0311
0.9918, 1.17111.1490 and 1.18201.1378, respectively.
Furthermore, the morphological type of the best-fitting template
12
Here we give the best-fit photo-z together with the values that
bracket the 68% range of the photo-z solutions.
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
1
0
22
24
26
22
24
26
1
0
Fig. 15. i′ −z′ vs z′ colour–magnitude diagram for all GROND sources in the XMMU J0338.7+0030 region with flux detections in the
i′ , z′ bands at a significance level ≥ 1σ, whatever the classification of the source by Le Phare (i.e., stars are included). Photometric
errors (1σ) are also shown. Candidate members of XMMU J0338.7+0030 are reproduced with red (green) filled circles if they
are within (outside) the bona fide cluster region (panel a). The three spectroscopic cluster members in the GROND photometric
catalogue are marked with blue filled circles there. Green asterisks represent the two QSO at a cluster-centric distance greater
than 1′ . Alternatively (panel b), photometric cluster members classified as galaxies are colour-coded according to their spectrophotometric type, which is E (red), Sbc(yellow), Scd (green), Im (cyan) or SB (blue). In each panel, the red short-dashed, solid and
long-dashed lines represent the 1σ, 3σ, and 5σ flux thresholds of our photometry (cf. Sect. 2.3.2). The red dotted–short-dashed line
represents the completeness limits in the i′ , z′ bands, as obtained from Fig. 12. As a reference, the blue solid line reproduces the
red-sequence defined by the inner regions of the 31 E+S0+S0/a galaxies that are located within 1′ from the X-ray centroid of the
cluster RDCS J0910+5422 at z = 1.106 (Mei et al. 2006a).
corresponds to an E galaxy for the spectroscopic members with
ID=2 and ID=16 and to an Scd galaxy for the spectroscopic
member with ID=15. This spectro-photometrically determined
classification is nicely consistent with the spectroscopic classification in Sect. 2.4.2. Furthermore, it is reasonable to conclude
that the two peaks at z ∼ 1.0–1.1 and z ∼ 1.2–1.3 in Fig. 14a are
caused by true cluster members, at least in part.
Therefore we selected as photometric (also candidate or fiducial hereafter) cluster members those sources with a photo-z in
the range 1.0110–1.2317, which is centred on the mean photometric redshift of XMMU J0338.7+0030 (< z >= 1.1214 ±
0.0957, 1σ), contains the previous three spectroscopic members
and, thus, has a width equal to ±1.15σ (or ±0.1 < z >). Fortyfour sources satisfy our criterion: fourteen (36) are within 1′ (2′ )
from the X-ray position of XMMU J0338.7+0030. Only two of
these 44 candidate cluster members are identified as QSOs according to their best-fit templates; they are at cluster-centric distances greater than 1′ . The remaining 42 fiducial cluster members are spectro-photometrically classified as E (7), Sbc (6), Scd
(11), Im (8) or SB (10) according to the templates that best fit
their individual SEDs and provide their fiducial photometric redshifts. As discussed in Appendix A, the photometric redshifts associated with these 42 galaxies are, in general, well-behaved and
exhibit low uncertainties.
17
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
3.3.2. The i′ − z′ vs z′ colour–magnitude diagram: evidence
for a red sequence
We now investigate if the previously selected candidate cluster members consistently define a red sequence in a suitable
colour-magnitude diagram. We considered the i′ , z′ bands because the rest-frame 4000 Å break spans the entire wavelength
range of the z′ band as the redshift of a source increases from
1.1 to 1.6. Figure 15 shows the distribution of all sources of
the GROND photometric sample in the i′ − z′ vs. z′ colour–
magnitude diagram. In the upper panel, the photometric members of XMMU J0338.7+0030 are marked in red or green if they
fall inside or outside the bona-fide cluster region, respectively.
In the lower panel, the same sources are colour-coded according to the best-fit template (if they are identified as galaxies).
About a dozen candidate cluster members identified as galaxies (including the three spectroscopic cluster members) suggest the existence of a red locus in the i′ − z′ vs. z′ colour–
magnitude diagram: i′ −z′ slightly decreases from ∼ 1.1 AB mag
to ∼ 1.0 AB mag as z′ increases from ∼ 21.5 AB mag to
∼ 24.0 AB mag. Furthermore, half of these galaxies are identified as ellipticals. This is consistent with the existence of a red
sequence (Arimoto & Yoshii 1987).
As a reference, the red sequence of RDCS J0910+5422 at
z = 1.106 (Mei et al. 2006a) is reproduced in both panels of
Fig. 15. Mei et al. obtained high-resolution imaging of this massive cluster with the Hubble Space Telescope Advanced Camera
for Surveys (HST ACS) in the F775W and F850LP bandpasses.
They computed total z850 magnitudes (SExtractor MAG AUTO)
and i775 − z850 colours within the effective radii of the individual galaxies in order to select candidate cluster members. The
use of these aperture colours was meant to avoid a selection bias
introduced by the potential presence of radial colour gradients.
Furthermore, Mei et al. fitted different i775 − z850 vs z850 colour–
magnitude relations to different sub-samples of morphologically
classified early-type galaxies. We have transformed their best-fit
relation for the 31 E+S0+S0/a galaxies within 1′ from the X-ray
centroid of RDCS J0910+5422 (see table 1 in Mei et al. 2006a)
into a red sequence in the i′ − z′ vs z′ colour–magnitude diagram by introducing corrections for the different transmission
functions of the corresponding HST ACS and GROND filters. In
spite of the uncertainties on these colour transformations (likely
of the order of 0.1 mag), the red-sequence of RDCS J0910+5422
describes the red locus of XMMU J0338.7+0030 rather well.
