c ESO 2009
Astronomy & Astrophysics manuscript no. GN˙ch2˙MED
February 3, 2009
Searching for massive galaxies at z ≥ 3.5 in GOODS-North
C. Mancini1,2,4 , I. Matute2 , A. Cimatti3 , E. Daddi4 , M. Dickinson5 , G. Rodighiero6 , M. Bolzonella7 , L. Pozzetti7
arXiv:0901.3341v2 [astro-ph.CO] 3 Feb 2009
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Dipartimento di Astronomia e Scienza dello Spazio, Università degli Studi di Firenze, Largo E. Fermi 3 50125, Firenze, Italy
e-mail: mancini@arcetri.astro.it
Osservatorio Astrofisico di Arcetri (OAF), INAF-Firenze, Largo E. Fermi 5, 50125 Firenze
Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, I-40127 Bologna, Italy
CEA-Saclay,DSM/DAPNIA/Service d’Astrophysique, 91191 Gif-Sur Yvette Cedex, France
NOAO, 950 N. Cherry Ave. P.O. 26732, Tucson, AZ 85726-6732, USA
Dipartimento di Astronomia, Università di Padova, Vicolo Osservatorio 2, I-35122, Padova, Italy
INAF-Bologna, Via Ranzani, I-40127 Bologna, Italy
Preprint online version: February 3, 2009
ABSTRACT
Aims. We constrain the space density and properties of massive galaxy candidates at redshifts of z ≥ 3.5 in the Great Observatories
Origin Deep Survey North (GOODS-N) field. By selecting sources in the Spitzer + IRAC bands, a highly stellar mass-complete
sample is assembled, including massive galaxies which are very faint in the optical/near-IR bands that would be missed by samples
selected at shorter wavelengths.
Methods. The z ≥ 3.5 sample was selected down to mAB = 23 mag at 4.5 µm using photometric redshifts that have been obtained by
fitting the galaxies spectral energy distribution at optical, near-IR bands and IRAC bands. We also require that the brightest band (in
AB scale) in which candidates are detected is the IRAC 8.0 µm band in order to ensure that the near-IR 1.6 µm (rest-frame) peak is
falling in or beyond this band.
Results. We found 53 z ≥ 3.5 candidates, with masses in the range of M⋆ ∼ 1010 − 1011 M⊙ . At least ∼81% of these galaxies
are missed by traditional Lyman Break selection methods based on ultraviolet light. Spitzer + MIPS emission is detected for 60%
of the sample of z ≥ 3.5 galaxy candidates. Although in some cases this might suggest a residual contamination from lower redshift
star-forming galaxies or Active Galactic Nuclei, 37% of these objects are also detected in the sub-mm/mm bands in recent SCUBA,
AzTEC and MAMBO surveys, and have properties fully consistent with vigorous starburst galaxies at z ≥ 3.5. The comoving number
density of galaxies with stellar masses ≥ 5× 1010 M⊙ (a reasonable stellar mass completeness limit for our sample) is 2.6× 10−5 Mpc−3
(using the volume within 3.5 < z < 5), and the corresponding stellar mass density is ∼ (2.9 ± 1.5) × 106 M⊙ Mpc−3 , or about 3% of
the local density above the same stellar mass limit. For the sub-sample of MIPS-undetected galaxies, we find a number density of
∼ 0.97 × 10−5 Mpc−3 and a stellar mass density of ∼ (1.15 ± 0.7) × 106 M⊙ Mpc−3 . Even in the unlikely case that these are all truly
quiescent galaxies, this would imply an increase in the space density of passive galaxies by a factor of ∼ 15 from z ∼ 4 to z = 2 , and
by ∼ 100 to z = 0.
Key words. Cosmology: observation, – Galaxies: formation – Galaxies: evolution– Infrared: galaxies
1. Introduction
The question of the galaxy mass assembly at early cosmological epochs is still open. In a ΛCDM Universe, the standard formation scenario predicts hierarchical growth of structures. Local galaxies would have been formed through repeated
merging events. The first semi-analytic galaxy-formation models
(Kauffmann & Charlot 1998) predicted that the assembly of the
most massive systems would only have been completed in the
most recent epochs (z < 1). Nevertheless, in recent years a substantial population of massive galaxies has been found beyond
z ∼ 1. Up to z ≃ 2.5 the most massive objects spectroscopically
confirmed are similar to the local Early Type Galaxies (ETGs):
old (age of ∼1 Gyr), passively evolving, and with M⋆ > 1011 M⊙
(Cimatti et al. 2004; McCarthy et al. 2004; Daddi et al. 2005;
Saracco et al. 2005; Kriek et al. 2006).
At higher redshifts the availability of multi-wavelengths data
from the new generations of deep surveys has allowed searches
for massive galaxy candidates up to z ≃ 5 − 6.5. The dropouttechnique pioneered by Steidel et al. (1996) to find LymanSend offprint requests to: aaa
break galaxies (LBGs) at z ≃ 3, has proven successful also
for identifying blue star-forming galaxies at higher redshifts
(z ∼ 4 − 6 Steidel et al. 1999; Dickinson 1998; Steidel et al.
2003; Giavalisco et al. 2004b; McLure et al. 2006; Yan et al.
2006).
A substantial amount of spectroscopic confirmations is available for LBGs at z ∼ 5 − 6 (Bunker et al. 2003; Stanway et al.
2004; Dickinson et al. 2004; Eyles et al. 2007; Stark et al. 2007;
Vanzella et al. 2008). This allowed early estimates of the stellar
mass density at those redshifts. For instance Yan et al. (2006)
measured the comoving stellar mass density at z ∼ 6 in the total
GOODS field and found it in good agreement with the simulations of Night et al. (2006). The stellar mass density found in
the GOODS-S field by Eyles et al. (2007) for a sample of idropout LBGs in the same redshift range is also consistent with
the Yan et al. (2006) results. However it is larger by a factor of
4 (∼ 2, if the different IMFs are taken into account) with respect to the Bower et al. (2006) semi-analytical model predictions. The general picture is, however, still controversial and further studies are necessary to better constrain galaxy formation
models. In fact, the Lyman-break technique can only be used
2
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
to select actively star-forming galaxies having sufficiently luminous UV emission to allow their detection and color selection.
Galaxies, and especially massive ones, could be faint or undetected in the optical and near-infrared bands. Such sources could
be either old systems with small quantities of dust and low levels of star-formation rate, or dust reddened starburst galaxies. If
such z > 3.5 objects really exist, they would be too faint to be
studied spectroscopically with the instrumentation that is currently available, and the only possible approach that can be used
to constrain their number density and their properties remains
a photometric one, based on the Spectral Energy Distribution
(SED) fitting analysis.
The most popular criterion used so far to select high-z
galaxies with red rest-frame colours is based on the redshifted
4000 Å/Balmer break, i.e. the typical features of galaxies with
evolved stellar populations. It was pioneered by Franx et al.
(2003) to pick out ‘distant red galaxies’ (DRGs) at z > 2
(J s − K s > 2.3, AB system). Recently it was extended by
Brammer & van Dokkum (2007) to identify DRGs up to z ∼ 3.7
(H − K > 0.9, AB system). By comparing the properties and the
space density of DRGs at z ∼ 3.7 and z ∼ 2.4 in the same field,
they found a stellar mass density of about a factor of 5 lower in
the higher redshift bin.
In the recent years the availability of Spitzer data has also
enabled us to search for galaxies with red rest-frame optical colors at high redshifts, and to better constrain their stellar masses
(Fontana et al. 2006; Dunlop et al. 2007; Rodighiero et al. 2007;
Wiklind et al. 2008; Eyles et al. 2005). Some of the objects identified may be among the most massive stellar systems found at
z ∼ 4 − 5. In particular, Rodighiero et al. (2007) extracted a
Spitzer+IRAC selected sample in the GOODS-South field, limited to galaxies undetected in the optical and close to the detection limit in the K-band. Their criterion is complementary with
respect to those adopted by previous studies (i.e. the K < 23.5
selection used by Dunlop et al. 2007) and is aimed at identifying high-z massive galaxies that are missed by conventional
selection techniques based on optical and near-IR observations.
They found a potential population of optically obscured massive
galaxies at z ≥ 4. In the same field Wiklind et al. (2008) found
11 K-selected z ≥ 5 massive and evolved galaxy candidates.
They selected these objects by applying color criteria aimed at
the identification of the 4000 Å/Balmer break between the K and
3.6 µm passbands, in combination with SED fitting.
It should be noted that in both of these works roughly half of
the high-z massive candidates are detected at 24 µm. The 24 µm
emission might be due to obscured AGN activity at high redshift,
but also from Polycyclic Aromatic Hydrocarbon (PAH) emission in dusty star forming galaxies at lower redshifts. In fact,
one has to take into account the degeneracy between reddening
and redshift while selecting high redshift sources based on photometry. Recent studies of sub-mm galaxies (SMGs) have shown
that moderate 24 µm emission can arise from massive starburst
galaxies at high redshift (z ≥ 3.5, Daddi et al. 2008; Greve
et al. 2008; Perera et al. 2008). Hence one of the main difficulties in selecting bona fide high-z galaxies through photometric
redshifts is to efficiently remove the contamination from dustreddened galaxies at lower redshift. As an example, one of the
most debated cases is the galaxy HUDF-JD2 in the Hubble Ultra
Deep Field. Mobasher et al. (2005) suggested that such a galaxy
is a massive (M ≃ 6 × 1011 M⊙ ), low-reddened, old galaxy (with
an age of ∼ 108 yrs and a z f orm > 9) at z ≃ 6.5. However, other
studies (e.g. Dunlop et al. 2007; Rodighiero et al. 2007; Chary
et al. 2007) suggest that HUDF-JD2 is instead a lower redshift
galaxy with very strong dust reddening (z ≃ 2.2 and AV ≃ 3.8,
Dunlop et al. 2007).