However, galaxies with very different spectro-photometric
types populate the red locus of XMMU J0338.7+0030 within
a cluster-centric distance of 1′ and not just early-type galaxies as in RDCS J0910+5422. This was also observed in
e.g. XMMU J1230.3+1339 at z = 0.975 (see fig. 7 in
Lerchster et al. 2011). Nevertheless the two coeval clusters
XMMU J0338.7+0030 and RDCS J0910+5422 seem to host
also analogus populations of star-forming galaxies that exhibits
bluer i′ − z′ colours with respect to the red, passively evolving
members by about 0.5 AB mag (cf. fig. 7 in Mei et al. 2006a).
For XMMU J0338.7+0030, these galaxies constitute about half
of the candidate members and are identified as Scd, Im or SB.
Furthermore, those within the bona-fide cluster region span the
magnitude range 22.9 ≤ z′ ≤ 24.9, whereas those farther out
can be as bright as z′ ∼ 21.7. These bright, star-forming, candidate cluster-member galaxies might be analogous to the spectroscopic member with ID=12 at a cluster-distance of 2.3′ (cf.
Table 1). Evidence of a population of bright star-forming disc
galaxies was found in RDCS J0910+5422 as well and within a
cluster-centric distance of 2′ (Mei et al. 2006a).
18
2
1
0.5
1
1.5
CCD (cluster-centric distance) [arcmin]
Fig. 16. Cumulative number density of the 44 photometric members of XMMU J0338.7+0030 as a function of their clustercentric distances. The mean number density of those members
within a concentric, circular annulus delimited by radii of 1′ and
2′ is reproduced together with upper and lower limits (red solid
and short-dashed lines, respectively) computed as in Gehrels
(1986).
In conclusion, the photometric redshifts determined in
Sect. 3.2 confirm that XMMU J0338.7+0030 is a cluster at z =
1.1 and suggest that its galaxy population is qualitatively similar
to that of the coeval cluster RDCS J0910+5422.
3.3.3. Projected galaxy distribution
In the entire region of XMMU J0338.7+0030 imaged with
GROND at all seven bands, there are 44 candidate cluster members, including three spectroscopic members (Sect. 3.2.1). Their
projected distribution as a function of cluster-centric distance
(up to 2′ ) is reproduced in Fig. 16. There are 14 photometric
members in the bona-fide cluster region and 22 in the concentric annulus of inner and outer radii equal to 1′ and 2′ that has
a three times larger area. If one assumes this annulus to represent the surrounding low-density region (i.e., the “field”), the
over-density of galaxies within 1′ from the X-ray position of
XMMU J0338.7+0030 has a significance of 4.3σ. The significance rises to 5.3σ if one assumes that the region external to the
circular area of 2′ radius centred on the X-tay position of the
cluster represents the coeval field environment. Considering that
a value of R200 equal to 88.8′′ (equivalent to ∼ 725 kpc) can be
estimated from the scaling relations in Fassbender et al. (2011c),
we conclude that the existence of an over-density of galaxies
at the same photometric redshift of XMMU J0338.7+0030 is
highly significant. This is consistent with the excess by a factor
of two in the Ks-band galaxy number counts (for the entire region imaged with GROND) at ∼ 20.5–22.5 AB mag with respect
to several computations for the field from survey areas/deep
fields (Fig. 12 in Sect. 3.1).
In addition, the projected distribution of the 44 photometric members is not concentrated towards the X-ray position of
the cluster. This is consistent only in part with the original inference from the OMEGA2000 z, H imaging that the 2-D dis-
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
tribution of the galaxies with very red z − H colours is offset
by about 20′′ (see Fig. 1 and Sect. 2.2.1). Indeed, the 2-D distribution of the 44 candidate cluster members in the z′ -image
of XMMU J0338.7+0030 (right panel of Fig. 4 in Sect. 2.3.1)
is broadly consistent with the weak, marginally extended X-ray
emission of the cluster. A large part of the candidate members is
distributed in the north–south direction. This is consistent with
the location of the spectroscopic member with ID=12, that lays
2.3′ away from the X-ray position of the cluster and to the north
(Sect. 2.4.2). The elongation of the X-ray emission from the
ICM suggests that XMMU J0338.7+0030 is still in an assembly
phase.
We identify as the brightest central galaxy (BCG) the brightest photometric member, which sits on the red locus of the
i′ −z′ vs. z′ colour–magnitude diagram of XMMU J0338.7+0030
(Fig. 15). The BCG is offset by ∼ 43′′ , which corresponds to a
projected distance of ∼ 350 kpc from the X-ray position of the
cluster for z = 1.1 and the adopted cosmology (Sect. 1). This
galaxy is consistently classified as elliptical by le Phare; however, it is very close to a bright star, which can affect its photometry. For this reason, the identified BCG was not selected as a
target for spectroscopy with FORS2 (see Sect. 2.4.1).
Conversely, the spectroscopic member with ID=16 is only
2.1′′ away from the X-ray position of XMMU J0338.7+0030:
this separation is smaller than the on-axis HEW (∼ 14′′ ) of
XMM-Newton. This galaxy exhibits a weak evidence for [O II]
line emission (cf. Fig. 11 and Table 1 in Sect. 2.4.2) but is identified as an E galaxy by le Phare. With K s = 21.19 ± 0.10
and i′ − K s = 3.36 ± 0.12 (i.e., K s = 19.40 ± 0.10 and
i′ − K s = 4.77±0.12 in the Vega magnitude system) it also meets
the selection criterion of EROs (footnote 11). Its classification
as a galaxy dominated by old, passively evolving stellar populations (i.e., an elliptical galaxy) or a dusty starburst is ambiguous
because it exhibits J −K s = 1.28±0.13 (i.e., J −K s = 2.17±0.13
in the Vega magnitude system; see Pozzetti & Mannucci 2000;
see also fig. 13 in Pierini et al. 2004a). This spectroscopic cluster member might be associated with part of the emission detected through XMM-Newton because about 15% of the I − Kselected EROs with K ≤ 20.1 Vega mag exhibit X-ray properties
consistent with those expected from luminous, obscured activegalactic-nuclei (e.g., Alexander et al. 2002). If so, the true X-ray
emission centroid of XMMU J0338.7+0030 might be different.