The principal aim of this paper is to study the properties
and space density of massive galaxies at z ≥ 3.5 in the Great
Observatory Origin Deep Survey-North (GOODS-N). This survey is especially suitable for our purpose because of the quality
and depth of the available multi-wavelength data. In particular it
has the advantage of a better coverage at sub-millimetric wavelengths with respect to GOODS-S. In addition, while similar
searches have been carried out in the GOODS-South field, none
has been performed so far in the northern hemisphere field of
the GOODS Survey. We chose to select the sample in the IRAC
channel 2 (4.5 µm) for two reasons. First, to pick out the most
massive systems. At z ≥ 3.5 the 4.5 µm band samples the restframe near-IR emission, which is well correlated to the galaxy
stellar mass (Kauffmann & Charlot 1998). Second, the 4.5 µm
band selection allows us to have a more complete sample with
respect to an optical or near-IR selection. This is one of the main
differences with the previous works: we attempted to construct
a sample as complete as possible, including objects undetected
in optical and near-IR bands, and hence missed by the optical
or K selection criteria used so far in the literature (Drory et al.
2004; Fontana et al. 2006; Dunlop et al. 2007; Wiklind et al.
2008; Brammer & van Dokkum 2007). As already mentioned,
Rodighiero et al. (2007) also used an IRAC band (3.6 µm) selection, but their additional conditions about the lack of optical
emission and the faint near-IR detection (K > 23, AB system)
make their sample incomplete by construction.
The structure of the paper is as follows. In Sect. 2 we present
the optical, near-IR and IR data available for the GOODS-N
field. In Sect. 3 we describe the IRAC selection and the photometry of the sample. Sect. 4 describes the results from the SED
fitting analysis with the measurement of photometric redshift
and the stellar mass estimates. In Sect. 5 we discuss the selection of z ≥ 3.5 galaxy candidates. In the same section we discuss the detections of the sample in the MIPS-24 µm and in the
sub-mm/mm. In Sect. 6 we study the effects of incompleteness
and the bias against low mass galaxies in our magnitude-limited
sample. We give estimates for the comoving number density and
stellar mass density for the full sample, and for the sub-sample
of MIPS-undetected galaxies. Sect. 7 presents the summary and
conclusions.
Throughout this paper, we adopt the following cosmological
parameters H0 = 70km s−1 Mpc−1 , ΩΛ = 0.7 and Ωm = 0.3, and
all magnitudes are given in the AB system.
2. The GOODS-N data.
This work is based on the analysis of the Great Observatory
Origins Deep Survey - North (GOODS-N) data. The GOODSN is a multi-wavelength deep survey centered in the Hubble
Deep Field North (HDFN 12h 36m 55 s,+62◦ 14′ 15′′ Giavalisco
et al. 2004b) with size of ∼ 10′ × 16′ . The field is part of
the GOODS Spitzer Legacy Program (PI: M. Dickinson) and
the Hubble Space Telescope (HST) Treasury Program (PI: M.
Giavalisco), divided in two fields (GOODS-N and GOODS-S),
from the northern and southern hemispheres (see Dickinson et al.
2003). The GOODS-N field observations span from the X-rays
to the sub-mm wavelengths. In particular, for the analysis presented in this paper, we make use of the following data sets:
– U band imaging, taken at the Kitt Peak National Observatory
(KPNO) 4m telescope, from the Hawaii Hubble Deep Field
North (H-HDFN) survey datasets (Capak et al. 2004).
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
– Optical imaging from the Hubble Space Telescope (HST)
with the Advanced Camera for Survey (ACS) in the
F435W (B), F606W (V), F775W (i), F850LP (z) pass-bands
(Giavalisco et al. 2004b).
– Near-Infrared imaging, in the J, H and K s bands, from
the KPNO-4m telescope (M. Dickinson private communication).
– Infrared observations from the Spitzer Space Telescope
(Werner et al. 2004) + Infra-Red Array Camera (IRAC;
Fazio et al. 2004)
– 24 µm observations from the Spitzer Space Telescope and
Multi-band Imaging Photometer for Spitzer (MIPS; Rieke
et al. 2004).
Specifics of the data are given below.
2.1. KPNO 4m U-band imaging
The H-HDFN Survey is a deep multi-colour imaging survey of
0.2 degrees2 , centered in the Hubble Deep Field North. The data
consist of deep images in the U, B, V, R, I, and z′ bands. Out of
these we used only the U-band images, due to the availability
of HST/ACS imaging from the B through z bands. The U-band
observations were taken in March 2002, using the KPNO-4m
telescope with the MOSAIC prime focus camera (Muller et al.
1998), covering a field of view of 36′ ×36′ with a seeing of 1.′′ 26.
We analyzed the part overlapping with the GOODS-N field (an
area of about 165 arcmin2 around the HDF-N). The U image
reaches a 5σ sensitivity limit of 27.1 mag.
2.2. ACS+HST optical imaging
The GOODS-N optical data from the Advanced Camera for
Surveys (ACS), on-board HST, are in the F435W (B), F606W
(V), F775W (i), F850LP (z) pass-bands (v1.0 data products).
The 5σ point source sensitivity limits are 28.5, 28.8, 28.1, 27.6
respectively, scaled from values reported in Giavalisco et al.
(2004b) to the full exposure time of ACS v1.0 images. The
HST/ACS high-resolution data is organized in 17 images, called
“sections”. Each section is an image 8 192 × 8 192 pixels in size,
drizzled from the native ACS pixel scale (0.′′ 05/pixel) to an image subsampled at 0.′′ 03/pixel. The huge size (∼ 40 000 × 40 000
pixels) of the combined HST images in the field made source extraction an extremely intensive computing task. Therefore we rebinned the final image using an 8 × 8 pixel kernel, requiring flux
conservation. The new re-binned mosaic obtained in this way
has a size of 5 120 × 5 120 pixels, and a pixel scale of 0.′′ 24/pixel.
2.3. KPNO 4m FLAMINGOS near-infrared imaging
We complemented the public data with near-IR imaging taken at
the KPNO-4m telescope with the Florida Multi-object Imaging
Near-IR Grism Observational Spectrometer (FLAMINGOS; observations by Mark Dickinson et al.). The FLAMINGOS images
in the J, H and Ks pass-bands, are mosaics of data taken at different pointings and orientations to cover the whole GOODSN. The average exposure times per band in the main part of
the GOODS-N area are 20250 s, 15770 s and 35440 s at J-,
H- and K s, respectively. The three images reach a 5 σ point
source sensitivities of 24.03, 23.77, 23.81 mag. The pixel-scale
is 0.′′ 316/pixel, and the seeing 1.′′ 27, 1.′′ 2 and 1.′′ 2, for the J, H,
and Ks pass-bands, respectively.
We performed some photometric checks before measuring source fluxes. We verified the zero-points reliability of
3
FLAMINGOS images, comparing the bright and unsaturated
star magnitudes with stars from the 2MASS catalogue.
2.4. Spitzer imaging at 3.6 to 24 µm
The infrared imaging is from Spitzer Space Telescope + IRAC
between 3.6 and 8.0 µm, and from Spitzer + MIPS at 24 µm.
The four IRAC channels, centered on 3.6 µm, 4.5 µm,
5.8 µm, 8.0 µm, have formal 5 σ limits for isolated point sources
of 26.1, 25.5, 23.5, 23.4 respectively. The mean FWHM of a
point source for the IRAC-bands are 1.′′ 66, 1.′′ 72, 1.′′ 88 and 1.′′ 98,
for channels 1, 2, 3 and 4 respectively. The initial image pixelscale of 1.′′ 22 is reduced to 0.′′ 6/pixel, after the dithering and drizzling process.
The MIPS public dataset includes calibrated maps and a catalogue of 24 µm sources with flux densities S 24 > 80 µJy.
The PSF was generated from isolated sources in the image, and
renormalized based on the aperture correction published in the
MIPS data Handbook (v2.1, section 3.7.5, table 3.12). Then an
independent PSF algorithm was run to extend the 24 µm sample
to fainter sources (see Rodighiero et al. 2006). By this procedure
the detection threshold was extended to S24 > 35 µJy. Since the
24 µm information can help to better constrain the SFR (Chary &
Elbaz 2001) and the nature of high-redshift sources, we also exploited a deeper 24 µm catalogue, reaching a 5σ limit of 20 µJy
(Chary et al. 2008 in preparation, Daddi et al. 2007).
3. Sample selection
In order to have an accurate measurement of the object fluxes in
all bands, we performed aperture photometry in each band using the SExtractor software (Bertin & Arnouts 1996). The initial
catalogue was built by selecting the sources in the 4.5 µm public image obtained with Spitzer Space Telescope + IRAC, down
to a limiting magnitude of m4.5 = 23.0 (2.3 µJy). Simulations
indicate that a sample selected in this way is complete at the
80% level at m4.5 ∼ 23.0 (M. Dickinson private communication),
where 20% of the sources are lost due to blending.
The infrared selection allows us to search for the presence of
massive galaxies up to very high redshifts. In fact for galaxies at
3.5< z <7 the rest-frame light emitted at λ > 0.6 µm is redshifted
into the IRAC 4.5µm band. Therefore, the 4.5µm selection is
directly sensitive to stellar mass, rather than to the ongoing star
formation activity. From a technical point of view, the choice of
selecting the sample at 4.5 µm represents the best compromise
amongst the IRAC bands in terms of image quality, blending
problems, and sensitivity.
In order to detect galaxies in the 4.5µm IRAC band and perform the photometry we used SExtractor with the following set
of parameters. The detection limit was set at ∼ 1 σ above the sky
background. A Gaussian filter was used to improve the detection
of faint extended objects. Due to the high crowding of IRAC images, and the large PSF, the minimum contrast parameter was set
to a small value of 5 × 10−9 , to help with source deblending.
We measured fluxes in 4.′′ 0 diameter apertures, as this size
allows us to minimize uncertainties in the photometry in the
Spitzer +IRAC bands (as also suggested by the SWIRE team1 ).