4. Discussion
Sections 2.1 and 2.2 have described data reduction and analysis
for the archival XMM-Newton X-ray observations and the nearIR follow-up imaging with OMEGA2000 that led to the selection of XMMU J0338.7+0030 as a candidate cluster at z > 0.8.
Available FORS2 observations indicate four galaxies within 2.3′
from the X-ray position of this cluster as spectroscopic members. This establishes the existence of a bound system of galaxies and hot plasma at < z >= 1.097 ± 0.002 (1σ) as illustrated in
Figs. 10 and 11 and Table 1 (Sect. 2.4.2). With an estimated total mass M200 ∼ 1014 M⊙ , XMMU J0338.7+0030 is in between
X-ray selected groups and clusters (Sect. 2.1.2).
There is some tension between the spectroscopic redshift
and the original estimate of z ∼ 1.45 ± 0.15 (1σ) that was
based on the tentative identification of red, passively evolving,
member galaxies in the z − H vs z colour–magnitude diagram
of XMMU J0338.7+0030 (Fig. 3 in Sect. 2.2.2). A honest but
wrong assumption (see Sect. 2.2.2) as well as systematics (see
Appendix B) are at the origin of the discrepancy.
1.5
1
0.5
5000
Fig. 17. Transmission curves of the GROND g′ , r′ , i′ , z′ , J, H, Ks
dichroic filters (black) plus normalized SEDs that represent a
galaxy dominated by an old, passively evolving stellar population (red) and a very young, dusty starburst galaxy (blue). See
the text for details.
In addition to the spectroscopic observations with FORS2,
we performed simultaneous optical/near-IR imaging of a region of the sky centred on XMMU J0338.7+0030 with GROND.
Several pieces of evidence consistently point to the existence of
an optical counterpart to the weak, marginally extended X-ray
emission detected with XMM-Newton in the area imaged at all
GROND g′ , r′ , i′ , z′ , J, H, Ks bands:
– the presence of an excess by a factor of two in the Ksband galaxy number counts at ∼ 20.5–22.5 AB mag with
respect to analogous computations for the field from survey
areas/deep fields (Fig. 12 in Sect. 3.1);
– the consistency between the spectroscopic redshift of the
cluster and the mean value of the photometric redshifts of the
three spectroscopic members imaged with GROND, which is
equal to 1.12 ± 0.09 (1σ);
– the selection of 44 sources in the photometric redshift range
1.01–1.23 that contains the three aforementioned cluster
members, out of which fourteen are within a cluster-centric
distance of 1′ and signpost a galaxy over-density at a significance of 4.3σ (Sect. 3.2.1);
– the existence of a red locus in the distribution of these
44 photo-z selected galaxies in the i′ − z′ vs z′ colour–
magnitude diagram, which is consistent with the red sequence of the cluster RDCS J0910+5422 at z = 1.106
(Fig. 15 in Sect. 3.2.2);
– the consistency between the projected spatial distribution of the 44 photometric cluster members and the
weak, marginally extended emission of the X-ray source
XMMU J0338.7+0030 (Fig. 4 in Sect. 2.3.2).
We acknowledge that the identification of three spectroscopic members out of the GROND photometric sample of 832
sources was essential to determine the photo-z of the cluster as
well as to select its candidate (photometric) members. Indeed,
the photo-z distribution of the sources within 1′ from the X-ray
position of the cluster exhibits two substantial excesses at photometric redshifts of 1–1.1 and 1.2–1.3 with respect to the photo-z
19
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
4
3
2
1
18
20
22
Ks [AB mag]
4
3
2
1
18
20
22
Ks [AB mag]
Fig. 18. i′ − K s vs K s colour–magnitude diagram for all GROND sources in the XMMU J0338.7+0030 region with flux detections
in the i′ , z′ , Ks bands at a significance level ≥ 1σ, whatever the classification of the source by Le Phare (i.e., stars are included).
Photometric errors (1σ) are also shown. In each panel, candidate members of XMMU J0338.7+0030 are reproduced according
to the corresponding panel of Fig. 15. Here the red short-dashed, solid and long-dashed lines represent the 1σ, 3σ and 5σ flux
thresholds in the i′ , Ks bands, respectively. The red dotted–short-dashed line represents the completeness limits in the i′ , Ks bands,
as obtained from Fig. 12. The black vertical dotted and horizontal long-dashed lines reproduce the characteristic magnitude of the
Ks-band luminosity function of cluster galaxies at z ∼ 1.2 (Strazzullo et al. 2006) and the i′ − K s colour–selection threshold for
EROs (Pozzetti & Mannucci 2000). As a reference, the blue solid and short-dashed lines reproduce the red sequences of the clusters
RDCS J0910+5422 at z = 1.106 (Tanaka et al. 2008) and RDCS J1252.9−2927 at z = 1.24 (Tanaka et al. 2009), respectively. The
candidate members of XMMU J0338.7+0030 that are classified as elliptical galaxies (red symbols in panel b) populate a locus that
is broadly consistent with the red sequences of the other two clusters.
distribution of the sources populating the remaining area imaged
by GROND at all seven bands (Fig. 14a in Sect. 3.2.1). This dicotomy is caused, at least in part, by the following reasons.
On one hand, the wavelength coverage yielded by the
GROND dichroic filters for a z = 1.1 source is not optimal.