In the 4.5 µm IRAC filter we applied an aperture correction of
0.25 mag that we independently computed from point-source objects, by measuring the total flux in 8.′′ 0 diameter apertures.
The final IRAC selected catalogue contains 4 142 objects
(m4.5 < 23.0).
1
http://data.spitzer.caltech.edu/popular/swire/20050603 enhanced v1/
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Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
3.1. Multi-band Photometry
We searched for the counterparts of the 4 142 IRAC-selected objects in the other datasets. We detected the sources and measured the photometry independently in each dataset and associated counterparts to the 4.5 µm IRAC detected galaxies using a
search radius of 1.′′ 0.
For the U-band we measured fluxes 4.′′ 0 (diameter) apertures.
We applied an aperture-correction of 0.1 mag, computed by considering the total flux in 8.′′ 0 diameter aperture. For undetected
sources we used the 3 σ flux upper limit of 26.7 mag. This value
was computed by measuring the standard deviation of the sky
signal at random position over the field.
For the HST/ACS bands circular apertures with diameters
of 2.′′ 0 were used. The following aperture-corrections for pointsources were found by measuring the total flux in 5.′′ 0 diameteraperture, for the B, V, i, z bands: 0.050, 0.040, 0.030, 0.045 mag,
respectively. For undetected objects we obtained 3 σ upper limits in the same way as described above for the U-band. We measured upper limits to be 26.3, 26.9, 26.3, 25.5, for the B, V, i and
z bands respectively.
The near-IR magnitudes were measured in 4.′′ 0 diameter
apertures, and corrected to total (within 8.′′ 0 diameter) by 0.1,
0.14, 0.15 mag for the J-, H- and Ks- band, respectively. The 3 σ
detection limits are 23.3, 23.0 and 23.1, for the J-, H- and Ksband.
In the other IRAC channels we used the same aperture diameters as in IRAC-4.5 µm (4.′′ 0), and the same approach to
compute aperture corrections. The final IRAC magnitudes were
obtained by applying aperture corrections of 0.23, 0.25, 0.40,
0.44 mag for 3.6, 4.5, 5.8 and 8.0 µm bands, respectively.
To check the reliability of our photometry we compared
the colors of stars with those expected based on the models of
Lejeune et al. (1997) in color-color diagrams involving one band
from each one of the three datasets (ACS/HST optical, NearInfrared/FLAMINGOS and infrared/IRAC-Spitzer). The case of
(b − J) vs (J − m3.6 ) is shown in Fig. 1. The star sequence is
clearly visible on the left side of the panel. As it appear from the
figure we found a very good agreement between the observed
colours (green circles) and the expected ones (red asterisks from
the “corrected” templates of Lejeune et al. 1997) for spectroscopically identified stars (public database of the TKRS survey
Wirth et al. 2004). Therefore, thanks to the very good concordance between optical, near-IR and IR datasets we did not have
to apply further photometric corrections to our data magnitudes.
The photometric errors given by SExtractor software are
generally underestimated, because the software does not take
into account the possible correlation between neighboring pixels
that can result from image processing. Hence we applied corrections to the photometric errors in each data-set. For the four
IRAC channels we added 0.1 mag quadratic to SExtractor errors.
This is a conservative choice to account for uncertainties on the
photometric zeropoints and on the aperture corrections, as also
suggested in Maraston et al. (2006). In the other filters we measured the signal in a number of random sky positions in the field,
within the same aperture used to perform galaxy photometry in
each band. Comparing the standard deviation of these measurements to the actual errors in the photometry of faint galaxies
inferred by SExtractor we derived scaling factors to apply to the
formal errors reported in the SExtractor catalogues.
Fig. 1. The color-color diagram (b − J) vs (J − m3.6 ). The star sequence
is clearly visible on the left side of the diagram. We found a very good
agreement between the observed colours (green circles) and the expected ones (red asterisks from the “empirically corrected” templates
of Lejeune et al. 1997) for spectroscopically identified stars (public
database of the TKRS survey Wirth et al. 2004).
4. Spectral Energy Distribution (SED) fitting
analysis
4.1. Photometric redshifts estimate
To estimate the photometric redshifts of our sample of galaxies we used the Hyperz code (Bolzonella et al. 2000). This code
compares the multi-wavelength photometry of each source to a
database of theoretical and/or empirical templates at different
redshifts. The best fit solution, for a given set of templates, is
derived through a standard χ2 minimization.
For the determination of the photometric redshifts we excluded the 24 µm band, because the templates of stellar populations that we used do not include dust emission. For a similar
reason we decided to omit also the 8 µm band, which includes
thermal dust and PAH (i.e. non-stellar) emission for the lower
redshift galaxies and can be contaminated by star formation or
AGN also at higher redshift (Daddi et al. 2007). We used the
template library of the observed SED constructed by Coleman
et al. (1980, hereafter CWW), extended in the UV and IR regions by means of Bruzual & Charlot (2003) synthetic spectra,
as explained in the Hyperz User’s manual2 . This set of templates
includes four types of spectra with characteristics corresponding to the different local types of galaxies: Elliptical/Lenticular
(E/S0), Spiral (Sbc and Scd) and Irregular (Im). The CWW library was complemented with a spectrum of a Young Star-Burst
(SB) galaxy, with constant star formation rate (SFR) and age of
0.1 Gyr, from the Bruzual & Charlot (2003) models.
We took into account the dust extinction by applying the
Calzetti et al. (2000) law. The extinction parameter (AV ) was allowed to vary from 0 to 0.8 in steps of d(AV ) = 0.1. Only a small
range of AV was allowed since the CWW templates include al2
http://webast.ast.obs-mip.fr/hyperz/
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
ready intrinsically reddened SEDs. The permitted redshift range
span the large interval of values of 0 < z < 10, with a step of
d(z) = 0.07.
We used the prescription of Madau (1995) to represent the
average Lyman-α forest opacity as a function of wavelength and
redshift.
4.2. Photometric redshift reliability
We checked the reliability of our photometric redshifts for 836
objects of the GOODS-N field having high quality spectroscopic
redshifts. Unobscured AGN were excluded from the list by
means of the X ray information from the publicly available X-ray
catalogue of the Chandra Deep Field-North SurveyAlexander
et al. (2003). The largest number of spectroscopic redshifts
(721) are from the public database of the Team Keck Treasury
Redshift Survey (TKRS; Wirth et al. 2004), and are accurate
to 100 kms−1 . The median redshift of this survey is rather low
(z = 0.65). In order to increase the number of available objects
with the highest spectroscopic redshifts, we complemented the
TKRS database with the galaxies found by Reddy et al. (2006)
in the redshift interval of 2.0 . z . 3.5 (108). We also used
some objects (5) up to z ≃ 5 from the NASA/IPAC Extragalactic
Database3 (NED).
Fig. 2. Comparison between photometric z phot and spectroscopic zspec
redshifts for 836 objects of our sample. The zspec -z phot relation has a
combined mean offset of (zspec − z phot )/(1 + zspec ) = 0.004 with an rms
scatter of σ [(zspec − z phot )/(1 + zspec )] = 0.09. For the flagged objects
(red open diamonds), appearing as catastrophic outliers we suggest the
lower redshift solution, as detailed in the text.
The comparison between photometric (z phot ) and spectroscopic (z spec ) redshifts is shown in Fig.2. After clipping 5 σ outliers we found a combined mean offset of (z spec − z phot )/(1 +
z spec ) = 0.004 and a rms scatter of σ [(z spec − z phot )/(1 + z spec)] =
0.09. This result was obtained by means of an iterative optimization procedure applied to the Hyperz results. It consists in repeating the SED fitting analysis for the same objects, fixing the
redshifts to the known spectroscopic values. For each filter the
median offset between empirical and “theoretical” magnitudes
(the latter deriving from the best fit SEDs) was obtained. The
largest photometric offsets that we found were for the U-, z- and
K-bands (+0.2, +0.09, +0.14, respectively) and | ∆mag|∼ 0.05
3
http://nedwww.ipac.caltech.edu/
5
Table 1. Outliers of our z phot − zspec comparison (Fig. 2). Their best
fit SEDs are shown in Fig. 3. The coordinates and the spectroscopic
redshifts are from NED database. The last column is the distance to the
NED position (arcsec).
id
#6955
#8734
#8903
RA
12:36:40.510
12:36:37.630
12:37:21.048
Dec
+62:13:34.90
+62:14:53.70
+62:15:01.73
z phot
1.48
0.68
0.98
zspec
3.826
4.886
4.762
dist(′′ )
0.27
0.16
0.41
for the others. Adding these corrections to the observed magnitudes, the rms of the ∆ z/(1+z spec ) quantity was slightly reduced.
The same offsets were finally applied to the whole sample for estimating the photometric redshifts.
Since the current work is focused on galaxies at z ≥
3.5 we have carefully examined three outliers (#6955, #8734
and #8903) in the photometric-spectroscopic redshift comparison with reported z spec ≥ 3.5. These are noted with red diamonds
in Fig. 2, and information about them is presented in Tab. 1. In
Fig. 3 we show the photometric SEDs for these three objects and
in Fig. 4 we show cutout images from U-band through 24 µm.