This is shown in Fig. 17, where the transmission curves of the
GROND bands are reproduced together with two synthetic SEDs
(from Pierini et al. 2004a) redshifted to z = 1.1 and normalized
at the effective wavelength of the z′ -band. One SED corresponds
to an SSP model with age of 4 Gyr and solar metallicity (red); it
represents a galaxy characterized by an old, passively evolving
stellar population. The other corresponds to a composite stellar
20
population model with constant star-formation rate (SFR), age
of 0.1 Gyr, half-solar metallicity, attenuated by dust as for a typical nearby starburst (blue); it represents a very young, dusty
starburst galaxy. The redshifted 4000 Å break, a fundamental
feature in redshift determination, falls in between the i′ -band
and the z′ -band. The redshifted [O II](λ = 3727 Å) emission
line falls at the red edge of the i′ -band, whereas the redshifted
[O III](λ = 5007 Å) and Hα(λ = 6563 Å) emission lines fall,
respectively, in regions of the z′ and J filters with poor transmission efficiency.
On the other hand, some ambiguity is favoured by the photometric uncertainty yielded by the present, unoptimized GROND
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
4
3
2
1
0
18
20
Ks [Vega mag]
4
3
2
1
0
18
20
Ks [Vega mag]
Fig. 19. J − K s vs K s colour–magnitude diagram for all GROND sources in the XMMU J0338.7+0030 region with flux detections
in the z′ , J, Ks bands at a significance level ≥ 1σ, whatever the classification of the source by Le Phare (i.e., stars are included).
Photometric errors (1σ) are also shown. In each panel, candidate members of XMMU J0338.7+0030 are reproduced according to the
corresponding panel of Fig. 15. Here the red short-dashed, solid and long-dashed lines represent the 1σ, 3σ and 5σ flux thresholds
in the J, Ks bands. The black vertical dotted and horizontal dotted–long-dashed lines reproduce the characteristic magnitude of the
Ks-band luminosity function of cluster galaxies at z ∼ 1.2 (Strazzullo et al. 2006) and the J − K s colour–selection threshold for
DRGs (Franx et al. 2003, see text). As a reference, the blue solid and short-dashed lines reproduce the red sequences of the clusters
RDCS J1252.9−2927 at z = 1.24 (Lidman et al. 2004) and XMMU J2235.3−2557 at z = 1.39 (Lidman et al. 2008), respectively.
The candidate members of XMMU J0338.7+0030 that are classified as elliptical galaxies (red symbols in panel b) populate a locus
that is broadly consistent with the red sequences of the other two clusters. However, candidate members that are classified as star
forming span the same broad range of colours, especially at magnitudes fainter than ∼ K s⋆ + 1.
observations, in particular at near-IR wavelengths (Fig. 7 in
Sect. 2.3.2). It is known that the photometric redshift technique
can be reliably extended from z ∼ 1 to z ∼ 2 only if deep nearand mid-IR photometry is available (e.g., Bolzonella, Miralles,
Pelló 2000; Ilbert et al. 2009).
In spite of the unfortunate mapping of the rest-frame SED
of a z = 1.1 galaxy yielded by the GROND channels and the
limited photometric accuracy of the present near-IR data, it is
remarkable that the photometric and spectroscopic estimates of
the redshift of XMMU J0338.7+0030 coincide within an uncertainty of 0.09 (cf. e.g. Lerchster et al. 2011). Indeed, the availability of spectroscopic redshifts for only 0.6% of the GROND
sources (and in the narrow redshift range 0.85–1.1) prevented
any meaningful training of the solutions of the photo-z code le
Phare.
Another exquisite possibility offered by the simultaneous
optical/near-IR imaging with GROND is the characterization of
the galaxy population of a high-z cluster on the basis of SEDs
- not of a single colour–magnitude diagram - in one and the
same study (cf. Mullis et al. 2005 and Lidman et al. 2008 for
XMMU J2235.3−2557; Lidman et al. 2004 and Tanaka et al.
2009 for RDCS J1252.9−2927; Mei et al. 2006a and Tanaka et
al. 2008 for RDCS J0910+5422). For instance, the distribution
in the i′ − z′ vs z′ colour–magnitude diagram of the 44 photo21
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
z selected members of XMMU J0338.7+0030 resembles that of
the members of the cluster RDCS J0910+5422 at z = 1.106, as
determined by Mei et al. (2006a), as shown in Fig. 15. At the
same time, the fiducial members of XMMU J0338.7+0030 that
are classified as elliptical galaxies populate a red locus in the
i′ − K s vs K s colour–magnitude diagram, which is broadly consistent with the red sequence of RDCS J0910+5422 determined
by Tanaka et al. (2008), as shown in Fig. 18b.
The rather neat progression from red, passively evolving, elliptical galaxies to blue, star-forming, late-type spirals and irregulars in Fig. 15b is not preserved in other colour–magnitude
diagrams that are used to constrain distance and properties of
the galaxy population of high-z clusters. The identification of a
red-sequence and the classification of a cluster member galaxy
as passively evolving or star forming become much more ambiguous in those colour–magnitude diagrams that involve broadband filters that do not map the redshifted 4000 Å break. This
is demonstrated by the distribution of the fiducial members of
XMMU J0338.7+0030 in the i′ − K s vs K s and J − K s vs
K s colour–magnitude diagrams (Figs. 18 and 19, respectively).
As a reference, we reproduce the red-sequences of the spectroscopically confirmed clusters RDCS J0910+5422 at z = 1.106
(Tanaka et al. 2008), RDCS J1252.9−2927 at z = 1.24 (Lidman
et al. 2004; Tanaka et al. 2009) and XMMU J2235.3−2557 at
z = 1.39 (Lidman et al. 2008)13 .