For the first two objects (#6955 and #8734) Dawson et al. (2001)
report Keck spectra showing isolated emission lines with no continuum detection (their quality class 4; see Tab. 2 and 3 of that
paper), which they identify as weak Ly-α emission. However object #6955 is detected at a significance > 3 σ limit in the U-band,
ruling out the proposed z = 3.826. More recent Keck/DEIMOS
spectra from 2005 and 2006 (M. Dickinson & D. Stern, private
communication) detected an emission line at 9273Å which is
likely to be [OII]3727Å at z = 1.488, consistent with our photometric redshift estimate for this object. Object #8734 shows a
red point source in the HST/ACS images, along with faint, bluer,
extended emission to the southeast. This is very likely to be a
superposition of a cool galactic star with a faint galaxy. Cowie
et al. (2004) report that this is a star based on DEIMOS spectroscopy. The faint, extended component is certainly a galaxy,
although it is clearly detected in the ACS B-band image, and
is therefore unlikely to be at z = 4.887 as reported by Dawson
et al. (2001). Overall, the photometry at all wavelengths redward
of the V-band is dominated by the point source component, and
the photometric SED for this object Fig. 3 clearly resembles that
of a cool, late-type star (probably an M-dwarf). Finally, for object #8903, Dawson et al. (2001) report z = 4.762. However,
ACS V − i and i − z colors for this galaxy are not consistent
with it being a V– dropout Lyman break galaxy, and fall within
the locus of ordinary foreground galaxies. The ACS color images show a faint, red, early-type galaxy, and our photometric
redshift estimate is z phot = 0.89, very close to that proposed by
Capak et al. (2004). Although Cowie et al. (2004) do not report
details about the spectral features that they observed, it is possible that faint [OII] emission at z = 0.88 was mistaken for faint
Lyα at z = 4.76.
4.3. Galaxy mass estimate
We used the code Hyperzmass to estimate the mass for each of
our sources. The Hyperzmass code works in a similar way to
Hyperz(see Pozzetti et al. 2007). In this task the redshifts were
fixed at the derived photometric values, or spectroscopic ones,
when available. The observed data points were fitted using both
the synthetic spectral models of Bruzual & Charlot (2003, hereafter BC03) and Maraston (2005, hereafter MA05). Both tem-
6
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
# 6955
# 8734
# 8903
Fig. 3. SEDs of the outliers in the zspec -z phot space (see objects flagged
in Fig. 2). Our best fits (black curves) produce photometric redshifts of
∼1.48, 0.68 and 0.98, respectively. We compare these results with the
best fits (red curves) obtained by fixing the redshifts to the spectroscopic
values taken from the literature (z = 3.826, z = 4.886, and z = 4.762;
NED database). Obviously, the black curves fit the observed data better
than the red ones. See also Tab.1
plate sets are composed by models of stellar populations with
Star Formation Rates (SFRs) exponentially decreasing with time
(ψ(t) = τ−1 exp−t/τ ). In our analysis we considered models with
different values of the time-scale τ (in unit of Gyr) : 0.1, 0.3, 1,
Fig. 4. Multi-wavelength identification of the three outlier objects in the
zspec -z phot diagram. The cut-outs have a size of 10′′ × 10′′ . North is up,
East is at left.
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
2, 3, 5, 10, 15, 30 and ∞. The last model corresponds to a stellar
population with a constant SFR.
For the BC03 models we used the Chabrier (2003) Initial
Mass Function (IMF). Instead, the MA05 models adopt the
Kroupa (2001) IMF, which is relatively similar to the Chabrier
IMF. To compare the different results we computed a systematic
correction to convert the masses obtained with Kroupa IMF to
Chabrier IMF (log MChabrier = log MKroupa − 0.04).
In the SED fitting analysis the extinction parameter, AV , was
allowed to span the range of values 0 ≤ AV ≤ 6, with a step of
0.1. This large reddening range is required given that synthetic
stellar population models are not intrinsically reddened as the
observed ones. We used AV = 6 as an upper limit for the reddening, since there is no evidence of objects with AV higher than
∼ 5. The galaxy age was left as a free parameter, spanning the
range from 5×6 yr to 11.7 × 109 yr (and always requiring ages
less than the age of the Universe at each redshift). To minimize
the number of free parameters we limited the fits to solar metallicity.
5. Massive galaxies at z ≥ 3.5
As previously discussed, evidence for populations of massive
galaxies at z > 3 has been found by several studies. However,
results are not yet conclusive since the number of objects is low,
and frequently biased against objects that are faint or undetected
in optical and near-IR bands. Moreover for most of these objects
no spectroscopic redshift information is available. Also, the possible contamination by lower redshift interlopers is a source of
uncertainty. To shed some light on the nature and the number
density of the high redshift population of massive galaxies we
selected a final sample of z phot ≥ 3.5 candidates, by applying
the following selection criteria (summarized in Table 2). Among
the initial 4 142 IRAC-4.5 µm selected galaxies we found 190
galaxies with photometric redshifts of z phot ≥ 3.5.
All such objects were carefully inspected for possible blending issues. One of the main problems in using Spitzer data in
crowded fields is, in fact, the blending between nearby sources.
Neighboring objects that can be well separated in optical and
near-IR images, might happen to be blended in the IRAC bands,
due to the limited spatial resolution of the Spitzer telescope.
Hence, in order to build a bona-fide sample of high-z galaxies
we had to take into account that a fraction of the IRAC-selected
objects could be affected by blending. We decided to visually inspect all the z ≥ 3.5 objects, and we excluded from the sample
any sources that the SExtractor software was not able to deblend.
To identify them we visually inspected examined the SExtractor
“aperture image” in each filter. This is a ‘check-image’ given as
output by the software, where the circular apertures are shown
for all the objects detected above the threshold level. We classified an object as blended if the IRAC position did not correspond
to a single source in the higher resolution optical and/or nearIR images (e.g. objects that are well separated in the optical or
near-IR images may be blended in the IRAC images due to their
poorer angular resolution). About the 25% of the pre-selected
z phot ≥ 3.5 objects were flagged as blended and therefore discarded.
By matching our catalogue with the publicly available X-ray
catalogue of the Chandra Deep Field-North Survey3, we found
14 objects that were detected in the hard X-ray band, and one
with soft X-ray flux. We excluded these 15 sources from the final
sample since X–ray emission indicates the presence of AGNs in
those galaxies, that can alter the observed flux and induce an
overestimate of the stellar masses.
7
Finally we introduced a further criterion, based on IRAC
colours. We considered as genuine z ≥ 3.5 candidates only those
sources showing an SEDs peak at the 8.0 µm observed frame
(1-2 µm rest-frame for z ≥ 3.5 galaxies) in the AB scale. The
reason is that the near-infrared emission of all but extremely
young galaxies show a distinct peak at 1.6 µm in the rest-frame,
that is redshifted in the 8.0 µm IRAC band for z ≥ 3.5 sources.
This peak corresponds to the combination of the Planck spectral peak of cool stars and the effects of a minimum in the H −
opacity in the stellar atmospheres. It has been used as photometric redshift indicator in several previous studies (e.g. Sawicki
2002; Berta et al. 2007). This criterion was applied because at
this step we expect that our high-z sample will be contaminated
by a substantial fraction of lower-redshift galaxies, because of
the intrinsic uncertainty of the photometric redshift estimate.
The degeneracy between reddening and redshift could introduce
lower-redshift contaminants in the final high-z sample (i.e. dust
reddened galaxies at lower redshift can be mistaken for higher-z
old stellar populations). Given that galaxies at z ≥ 3.5 represent
only a minority (∼ 4%) of the galaxy sample selected at 24µm,
while most of the objects are at z < 2, from a statistical point
of view we could expect a more substantial contamination by
lower-z sources entering our z > 3.5 redshift selection, largely
overbalancing the number of genuine z > 3.5 galaxies lost as
due to errors in their photometric redshifts. In addition, as already pointed out, the 8 µm data-point was excluded from the
SED fitting analysis because for objects at z < 1.8 it could sample the PAH emission lines for dusty galaxies, and the stellar
population models that we used do not include dust emission. In
this way, by excluding the 8 µm band from the fitting procedure,
we may fail to distinguish galaxies at z ∼ 2 − 3 where the 1.6 µm
peak lies in the 5.8 µm band from those at z > 3.5. Requiring the
SED to rise from 5.8 to 8 µm can help to exclude these possible
lower-redshift contaminants.
The parameterization of the colour criterion we used is the
following:
(m8.0 ≤ m5.8 + 0.1) ∩ (m8.0 ≤ m4.5 + 0.1)∩
∩(m8.0 ≤ m3.6 + 0.1)
(1)
the 0.1 mag terms are introduced to account for the median uncertainty in the IRAC photometry of ∼ 0.1.
In the two panels of Fig. 5 we show the expected m5.8 − m8.0
colour as a function of redshift for different stellar population
models. The top panel shows Simple Stellar Populations (SSPs)
with three different ages from 0.1 to 1 Gyr, and the bottom
panel shows a constant star formation rate model with fixed
age of 0.1 Gyr and reddening from E B−V =0 to 0.8. The templates shown are from the Maraston stellar population models
(Maraston 2005). These models, however, are not supposed to
be used for λ > 2.5 µm where they display a strong, unphysical, discontinuity. This discontinuity affects the IRAC colours
at 0 < z < 2.7. Hence we tested the reliability of this colour
criterion also with BC03 templates, that are not affected by this
problem. We verified that the expected IR colours for BC03 stellar population models at z < 2.7 are equally bluer than the colour
cuts defined by the equation 1.
From the upper panel of Fig. 5 it is clear that the 1.6 µm
peak criterion may lead to exclude some blue and unreddened
(EB−V = 0) galaxy at z > 3.5 with age <0.1 Gyr. This is because galaxies dominated by young stellar populations do not
show the 1.6 µm peak in their SEDs (see Sawicki 2002). In fact,
due to this criterion we excluded from the final sample a spectroscopic confirmed LBG (object #5455, from Spinrad et al. 1998,
8
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
with z spec = 5.34 and z phot = 5.18), for which the best fit result predicted age ∼ 7 Myr, and mass ∼ 1010 M⊙ . Nonetheless
we decided to apply this criterion as a conservative way to obtain a reliable sample of the most massive galaxies at z ≥ 3.5.
The objects missed in this way are very few (see next section),
and are also the least massive ones. Their inclusion would not
strongly affect our estimate of the comoving stellar mass density
(for more details see Sect. 5.2, 6.1 and 6.2).
Table 2. Sample selection criteria. In the left and right columns the subsequent steps to select the final sample and the corresponding number
of objects are shown, respectively.
Selection Criteria
m4.5 ≤ 23
z phot ≥ 3.5
No hard X-ray emission
IRAC/channel 4 peak
Number of Objects
4142
140
125
53
This criterion is very efficient in excluding SSPs galaxies
with redshifts lower than ∼ 2.5. On the other hand, we should
point out that it cannot completely remove the contamination by
interlopers at 2.5 < z < 3.5, as SSPs models with ages > 1 Gyr
and 3 < z < 3.5, and dusty star forming galaxies at 2.5 < z < 3.5
can still satisfy this criterion (see the bottom panel of the same
figure).
Finally it should be noted that the adopted colour criterion
would be met also by the so-called ‘IRAC power-law sources’.
These objects are generally selected by requiring a rising SED
over the four IRAC bands, and they are believed to be generally
z < 2.5 AGNs with a hot dust component swamping the stellar
emission (Alonso-Herrero et al. 2006; Donley et al. 2007). Since
Donley et al. (2007) selected a sample of (62) IRAC powerlaw galaxies in the Chandra Deep Field Nord (CDFN), whose
27 are in the GOODS-N field, we investigate the overlap with
their sample. We found 10 sources in our 4.5 µm IRAC-selected
(m4.5 ≤ 23) sample (of 4142 objects) in common with the ‘IRAC
power-law’ sample. All these sources but one have the infrared
part of the SED peaked at 8.0 µm, but most of them had been excluded from our z ≥ 3.5 final sample because of their lower photometric redshifts. Only one object of the Donley et al. (2007)
sample fulfill all of our selection criteria. This is object #14793
in our sample (see Tab. 4 and Fig.14), corresponding to the object CDFN:[DRP2007]19080 in Donley et al. This galaxy has
no X-ray emission (< 4.41 × 10−16 erg s−1 cm−2 ) and a rather
high 24 µm flux (111 ±3.5µJy). The high redshift solution is still
consistent with these properties.
As result we obtained a final sample of 53 galaxies (∼ 43%
of the pre-selected z ≥ 3.5 sample), that we consider to be valid
candidates at z ≥ 3.5. About the 12% of the 125 pre-selected
high-z candidates have been discarded due to the low S/N ratio in
the 8.0 µm-band, but we cannot exclude that some of them could
be at high redshift. The other 45% is likely mainly composed
either by younger systems (age < 0.1 Gyr) at the same redshift,
or by lower redshift z < 3.5 contaminants. We will show in the
following that the former are only a minority (see Sect. 5.2). In
Table 6, the multi-band photometry of the final sample is shown.
Fig. 5. The two panels show the m5.8 − m8.0 colour in function of redshift
for the Maraston stellar population models (Maraston et al. 2006). Top
panel: Single-burst Simple Stellar Populations (SSPs) with three different ages (0.1, 0.5 and 1 Gyr). Bottom panel: 0.1 Gyr stellar populations
with constant star formation rate (SFR) and four different extinction
values (E B−V =0.0, 0.3, 0.5, 0.8)
.
As a further test on the quality of this sample we checked
how many candidates satisfy the empirical blending criterion
tested on extensive simulations by the GOODS team and used
by Daddi et al. (2007) and Dickinson et al. (2008 in prep.). This
is based on the measurements of the angular separation (∆θ) between IRAC positions and the K-band counterparts (we used
also optical coordinates for K-undetected sources). According
this criterion, if a galaxy has ∆θ > 0.′′ 5, it is likely contaminated
by blending in the IRAC bands. However we should point out
that this criterion proved to be successful on objects detected at
the 5-10 σ in both of the K and IRAC bands. For sources with
fainter optical and near-IR detections the shift of the centroid
position throughout the different bands could be larger due to
higher noise. Hence a larger ∆θ could derive from coordinate
fluctuations rather than from blending contamination. Moreover
due to the extremely red colours of our candidate high-z galaxies, this criterion is not extensively applicable to our full sample being half of it both optical and near-IR undetected. Out
of the sources with at least a reliable optical or near-IR detection (>3-5 σ) we verified that the majority (18/25) did pass
this criterion. On the other hand we found 7 galaxies (#1098,
#2172, #2796, #13857, #15268, #15541,#15761) with ∆θ > 0.′′ 5.
However for these objects the lack of visual evidence of blending led us to conclude that the quite large ∆θ separation, is a S/N
issue. Moreover for two of these galaxies the confirmed spectro-
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
scopic redshifts (#13857, #15541, see Sect. 5.4) prove that the
measured IRAC photometry leads to reliable SED fitting results.
5.1. Comparison between the BC03, MA05 models
The SED fitting analysis with BC03 and MA05 templates gives
somewhat discordant results for both galaxy masses and ages
estimate. This is an issue recently debated by several authors
(e.g. Maraston et al. 2006; Kannappan & Gawiser 2007; Bruzual
A 2007), the major source of this discrepancy being the different
treatment of the Thermally Pulsing Asymptotic Giant Branch
(TP-AGB) phase of stellar evolution in the population models.
In the MA05 models stars in the TP-AGB phase contribute a
dominant source of bolometric and near-infrared light for stellar
populations in the age range 0.2 to 2 Gyr, much larger than is
found in the BC03 models. For those models the TP-AGB phase
has been calibrated with local stellar populations. Another difference is in the treatment of convective overshooting. In their work
Maraston et al. (2006) tested both the BC03 and MA05 models
on a sample of high-z galaxies, with reliable spectroscopic redshifts (1.4 . z . 2.5) in the Hubble Ultra Deep Field (HUDF).
They found that the results of MA05 models imply younger ages
by a factors up to 6 and lower stellar masses, by 60% on average, with respect to BC03 (and in general to the others models of
population synthesis). The overestimate of BC03 galaxy stellar
masses, produced by the lack of the TP-AGB contribution to the
integrated luminosity, become considerable for evolved stellar
populations with ages between 0.5 and 2 Gyr.
In Fig. 6 we compare the stellar masses estimated for the
z ≥ 3.5 galaxies with the two different template libraries (BC03
vs MA05). It appears that the BC03 models tend to overestimate
galaxy masses of our sample with respect to the MA05 ones, by
0.15 dex on average (see the smaller panel on the top of Fig. 6),
in good agreement with the previous studies (e.g. Maraston et al.
2006; Wuyts et al. 2007; Cimatti et al. 2008; Werner et al. 2004).
In the rest of this paper we will adopt the MA05 stellar templates to compute the galaxy stellar masses and the comoving
stellar mass density. However, in some cases we also use the
BC03 models results for comparisons with the literature.
Fig. 6. Comparison between estimated masses by means of different
template libraries for our final sample of 53 objects: log M(BC03) vs log
M(MA05). It is visible that BC03 models tend to overestimate galaxy
masses with respect to the MA05 ones. The inserted panel shows the
distribution of the log M(BC03)-log M(MA05) relation as a function of
the galaxy mass.
9
5.2. Comparison with Lyman break selection techniques
One of the most popular method used to identify high-z galaxies
is the ‘dropout’ technique, a colour selection based on the redshifted 912 Å Lyman-break caused by neutral hydrogen in the
galaxy SEDs. This technique was introduced by Guhathakurta
et al. (1990) and Steidel & Hamilton (1992) to select Lyman
Break Galaxies (LBG) at z ≃ 3 (u-band dropout). It has been
largely used also to identify galaxies in the redshift range z ∼
4 − 6, as b-, v- or i-dropout (Steidel et al. 1999; Dickinson 1998;
Steidel et al. 2003; Giavalisco et al. 2004b). However this colour
criterion is strongly biased against red massive galaxies at high
redshift, that are faint or totally undetected in optical and near-IR
bands.
To obtain a quantitative estimate of the fraction of red objects missed by combining optical magnitude limited selection
and dropout techniques, we matched our final sample of IRACselected high redshift galaxies (53 objects) with a sample of Band V-dropout LBGs in the GOODS-N field from Giavalisco
et al. (2004a). We found that only 7 of our massive high-z
candidates are included in the B-dropout sample, and none in
the V-dropout sample. This means that the remaining IR bright
(m4.5 < 23) z ≥ 3.5 candidates have been missed by those selection criteria, due to their faint emission at optical and near-IR
wavelengths (most of them having z850 > 27).
While the vast majority of our candidates are missed by the
dropout selection technique, there could also be a significant
population of massive galaxies with m4.5 < 23 that are identified through the dropout technique and that have been missed by
our selection criteria. We tested for the presence of such galaxies
using again the sample of Giavalisco et al. (2004a).
Giavalisco et al. (2004a) listed 684 LBGs candidates (Bdropout + V-dropout). Only 7 of those are also part of our sample of z > 3.5 candidates. Only 32 of the Giavalisco et al. galaxies have m4.5 < 23 (15 B-dropout and 17 V-dropout). Of these,
20 were excluded from our sample because of their photometric redshifts z < 3.5 (8 B-dropout and 12 V-dropout). One more
object (V-dropout) was excluded because of the presence of Xray emission and 4 objects (V-dropouts) were excluded because
of the lack of an IRAC-8.0 µm peak. One of these 4 V-dropout
galaxies has a confirmed spectroscopic redshift of z = 5.2 (see
also Sect. 5). It is reasonable to suppose that all the 4 V-dropouts
galaxies that we excluded are actually genuine z > 3.5 galaxies
with young ages, lacking a distinct peak in the IRAC-8.0 µm
band. Even taking them into account would increase our sample
of z > 3.5 galaxy candidates by only 7% and would reduce to
81% the fraction of z > 3.5 massive galaxies that are lost by the
UV dropout selection techniques.
It is also interesting to make a comparison with the Lymanbreak galaxies samples selected by Eyles et al. (2007) and Stark
et al. (2007) in the GOODS-South field. The former work reports 6 galaxies at z ∼ 6 with masses in the range 1 − 2.4 × 1010
M⊙ , and 4.5 µm magnitude m4.5 >23. Since the authors used
Salpeter IMF and BC03 models to derive galaxy SEDs, we have
to consider that galaxy masses for these 6 objects are overestimated by a factor of ∼ 2.4 (2.4=1.4 × 1.7, being 1.4 and 1.7
the conversion factors from BC03 to MA05 and from Salpeter
to Chabrier IMF, respectively Pozzetti et al. 2007; Cimatti et al.