A galaxy classification based on either spectro-photometry
or morphology indicates that there are by far not as many
early-type galaxies within a cluster-centric distance of 1′
in XMMU J0338.7+0030 as in RDCS J0910+5422: 3 vs 31
(Sect. 3.2.2). A high percentage of passively evolving galaxies
among the brightest members of a cluster should not necessarily be expected if the cluster is observed at an early stage of its
halo assembly (e.g., Tanaka et al. 2008; see also Braglia et al.
2009 for the most massive clusters at z ∼ 0.3). This could be the
case of XMMU J0338.7+0030, according to the projected distribution of its candidate member galaxies (Figs. 4 and 16).
The spectroscopically confirmed, optically-selected protocluster Cl 0332−2742 at z = 1.6 provides evidence of the buildup of the red sequence during its assembly (Kurk et al. 2009).
Conversely, the X-ray luminous cluster XMMU J2235.3−2557
at z = 1.393 (Strazzullo et al. 2010) exhibits a tight redsequence of massive galaxies, with overall old stellar populations, generally early-type morphology, typically showing earlytype spectral features and rest-frame far-UV emission consistent
with very low SFRs. The massive cluster XMMU J1230.3+1339
at z = 0.975 exhibits a richly populated red sequence, but
also a few, spectro-photometrically classified, early-type galaxies that potentially host some residual star-formation activity
(Lerchster et al. 2011). Consistently, the X-ray selected cluster RDCS J0910+5422, exhibits a red sequence although its
early-type galaxy population appears to be still forming (Mei
et al. 2006a; Tanaka et al. 2008). With a total mass of about
1014 M⊙ (Tanaka et al. 2008), RDCS J0910+5422 is as massive as XMMU J0338.7+0030 and four times less massive than
XMMU J1230.3+1339 at z = 0.975 (Fassbender et al. 2011a;
Lerchster et al. 2011).
If XMMU J0338.7+0030 lacks passively evolving galaxies,
it hosts a high percentage of star-forming systems (late-type
13
The observed red-sequences and colour thresholds for i′ − K s selected EROs (Pozzetti & Mannucci 2000) and J − K s selected distant
red galaxies (DRGs, Franx et al. 2003) reproduced in the two figures
were corrected for the different filter transmission functions, assuming
a 4 Gyr-old SSP with solar metallicity (see Pierini et al. 2004a).
22
galaxies or starbursts), judging from the spectro-photometric
classification of its candidate members (Fig. 15b). Nine of its
10 candidate members that are spectro-photometrically classified as starbursts are located well beyond a cluster-centric distance of 1′ (right panel of Fig. 4). Consistently, the spectroscopically confirmed member at a cluster-centric distance of 2.3′
(ID=12) is a bright galaxy with a quite strong [O II] line emission (Sect. 2.4.2). The other bright spectroscopic member with
evidence of [O II] line emission (ID=15) is a photometric member as well but is only 50′′ away from the cluster centre and sits
on the red sequence in Fig. 15 as well as the two spectroscopic
members classified as elliptical galaxies (ID=2 and ID=15) at
cluster-centric distances of 28′′ and 2.1′′ , respectively.
Interestingly, there is evidence of sustained star-formation
activity at the bright end of the galaxy luminosity function in
the X-ray luminous cluster XMMU J1007.4+1237 at z = 1.555
(Fassbender et al. 2011b) and in dense environments at z ∼ 1
(e.g., Elbaz et al. 2007; Gerke et al. 2007; Cooper et al. 2008).
Several [O II] line emitters are found in the luminous, red galaxy
population of XMMU J0302.2−0001 at z = 1.185 with a total
mass of a few times 1014 M⊙ (S̆uhada et al. 2011). Spectroscopic
cluster members with [O II] line emission can populate the locus of old, passively evolving galaxies or be even redder than
the red sequence (see XMMU J2235.3−2557; Lidman et al.
2008). Consistently, a non negligible fraction (20%) of the luminous infrared galaxies in 16 optically-selected galaxy clusters
at 0.4 < z < 0.8 sits on the corresponding red sequences (Finn
et al. 2010), the remaining 80% laying below. This is no surprise when the effect of dust attenuation on the observed SED
of a galaxy is computed in a physical way, tailored to its starformation mode14 (cf. Pierini et al. 2004a, 2004b, 2005).
In addition, star-forming galaxies at z ∼ 1–2.5, selected in
the rest-frame ultraviolet (UV) and with stellar masses of 1010 –
1011 M⊙ , exhibit evidence of complex configurations of stars
and dusty interstellar medium (e.g., Noll et al. 2007, 2009).
This points to the existence of dusty gas outflows, likely driven
by feedback from supernovae, at high redshifts (cf. Steidel et
al. 2010), but gravitational interactions cannot be excluded15.
Extended distributions of dust not associated with the main bodies of galaxies have been discovered in the nearby Stephan’s
Quintet (Natale et al. 2010) and M81 triplet of galaxies (Walter
et al. 2011). Walter et al. (2011) highlight the importance of tidal
stripping for the metal enrichment of the intergalactic medium
at high redshifts in addition to outflows. Here we note that the
additional cooling of the ICM on dust grains impacts on the Xray scaling properties of groups and clusters of galaxies (e.g., da
Silva et al. 2009; Natale et al. 2010). Furthermore, early preheating of the ICM is likely responsible for the observed evolution
of the X-ray scaling relations of galaxy clusters out to z ∼ 1.5
(Reichert et al. 2011).
In conclusion, XMMU J0338.7+0030 appears as a bound,
low-mass system of galaxies at z = 1.1 that is caught in an early
assembly phase and where the shaping of the bright end of the
galaxy luminosity function and nurturing effects on galaxy evolution (see Boselli & Gavazzi 2006 for a review) can be sought.
14
Assumptions on the relative distribution of dust and stars as well as
on the extinction law seem to impact the estimates of SFR and stellar
mass in a systematic way (e.g., Küpcü Yoldaş et al. 2007).
15
Hence, some complexity in the relation between bolometric infrared luminosity associated with re-emission by dust at mid-IR through
sub-millimeter wavelengths and colour temperature of the warm and
cold dust components can be expected. This could indeed have already
been observed (cf. Hwang et al. 2010).
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
A full understanding of the properties of the galaxy population
and ICM of this cluster requires a more in-depth study based on
spectroscopy and multi-wavelength imaging, which offer better
statistics and/or superior quality.
5. Conclusions
This paper presents, in particular, results from the first pointed
observations of a high-z cluster with GROND, the seven-channel
imager mounted at the MPI/ESO 2.2m telescope of La Silla,
Chile. The target was the X-ray selected, weak, marginally
extended source XMMU J0338.7+0030 in the XMM-Newton
Distant Cluster Project (XDCP) survey.
Follow-up imaging with OMEGA2000 at the 3.5m CAHA
telescope enabled a comparison of the reddest z − H colours of
the galaxies within a cluster-centric distance of 45′′ with predictions from simple stellar population evolutionary models, which
suggested a redshift of z = 1.45 ± 0.15 (1σ). Later available
VLT/FORS2 spectroscopy identified XMMU J0338.7+0030 as
a galaxy cluster at z = 1.097 ± 0.002 (1σ) with four spectroscopic members. From the determination of the flux in the
soft 0.5–2 keV band, the bolometric luminosity of the cluster
was inferred. The ensuing X-ray estimate of the total mass of
XMMU J0338.7+0030, based on a luminosity scaling relation,
is equal to M200 ∼ 1014 M⊙ .
About six hours of simultaneous imaging in the GROND
g′ , r′ , i′ , z′ , J, H, Ks bands enabled us to extract photometry for
832 sources detected down to z′AB ∼ 26 (1σ). Existing coverage
of the same region by the shallower Sloan Digital Sky Survey
and 2 Micron All Sky Survey allows a robust photometric calibration to be obtained for the optical and near-infrared (IR) channels of GROND, respectively. This is the basis for the application of the photometric redshift technique, as implemented in the
publicly available code le Phare.
With the GROND data, we confirmed the existence of an
optical counterpart to XMMU J0338.7+0030 on the basis of
five consistent and independent pieces of evidence. In particular, the Ks-band galaxy number counts for the cluster region
imaged with GROND exhibit an excess by a factor of two
at 20.5–22.5 AB mag with respect to those from survey areas/deep fields. In addition, the photometric redshifts of the
three spectroscopic members imaged with GROND (1.12 ± 0.09,
1σ) is consistent with the spectroscopic redshift of the cluster. Furthermore, the distribution of the 44 candidate members
of XMMU J0338.7+0030 (with photometric redshifts within the
range 1.01–1.23) in the i′ − z′ vs z′ colour–magnitude diagram
exhibits a red locus and a bluer one that are consistent with the
red sequence and the distribution of the star-forming galaxy population of the similarly massive cluster RDCS J0910+5422 at
z = 1.106.
XMMU J0338.7+0030 seems to host a galaxy population
that can still undergo significant bursts of star-formation activity,
as also found in other XDCP high-z clusters. Part of these starforming galaxies can exhibit colours that are at least as red as
those of passively evolving galaxies, owing to dust attenuation,
in agreement with results in the literature. A comparison of the
projected distributions of the X-ray emitting plasma and the photometric cluster members suggests that XMMU J0338.7+0030 is
likely caught in an early phase of its assembly: its star-forming
galaxies fall in and face an environment that rapidly changes.
Finally, we acknowledge that the identification of three spectroscopic members out of the 832 GROND sources was essential to determine the photo-z of the cluster as well as to select
its candidate (photometric) members. However, it is remarkable
that these two estimates of the redshift of XMMU J0338.7+0030
coincide within an uncertainty of 0.09, given that the so-called
4000 Å break (a fundamental feature for photo-z determination)
falls in between the GROND i′ and z′ channels for a z = 1.1
galaxy and the present unoptimized observations with GROND
yielded a limited photometric accuracy in the near-IR bands. We
also stress that no training of the solutions of the photo-z code
was performed, because the available spectroscopic information
is limited to only 0.6% of the GROND sources.
For these reasons, this unique imager is useful for establishing optical counterparts to X-ray selected clusters, even in absence of spectroscopic priors. This is particularly true for high
throughput surveys for clusters like the eROSITA all-sky survey,
which is expected to mostly deliver massive clusters at intermediate redshifts. These clusters should host a wealthy population
of old, passively evolving galaxies, which define a neat red sequence in at least one of the colour–magnitude diagrams that can
be built out of the GROND optical/near-IR photometry obtained
in less than two hours of observations.
Acknowledgements. DP thanks the anonymous referee for her/his insightful
comments, which led to a signficant improvement on the robustness and presentation quality of the results in the final version of the paper.
This research was supported by the DFG cluster of excellence “Origin and
Structure of the Universe” (http://www.universe-cluster.de) through EXC project
number 153, the DFG under grants Schw 536/24-1, Schw 536/24-2, BO 702/163, and the DLR under grants 50 OR 0405 and 50 QR 0802.
RS acknowledges support by the DFG in the program SPP 1177.
HQ thanks the FONDAP Centro de Astrofı́sica for partial support.
FZ acknowledges support from and participation in the International MaxPlank Research School on Astrophysics at the Ludwig-Maximilians University.
DP acknowledges useful feedback from O. Ilbert.
DP acknowledges the kind hospitality at the Max-Planck-Institut für extraterrestrische Physik (MPE).
This research has made use of observations collected at the European
Organisation for Astronomical Research in the Southern Hemisphere (ESO),
Chile (079.A−0634).
This research has made use of observations collected at the Centro
Astronómico Hispano Alemán (CAHA) at Calar Alto, Spain operated jointly
by the Max-Planck-Institut für Astronomie and the Istituto de Astrofı́sica de
Andalucı́a (CSIC).