2008, see also Sect. 5.1 and Sect. 6.2). When rescaled, masses
span the range 0.4 − 1 × 1010 M⊙ , with only one galaxy with
M∼ 1010 M⊙ . Lyman Break Galaxies similar to the ones found
by Eyles et al. (2007) would not be included in our sample, since
these are fainter than our magnitude cut m4.5 = 23. This issue is
10
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
in agreement with the lower masses found for these LBGs with
respect to our high-z sample.
The latter work also reports 6 galaxies, spectroscopically
confirmed at z ∼ 4.4 − 5.6. We used the same conversion factor
to rescale galaxy masses from BC03 and Salpeter IMF and we
found that only three galaxies of the Stark et al. (2007) sample
have masses > 1010 M⊙ . Moreover only two out of these three
sources have m4.5 < 23.
Hence only 4 galaxies out of the 12 LBGs at 4.4 ≤ z ≤ 6
from the two works, fall within our mass bracket (M∼ 1010 −
1011 M⊙ ), and only two would meet our magnitude selection
(m4.5 < 23). The two IRAC brightest and most massive sources
from Stark et al. (2007) have ages ∼ 150 Myr, if rescaled by a
factor of 6 to account for the overestimate produced by BC03
models (Maraston et al. 2006), and E B−V ∼ 0.0. We can suppose that they are similar to the youngest unreddened population excluded from our final sample by the 8 µm peak criterion
(see Sect. 5 and Fig. 5, top panel), such as the 4 V-band dropout
galaxies of the sample of Giavalisco et al. (2004b). The other
two galaxies with masses ∼ 1010 M⊙ and fainter than our IRAC
magnitude selection would be even younger (ages < 100 Mry).
In conclusion, it seems that we are not missing a significant
population of sources with our selection criteria. A few blue star
forming galaxies with young ages are indeed excluded but these
objects have generally low stellar masses that would not meet
our stellar mass completeness limits (M⋆ ≥ 5 × 1010 M⊙ ; see
Sect. 6.1 and 6.2).
them show colours consistent with older starlight (0.3–0.5 Gyr).
On the contrary, a smaller part of the sub-sample (7 objects)
shows bluer colours, more compatible with younger SSPs (0.1–
0.3 Gyr), probably implying ongoing star formation activity.
The SED fitting analysis and the lack of mid-IR 24 µm emission could thus suggest that many of the MIPS-u galaxies might
be ‘quiescent’, non dusty candidates at z ≥ 3.5. However, we
should not forget that the MIPS observations in the field are not
deep enough to unequivocally constrain the lack of star formation in these galaxies. The 24 µm emission is generally used
as SFR indicator in high-redshift galaxies (z ∼ 2 to 3; Daddi
et al. 2005; Papovich et al. 2007; Yan et al. 2007; Rigby et al.
2008). Following Chary & Elbaz (2001) we can estimate that a
source at z ∼ 4 with a 24 µm flux of 20 µJy will have a total
IR luminosity of ∼ 1.5 × 1013 L⊙ , corresponding to a SFR of
∼ 1500 M⊙ yr−1 for a Chabrier IMF (Kennicutt 1998). Therefore
the 24 µm non-detection imposes only a very loose upper limit to
the SFR. However, the 24 µm non-detection is a necessary condition for quiescent systems (given the MIPS flux limit). In the
following analysis we will use the MIPS-u galaxy sample to derive an upper limit to the space density and stellar mass density
of ‘quiescent’ candidates at z ∼ 4 (Sect. 6.3).
5.3. 24 µm emission
We used observations at 24 µm with Spitzer +MIPS to give some
constraints on the “activity” of our high redshift galaxy candidates. Detection of 24 µm flux for z > 3.5 galaxy candidates
could be explained in terms of radiation coming from star formation activity lately re-processed and re-emitted by the dust
(emission lines from PAH molecules), or with the presence of
an highly obscured AGN in relatively quiescent galaxies. We divided our sample of high redshift candidates in two sub-samples:
MIPS-detected (MIPS-d) and MIPS-undetected (MIPS-u) samples and discuss them separately in the following.
5.3.1. The MIPS-u sample
21 objects in our sample are undetected in MIPS down to the
limit of 20 µJy. The best fit Spectral Energy Distributions (SEDs)
of the MIPS-u sub-sample is presented on the left panels of
Fig. 13. The right panels of the same figure show the χ2ν distribution as a function of redshift for each of the sources. We also
report the most probable photometric redshift solution (z phot ).
The best fitting parameters and the estimated galaxy masses are
presented in Table 3. Most of the MIPS-u redshifts are spread
around a median value of z ∼ 4, in a redshift range z=3.5–5. We
find that the best fitting SEDs give χ2ν < 2 for 20 out of 21 objects and no degeneracy on the redshift solution for the majority
of the sources. On the contrary, we find also lower redshift solutions within the 90% confidence intervals of χ2ν distribution for
5 objects (# 4008, # 9561, # 12327 and #14130), caused mainly
by the redshift-reddening degeneracy (see Tab. 3 and Fig. 13).
To further constraints on the nature of the stellar populations
of MIPS-u galaxies we compared their z − m4.5 colours with the
expected ones for MA05 SSP models with different ages ( 0.1,
0.3 and 0.5 Gyr). The results are shown in Fig. 7. Although the
MIPS-u objects span a large range of z − m4.5 colours, most of
Fig. 7. Comparison between the observed z − m4.5 colours of the MIPSu galaxies (black open diamonds) and the ones predicted at the same
redshift for SSPs models (MA05) with different ages (black curves in
the figure, with age of 0.1, 0.3 and 0.5 Gyr, respectively, from the lower
part upward). Most MIPS-u objects have z−m4.5 colours consistent with
SSPs with ages in the range of 0.3–0.5 Gyr. On the contrary, a smaller
part of the sub-sample (7 objects) shows bluer colors, more compatible
with younger SSPs (0.1–0.3 Gyr), probably exhibiting star formation
activity.
5.3.2. The MIPS-d sample
Out of the 53 galaxies of our high-z sample, more than half
(32) are detected at 24 µm. The best fitting SED results for the
MIPS-d objects and their relative χ2ν distributions within the 90%
confidence intervals are shown in Fig. 14. In Table 4 we report
the best fitting parameters for the MIPS-d sample. The quality
of the fit for the MIPS-d SEDs is very good, with χ2ν values
below 1.5 for almost all objects (except for #5511, which has
χ2ν = 2.46). However most galaxies in the sub-sample present degenerate redshift solutions, caused by the different models used
and the amount of reddening present (# 702, # 2560, # 2796,
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
#ID
#522
#1098
#2574
#3081
#3302
#4008
#5073
#5460
#6876
#7042
#7286
#7309
#7785
#8403
#9537
#9561
#11682
#12327
#14130
#14260
#15268
z phot
3.928
4.369
3.620
3.543
4.292
9.024
4.481
4.236
3.620
3.774
3.564
3.613
3.970
4.264
4.103
4.026
4.320
4.971
4.432
4.152
4.775
χ2ν
0.410
1.465
0.307
0.466
0.306
0.201
0.738
0.854
0.683
0.646
5.582
1.267
1.964
2.018
0.150
0.838
0.367
0.202
0.315
1.243
2.534
Pχ2ν
94.28
14.55
97.98
91.26
98.01
99.63
68.90
57.65
74.09
77.54
0.00
24.30
3.28
2.76
99.89
59.14
96.11
99.41
97.06
25.71
0.47
AV
0.80
0.20
0.40
0.70
0.30
0.40
0.20
0.00
0.20
0.50
0.10
0.00
0.30
0.00
0.40
0.00
0.00
0.70
0.10
0.50
0.80
zin f −90%
3.060
4.250
2.570
3.340
3.620
2.150
4.250
3.480
2.220
3.340
2.850
3.130
3.900
4.110
3.200
2.080
4.040
2.920
1.590
4.040
4.390
zsup−90%
5.230
4.460
4.110
3.760
4.600
10.060
4.670
4.390
4.040
3.900
3.620
3.830
4.040
4.390
4.390
4.320
4.530
7.750
5.090
4.250
5.090
z2−phot
10.060
4.922
6.098
0.456
10.060
3.753
3.417
3.683
3.172
3.529
3.193
3.417
0.540
3.347
6.651
3.396
2.941
3.256
3.067
0.652
7.260
Pχ2ν ,2
43.52
0.04
0.47
0.00
0.00
99.57
20.62
50.87
72.67
75.10
0.00
22.90
0.00
0.57
0.00
51.28
64.89
96.17
96.30
0.01
0.07
11
log M(MA05)
9.960
10.143
10.581
9.816
10.565
10.069a
11.073
10.402
10.485
9.723
10.610
9.723
10.391
10.710
10.801
10.412
10.186
11.185
10.235
9.935
10.373
Table 3. Best fitting parameters for the MIPS-u sample. In the order: identification number, z (best fit solution), χ2ν , probability relative to z,
reddening, upper and lower bound for the 90% confidence interval, z2−phot (second best solution), probability relative to z2−phot . We should point
out that, for galaxies with z phot ∼ 8 − 10, the best fit redshifts result uncertain, and the equally probable solution range (zin f − zsup ), within the 90%
of confidence, is very large. Also for some of these objects -marked by the footnotea - the hyperzmass procedure was not able to find any physical
solution by fitting the data points with MA05 models the extreme redshifts.
a
This value for the galaxy mass was found for the secondary solution of photometric redshift, z = 3.753, because the hyperzmass software did not find a plausible solution at z = 9.024.