This work has made use of the SDSS database. Funding for the SDSS
and SDSS-II has been provided by the Alfred P. Sloan Foundation, the
Participating Institutions, the National Science Foundation, the U.S. Department
of Energy, the National Aeronautics and Space Administration, the Japanese
Monbukagakusho, the Max Planck Society, and the Higher Education Funding
Council for England. The SDSS Web Site is http://www.sdss.org/.
The SDSS is managed by the Astrophysical Research Consortium for
the Participating Institutions. The Participating Institutions are the American
Museum of Natural History, Astrophysical Institute Potsdam, University of
Basel, University of Cambridge, Case Western Reserve University, University
of Chicago, Drexel University, Fermilab, the Institute for Advanced Study,
the Japan Participation Group, Johns Hopkins University, the Joint Institute
for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and
Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences
(LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for
Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New
Mexico State University, Ohio State University, University of Pittsburgh,
University of Portsmouth, Princeton University, the United States Naval
Observatory, and the University of Washington.
This publication makes use of data products from the Two Micron All
Sky Survey, which is a joint project of the University of Massachusetts and
the Infrared Processing and Analysis Center/California Institute of Technology,
funded by the National Aeronautics and Space Administration and the National
Science Foundation.
This research has made use of the NASA/IPAC Extragalactic Data base
(NED) which is operated by the Jet Propulsion Laboratory, California Institute
of Technology, under contract with the National Aeronautics and Space
Administration.
23
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
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D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
1.5
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1
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2
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Fig. A.1. Distribution of the 68% range of the photo-z solutions (∆z) as a function of the associated best-fit photo-z for
the 436 GROND sources identified as galaxies (empty circles).
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with red (green) filled circles if they are within (outside) the bona
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Appendix A: Photometric redshifts: accuracy and
reliability
Here we discuss the accuracy of the photometric estimates of
the distances to the individual sources in the 3.9 × 4.3 arcmin2
region of XMMU J0338.7+0030 imaged with GROND that is
achieved through the GROND multi-band photometry and the
photo-z code le Phare (see Sect. 3.2). At variance with works on
survey areas/deep fields, we do so on the basis of photometric
redshifts, mostly, because spectroscopic ones are available for
only 0.6% of the GROND photometric sample (Sect. 2.4.1).
In particular, we reproduce the behaviour of the 68% range
of the photo-z solutions yielded by le Phare, ∆z16 , as a function of the best-fit photo-z or z′ -band magnitude for the 436
sources out of 832 that are classified as galaxies by the best-fit
solution (Figs. A.1 and A.2, respectively). The CFHTLS galaxy
templates without emission lines are used. The 42 galaxies selected as fiducial cluster members (i.e., with 1.01 ≤ z ≤ 1.23,
see Sect. 3.3.1) are colour-coded as in Fig. 15 (Sect. 3.2.2).
Overall, the distribution of these photometric redshifts is
well described by a straight line of equation ∆z ∼ 0.05 × (1 + z)
up to z ∼ 1.5; only 2 photo-z estimates (i.e., 0.4%) behave as outliers, in the sense that they exhibit a value of ∆z > 0.4 × (1 + z).
Unsurprisingly, photometric redshifts lower than 1 are assigned
16
Given its definition, ∆z corresponds to twice the value expected for
a Gaussian photo-z distribution centred at the best-fit value.
16
18
20
22
24
26
Fig. A.2. Distribution of ∆z as a function of the z′ -band magnitude for the same GROND sources displayed in Fig. A.1.
Symbols have the same meaning as well.
1
0.5
0.5
1
Fig. A.3. Comparison of photometric redshifts for the 24 galaxies that are in common between our GROND photometric catalogue and the SDSS DR7 one (Sect. 2.4.1). For the SDSS photoz’s, uncertainties correspond to ±1σ values. The red line represents equality.
with an accuracy that is better than ∆z ∼ 0.05 × (1 + z), whereas
photometric redshifts between 1.5 and 3 exhibit a mean accuracy
equal to ∆z ∼ 0.1 × (1 + z) but with a large scatter. In addition, an
accuracy of 0.2 or better characterizes the photometric redshifts
attributed to galaxies that are brighter than z′ ∼ 23.5, which corresponds to a 10σ detection (see Sect. 2.3.2).
The candidate member galaxies of XMMU J0338.7+0030
tend to exhibit the lowest values of ∆z among the galaxies with
the same z′ -band magnitude (Fig. A.2). However, three fiducial
25
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
1.2
1.1
1
0.9
0.9
1
1.1
1.2
Figure B.1 shows the z′ − H vs H colour–magnitude diagram of the same cluster obtained from the GROND simultaneous multi-band imaging, where total magnitudes are computed following method B (see Sect. 2.3.2). Its comparison with
the analogous diagram in Fig. 3 (Sect. 2.2.2) reveals that the
spectroscopic members with ID=2 and ID=16 exhibit z′ − H
colours that are about 0.8 Vega mag and 0.6 Vega mag bluer than
their z − H colours listed in Table 1 (Sect. 2.2.2). Therefore,
they lie closer to the spectroscopic member with ID=15 (with
z′ − H = 2.24 Vega mag, in full agreement with its z − H colour
listed in Table 1) in Fig. B.1 than in Fig. 3. In addition, the existence of a red sequence is suggested in Fig. B.1. If we compute total magnitudes following method A, the spectroscopic
members with ID=2 and ID=16 exhibit z′ − H colours that are
about 0.4 Vega mag bluer than their z − H colours in both cases,
whereas the spectroscopic member with ID=15 exhibits a z′ − H
colour that is about 0.3 Vega mag redder than its z−H colour this
time. The H-band magnitudes of these three galaxies can differ
by up to ±0.5 Vega mag with respect to those listed in Table 1,
as a function of object and method (A or B).
Fig. A.4. Comparison of photometric and spectroscopic redshifts
for the five sources in the GROND photometric catalogue observed with FORS2 (Sect. 2.4.1). For each spectroscopic value,
we show the 68% range of the photometric estimates. The red
line represents equality.
members within a cluster-centric distance of 1′ exhibit values of
∆z between 0.3 and 1.1: one is an outlier (Fig. A.1).
As a consistency check of our set of photometric redshifts,
we compared photo-z’s for 24 GROND sources with counterparts in the SDSS DR7 (see Sect. 2.3.2). The comparison
SDSS photometric redshifts were calculated using a Neural
Network method (Oyaizu et al. 2008). The agreement between
these two independent sets of photo-z estimates is good up to
z ∼ 1. As expected, our values have significantly lower uncertainties since they are based on medium–deep photometry in
the g′ , r′ , i′ , z′ , J, H, Ks bands (e.g., Bolzonella, Miralles, Pelló
2000). However, the u-band photometry that is available from
the SDSS shallower imaging in the u, g, r, i, z bands (Abazajian
et al. 2009 and references therein) better helps constraining
photo-z’s of galaxies at z ≤ 0.4 (Bolzonella, Miralles, Pelló
2000).
Finally, we reproduce photometric redshifts vs spectroscopic
ones for the five extragalactic sources in the GROND photometric catalogue that were observed with VLT-FORS2 (see
Sect. 2.4.1) in Fig. A.4. In spite of a slight tendency towards
higher values, our photometric redshifts, computed without any
training of the solutions of the photo-z code le Phare, agree with
the spectroscopic ones within ±0.1.
Appendix B: On the z′ - H vs H colour–magnitude
diagram of XMMU J0338.7+0030
Here we investigate the origin of the discrepancy between the
spectroscopic redshift of XMMU J0338.7+0030 (z = 1.097 ±
0.002, Sect. 2.4.2) and the value that was suggested from the
analysis of the z − H vs H colour–magnitude diagram obtained
from the CAHA imaging (z ∼ 1.45 ± 0.15, Sect. 2.2.2). There
total magnitudes correspond to Kron aperture magnitudes i.e.,
computed as SExtractor MAG AUTO (Sect. 2.1.2).
26
The method adopted to compute total magnitudes certainly
impacts the estimated brightness of sources like these three
galaxies, which are at the limit bewteen being considered partly
resolved or point-like for resolutions of 1.1–1.3′′. However, it is
likely the stacking of individual frames without PSF-matching in
the case of the OMEGA2000 images that explains the previous
differences in colours.
We note that an SSP model with Salpeter IMF and solar
metallicity exhibits z − H ∼ 2.4 Vega mag when observed at
z = 1.1, for a formation redhift between 3 and 10 (fig. 3 in
Fassbender et al., 2011c). This colour is about 0.8 Vega mag
bluer than the z − H colour of the same model observed at
z = 1.45 (Fig. 3). It also agrees well with the distribution of the
spectro-photometrically classified elliptical galaxies in Fig. B.1,
modulo (small) transformations between different z, H filters.
At the same time, it is not inconsistent with the distribution of
the spectroscopic members with absent/weak [OII] line emission
D. Pierini et al.: I. The galaxy population of XMMU J0338.7+0030 at z = 1.1
4
2
0
18
20
22
H [Vega mag]
4
2
0
18
20
22
H [Vega mag]
Fig. B.1. z′ − H vs H colour–magnitude diagram for all GROND sources in the XMMU J0338.7+0030 region with flux detections
in the z′ , H bands at a significance level ≥ 1σ, whatever the classification of the source by Le Phare (i.e., stars are included).
Photometric errors (1σ) are also shown. As in Fig. 15, candidate members of XMMU J0338.7+0030 are reproduced with red
(green) filled circles if they are within (outside) the bona fide cluster region (panel a). The three spectroscopic cluster members in the
GROND photometric catalogue are marked with blue filled circles there. Green asterisks represent the two QSO at a cluster-centric
distance greater than 1′ . Alternatively (panel b), photometric cluster members classified as galaxies are colour-coded according to
their spectro-photometric type, which is E (red), Sbc(yellow), Scd (green), Im (cyan) or SB (blue). In each panel, the red shortdashed, solid and long-dashed lines represent the 1σ, 3σ, and 5σ flux thresholds in the z′ , H bands (cf. Sect. 2.3.2). Photometric
members classified as elliptical galaxies define a narrow red locus in this colour–magnitude diagram.
(ID=2 and ID=15) in Fig. 3, taking into account the photometric
uncertainties.
1
Max-Planck-Institut für extraterrestrische Physik (MPE),
Giessenbachstrasse, D-85748 Garching, Germany
2
University Observatory Munich, Scheinerstrasse 1, D-81679
Munich, Germany
3
Max-Planck-Institut für Plasmaphysik, Boltzmannstrasse 2, D85748 Garching, Germany
4
Caltech, 1200 East California Blvd, PMA 249-17, Pasadena, CA
91125, USA
5
Laboratoire AIM, IRFU/Service d’Astrophysique - CEA/DSM CNRS - Université Paris Diderot, Bât. 709, CEA-Saclay, F-91191
Gif-sur-Yvette Cedex, France
6
European Southern Observatory (ESO), Karl-Schwarzschild-Strasse
2, D-85748 Garching, Germany
7
European Space Astronomy Centre (ESAC), P.O. Box 78, 28691
Villanueva de la Cañada, Madrid, Spain
8
Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D14482 Potsdam, Germany
9
Department
of
Physics,
Ludwig-Maximilians-Universität,
Scheinerstrasse 1, D-81679 Munich, Germany
10
Excellence Cluster Universe, Boltzmannstrasse 2, D-85748
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