# 3393, # 3850, # 5157, # 5367, # 6099, # 9301, # 9306, # 10176,
# 11304, # 13129, # 14105).
It is difficult to understand the nature of these objects, given
that often high and lower photometric redshift solutions result
equally probable. In these cases the 24 µm emission could be
interpreted in different ways, depending on the redshift of the
objects and on the reddening parameter. Assuming that z ≥ 3.5 is
correct, and taking into account the lack of hard-X ray emission,
the MIPS flux could ensue from highly obscured AGNs hosted in
relatively quiescent galaxies. However, still consistently with the
high redshift solution, the 24µm emission could be also due to
star formation at extreme high rates. In the next section we will
examine this possibility using sub-mm/mm photometry from the
SCUBA ‘supermap’(Borys et al. 2003; Pope et al. 2005), and the
MAMBO (Greve et al. 2008) and AzTEC (Perera et al. 2008)
imaging surveys of GOODS-N.
On the other hand if the lower redshift solution (z ∼ 2 − 3) is
considered, the 24 µm emission could be explained in terms of
PAH emission from dust heated by star formation activity, and
those objects would be accounted as dusty star-burst galaxies at
z ∼ 2 − 3.
ration of their bright phase (<< 100 Myr; Greve et al. 2005).
These objects could represent the common early phase in the
formation of massive elliptical galaxies, and hence they might
be crucial link to understand the massive galaxy formation process.
The numerous sub-mm datasets available in the GOODS-N
field can be used to improve the constraints on the nature of our
z ≥ 3.5 candidates.
We made direct comparison with three sub-mm maps of the
field:
5.4. Sub-mm detections : a large population of massive
starburst galaxies at z ∼ 4?
We used a simple approach, looking for overlap of our sample with the position of the submm/mm selected galaxies in the
above works. We have used radial separation limits of 7”, 6” and
9”, to search for matches with galaxies in the SCUBA, MAMBO
and AzTEC, respectively.
We found eight of our high-z candidates in the error box of
the SCUBA ‘supermap’ sources (#5157, #6099, #6463, #8520,
#10176, #11682, #13857, #15541), but only seven (except
#11682) correspond to the IRAC counterparts identified by Pope
et al. (2006). For two of these galaxies, GN20 and GN20.2,
(objects #15541, #13857 in our sample) spectroscopic redshifts
Sub-mm/mm selected galaxies (hereafter SMGs) are the brightest star forming galaxies known, being much more more luminous than the local ultra-luminous IR galaxies -ULIRGs- with
LIR ≫ 1012 L⊙ . These galaxies are massive young objects seen
during their formation epochs, with very high SFR (Lilly et al.
1999; Scott et al. 2002), about one order of magnitude larger than
that of typical systems with similar masses (Daddi et al. 2007).
They are also fairly rare objects, probably due to the short du-
– the Sub-millimeter Common User Bolometer Array
(SCUBA, Holland et al. 1999) ‘supermap’ of 35 SMGs
selected with S/N≥ 4 at 850 µm (Borys et al. 2003; Pope
et al. 2005, 2006);
– the Max Planck Millimeter Bolometer Array (MAMBO,
IRAM 30-m telescope, Bertoldi et al. 2000) 1200 µm map
from Greve et al. (2008), composed by 27 sources detected
with 3.5 ≤ S/N < 4 and 20 with S/N ≥ 4;
– the AzTEC (Wilson et al. 2008, ,15 m James Clerk Maxwell
Telescope -JCMT) 1100 µm map of 28 sources detected with
S/N ≤ 3.75 (Perera et al. 2008).
12
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
ID
# 588
# 702
#1720
#2172
#2560
#2796
#2894
#3393
#3850
#5157
#5367
#5511
#6099
#6245
#6463
#8520
#9007
#9301
#9306
#9782
#10176
#11304
#12699
#13129
#13857
#14505
#14668
#14722
#14793
#15541
#15761
#15771
z phot
4.285
8.814
3.522
3.711
3.830
10.060
3.893
5.503
4.166
4.740
4.355
4.152
5.251
5.062
9.437
3.683
3.669
8.226
8.660
3.928
5.818
4.145
3.543
7.960
3.746
3.564
4.600
4.145
4.474
3.949
4.292
3.564
χ2ν
0.785
0.067
0.526
0.536
0.044
0.279
0.628
0.477
0.063
0.344
0.494
2.521
0.367
0.108
0.193
0.573
1.117
0.059
0.127
1.472
0.002
0.378
1.760
0.030
0.863
0.014
0.209
1.816
1.225
1.216
0.177
0.360
Pχ2ν
64.36
99.99
87.33
86.56
100.00
98.59
79.14
90.59
100.00
96.90
89.51
0.50
96.10
99.98
99.68
83.71
34.42
100.00
99.95
14.28
100.00
95.67
6.21
100.00
56.74
100.00
99.56
5.23
27.41
27.45
99.78
96.35
AV
0.60
0.40
0.60
0.70
0.40
0.80
0.70
0.80
0.70
0.00
0.80
0.50
0.10
0.00
0.10
0.00
0.60
0.20
0.80
0.20
0.20
0.80
0.30
0.20
0.10
0.80
0.10
0.40
0.00
0.40
0.10
0.20
zin f −90%
3.690
1.450
2.850
3.550
2.780
3.360
2.990
3.830
3.410
4.110
3.900
4.040
3.200
4.460
3.690
3.340
2.780
2.640
2.360
3.480
3.550
3.550
3.200
3.410
3.270
2.920
1.870
4.110
4.040
3.690
3.900
3.060
zsup−90%
4.740
10.060
4.250
4.180
8.520
10.060
4.460
5.860
9.290
7.400
4.880
4.250
8.450
5.370
10.060
3.830
4.110
10.060
10.060
4.320
9.570
8.100
3.690
10.060
4.040
10.060
5.230
4.180
5.160
4.110
4.530
3.830
z2−phot
9.388
3.340
3.921
4.117
7.274
3.536
10.060
5.167
8.009
6.504
10.060
1.310
4.999
2.934
4.215
3.137
2.969
8.037
8.219
8.723
5.517
5.461
3.354
8.296
3.403
7.757
4.789
0.617
9.185
0.680
6.567
6.854
Pχ2ν ,2
0.05
99.99
84.30
70.42
100.00
95.36
0.37
84.73
99.90
92.59
0.03
0.00
96.02
81.31
99.64
44.99
34.41
100.00
99.92
0.02
100.00
89.78
5.62
100.00
43.23
100.00
99.53
0.36
0.01
0.00
0.00
0.00
log M(MA05)
9.973
10.563
10.399
11.097
10.762
10.870a
10.441
10.647
10.940
11.433
10.806
10.551
11.178
10.662
9.973a
10.730
10.824
11.035
11.011
11.116
11.280
10.759
10.558
11.056
10.707
10.025
10.177
10.180
11.411
11.146
11.126
11.283
Table 4. Best fitting parameters for the MIPS-d sample. In the order: identification number, z (best fit solution), chi2ν , probability relative to z,
reddening, upper and lower bound for the 90% confidence interval, z2−phot (second best solution), probability relative to z2−phot . For the objects with
extremely high redshift best fit solutions, the same caveat we gave in Tab. 3 should be taken into account
.
a
These values for the galaxy masses were found for the secondary solutions of photometric redshift, because the hyperzmass software did not find a acceptable solution at z ∼ 9 − 10.
around z ∼ 4 were recently determined by Daddi et al. (2008,
hereafter D08). Those objects are among the most distant SMGs
spectroscopically confirmed known at the present. Our photometric redshifts for these galaxies are consistent within the error
(∼ 0.2) with the spectroscopic ones (being 3.75 and 3.95 for
GN20 and GN20.2 respectively). It is interesting to remark that
D08 discuss the evidence of a galaxy proto-cluster at z ∼ 4 in the
GN20 and GN20.2 area, due to the finding of an over-density of
B-dropout galaxies. Four objects in total from our z > 3.5 sample
(#13857, #14722, #15268, #15541) are also within a 25” radius
centered on the coordinates of GN20. This represents an overdensity of a factor of 18 with respect to the expected number
density of IRAC-selected galaxies in the field. Massive galaxies
seem to be tracing this proto-cluster structure at z = 4.05.
We found five sources of our high-z sample matching the
MAMBO galaxies within the correlation distance. Out of them
only #15541 (GN20) is also SCUBA-detected. In addition, 8
objects from the AzTEC survey correspond to galaxies in our
z > 3.5 sample, within the adopted search radius. Three of them
are also SCUBA detected, and three are in common with the
MAMBO survey from G08.
Candidate z > 3.5 galaxies in our sample that are consistent with being counterpart of SCUBA, MAMBO and/or AzTEC
sources are listed in Tab. 5.
It should be noted that the bulk of the sub-mm counterparts, except the objects #522 (GN1200.5/AzGN06) and #7785
(GN1200.46), are 24 µm-detected (MIPS-d sample). The latter is
also a less secure MAMBO detection, being among the sources
with a S/N below the 4.5 σ limit, and is not part of the ‘robust
deboosted catalogue’.
We can check the reliability of the high-z solution for
the sub-mm detections by comparing their S850 /S24 vs S24 /S8.0
colours with the ones found by D08 for the two confirmed SMGs
galaxies at redshift ∼ 4 (GN20 and GN20.2). We show this comparison in Fig. 8, where GN20 and GN20.2a are represented
as black bold diamonds. As suggested by D08, starburst galaxies at z ∼ 4 would have relatively blue Spitzer MIPS-IRAC
colours, similarly to GN20 and GN20.2a (log(S24 /S8.0 ) < 0.7,
log(S24 /S4.5 ) < 1.11). Moreover, because of the negative submm k-correction the S850 /S24 ratio should increase with redshift.
Hence we expect that starburst at z ∼ 4 would occupy the upperleft part of the diagram in Fig. 8, with log(S850 /S24 )> 2 (such as
GN20 and GN20.2a).
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
ID
(this paper)
#588
#3850
#5367
#14505
#522
#7785
#11304
#6099
#10176
#5157
#8520
#6463
#15541
#13857
SCUBA
name
...
...
...
...
...
...
...
GN03
GN09
GN11
GN12
GN18
GN20
GN20.2a
MAMBO/AzTEC
name
.../AzGN12
.../AzGN20N
.../AzGN27N
GN1200.9/AzGN28
GN1200.5/AzGN06
GN1200.46/...
GN1200.13/...
...
.../AzGN31
...
.../AzGN08
...
GN1200.1/AzGN01
...
z phot
(this paper)
4.28
4.17
4.35
3.56
3.93
3.97
4.14
5.25
5.818
4.74
3.68
9.437/4.215
3.949
3.746
zmin90%
zmax90%
3.69
3.41
3.90
2.92
3.06
3.90
3.55
2.95
3.55
4.11
3.34
3.7
3.69
3.27
4.74
9.29
4.88
10.06
5.23
4.04
8.10
10.0
10.0
7.40
3.83
10.0
4.11
4.04
z phot
(P06)
...
...
...
...
...
...
...
2.0
2.4
2.3
3.1
2.2
2.95
2.83
zradio−IR
(D08)
...
...
...
...
...
...
...
1.9
13.42
2.45
3.20
2.50
4.055
4.051
S850
[mJy]
...
...
...
...
...
...
...
1.8
8.9
7.0
8.6
3.2
20.3
9.9
13
S1200/1100
[mJy]
3.07
2.79
2.31
2.7/2.31
3.8/4.13
2.1a
2.2
...
2.13
...
3.83
...
9.3/10.69
...
d
[arcsec]
5.2
2.3
6.5
4.1/4.2
2.9/1.1
4.5
0.8
2.5
3.9/5.3
2.4
5.16/8.7
3.2
1.3/1.9/1.4
6.2
Table 5. SCUBA, MAMBO and AzTEC counterparts found in our z ≥ 3.5 of MIPS-d detected sources. In the columns we report: the ID number
of the objects (this paper); the names from the sub-mm surveys; photometric redshift estimated in this paper and their 90% confidence intervals;
photometric redshifts by Pope et al. (2006, P06); radio-IR redshifts by Daddi et al. (2008, D08) - with the two spectroscopic values in boldface; the SCUBA, MAMBO and AzTEC fluxes. The last column is the distances between the proposed IRAC-4.5 µm counterparts and the sub-mm
positions.
a
Not deboosted flux
Out of the SCUBA sources (black diamonds), two
(#10176/GN09 and #8520/GN12) have mid-IR colours comparable with GN20 and GN20.2a, and log(S850 /S24 )> 2). This finding can be seen as a further confirmation of the z > 3.5 redshift
estimated for these objects. It is also in agreement, within the
99% confidence interval, with the radio-IR redshifts estimated
by D08 (see their Tab. 3). In the same figure also the ‘SCUBAblank’ MAMBO (red triangles) and AzTEC (green crosses) detections are shown. To represent them in this diagram we scaled
the S1200 and S1100 fluxes to S850 , multiplying by a factor of 2, i.e.
the typical S850 /S1200 ratio found for GN20 and GN20.2 (D08).
This is also consistent with the S850 /S1100 flux ratio derived by
Perera et al. (2008, 2.08±0.18). For the MIPS-u #522 and #7785
we show the limits in the two-colours diagram, computed with
S24 =20 µJy (the 5 σ 24 µm-MIPS flux lower limit). Also the
sub-mm-IR colours of these sources are consistent, within the
error bars, with the expected colours for high-z SMGs. For comparison we show also the positions occupied in Fig 8 by the
other sub-mm galaxies from the (Pope et al. 2006) sample with
secure IRAC counterparts, that do not overlap with our sample. Almost all these sources (violet small squares) are spectroscopically confirmed at z ∼ 1 − 2.5 (see Pope et al. 2006).
The evidence that most of them occupy the lower-right part of
Fig 8 strongly support the above statements. Other two galaxies from the SCUBA ‘supermap’ have colours similar to GN20
and GN20.2, i.e. GN10, and GN22. The first one is claimed by
several authors (D08, Dannerbauer et al. 2008; Wang et al. 2007,
2009) to be a z > 3.7 starburst, although (Pope et al. 2006) report
a z = 2.2 photometric redshift. The second one is a spectroscopically confirmed galaxy at z = 2.509 (Chapman et al. 2005; Pope
et al. 2006). It represents the only case in the SCUBA sample
of a lower-redshift sub-mm/mm emitter with both S850 /S24 ratio
and IRAC colours similar to spectroscopic confirmed z ≥ 3.5
SMGs. Nevertheless from a statistical point of view the diagram
shown in Fig 8 seems to be a valid diagnostic to prove high-z
starburst for sub-mm/mm sources.
Hence we can conclude that, for ∼ 78% of the galaxies
in our sample that are likely sub-mm/mm detected, these results support the hypothesis of extreme SFR activity at high-z.
Instead the sub-mm (SCUBA) sources lying in the right part of
Fig. 8(objects #5157, #6099 and #6463 -∼ 22% of the sub-mm
detections) are more probably at lower redshifts, as proposed by
Pope et al. (2006).
Fig. 8. S850 /S24 vs S24 /S8.0 colours for the SCUBA(black diamonds),
MAMBO(red triangles) and AzTEC(green crosses) detections of our
high-z sample. The comparison with the GN20 and GN20.2 (Daddi
et al. 2008) seems to support the high-z hypothesis for most of these
sources (left part of the diagram; see Sect.5.4). The solid black line represent the colour limit suggested by (Daddi et al. 2008) for sub-mm
galaxies at z & 4 (log(S24 /S8.0 ) < 0.7). The small violet squares are the
SCUBA ‘supermap’ sources that do not overlap with our high-z sample,
most of them spectroscopically confirmed at z ∼ 1 − 2.5.
5.5. Mid-IR vs IR colours
The properties and the overall spectral energy distributions make
the MIPS-d galaxies comparable to one of the most debated
objects in the literature, HUDF-JD2. This object, identified as
14
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
Fig. 9. Infrared colours m8.0 − m24 vs m3.6 − m8.0 of our high-z sample of galaxies (black diamonds) compared with the objects found by
Rodighiero et al. (2007) (red open triangles). The bold red triangle represents HUDF-JD2 (Mobasher et al. 2005). Bold black open diamonds
are the SCUBA detected galaxies of our sample. The two objects with
confirmed z ∼ 4, GN20 and GN20.2, are highlighted (filled black diamonds). The MIPS-undetected objects are shown as upper limits in the
m8.0 − m24 colour: black and red arrows for our sample and Rodighiero
et al. (2007) sample, respectively. Two evolutionary colour tracks of an
ULIRG (violet dot-dashed curve; Chary & Elbaz 2001), and of a galaxy
hosting an obscured AGN (dashed green curve; Seyfert 2 galaxy NGC
1068), with increasing redshift values, are also shown. The black horizontal line represents the upper limit suggested by D08 and also shown
in Fig. 8 for colours of starburst galaxies at z ≥ 3.5.
a z ∼ 3.4 galaxy by Yan et al. (2004) and Chen & Marzke
(2004), was recently suggested by Mobasher et al. (2005) as
a massive post star-burst galaxy at z ≃ 6.5 hosting an obscured AGN. However subsequent studies (Dunlop et al. 2007;
Rodighiero et al. 2007; Chary et al. 2007) claimed that HUDFJD2 is more likely a lower redshift (z ∼ 1.5-2.2) star forming
galaxy with very strong dust reddening. Our galaxies are also
similar to candidate z > 4 galaxies found by Rodighiero et al.
(2007) and Wiklind et al. (2008). In Fig. 9, the infrared colours
(m8.0 − m24 vs m3.6 − m8.0 ) of our high-z sample (open black
diamonds) are compared with the objects found by Rodighiero
et al. (2007, open red triangles). The bold red triangle represents
HUDF-JD2 (Mobasher et al. 2005). Bold black open diamonds
are the sub-mm detected galaxies of our high-z sample. The two
objects with confirmed z ∼ 4, GN20 and GN20.2, are represented by filled black diamonds. The MIPS-undetected objects
are shown as upper limits in the m8.0 − m24 colour (black and
red arrows for our sample and the sample of Rodighiero et al.
2007, respectively). The evolutionary colour tracks of an ULIRG
(Chary & Elbaz 2001) and of a galaxy hosting an obscured AGN
(the Seyfert 2 galaxy NGC 1068) with increasing redshift values
are also shown. It is clear, though, that such locally calibrated
templates are not very useful to interpret the colors of distant
galaxies. This can be argued just considering that the observed
m8.0 −m24 colours of GN20 and GN20.2 are ∼ 1 mag redder with
respect to the ULIRG track at z ∼ 4. As discussed in Daddi et
al. (2008), this is most likely due to a lower stellar mass to light
ratio in the distant galaxies.
The black horizontal line represents the upper limit suggested by D08 and also shown in Fig. 8 for colours of starburst galaxies at z ≥ 3.5 (log(S24 /S8.0 ) < 0.7, in AB magnitude:
m8.0 − m24 < 1.7; see also Sect. 5.4). If the high-z solution is
correct, objects bluer than this limit should be strongly dominated by star formation, such as GN20 and GN20.2. The issue
that most of the the sub-mm detected galaxies in our sample are
down to the black line in the figure also suggest that they have
properties similar to the two starbursts spectroscopically confirmed at z ∼ 4. The MIPS-d sources which have similar colours
but are lacking in a sub-mm counterpart could be also high-z
starbursts. Alternatively, the lack of sub-mm detection should be
justified either by the shallow sensitivity of the currently available sub-mm instruments (SCUBA, MAMBO, AzTEC), or by
particularly “warm” SEDs for this objects.
On the other hand, the z ≥ 3.5 candidates with m8.0 − m24 >
1.7 in Fig. 9 could represent a different population. Their redder colours, more similar to the bulk of the Rodighiero et al.
(2007) sample, could be due to heavily obscured AGN, as also
suggested by Rodighiero et al. (2007) for most of their sources,
or they might be z < 3.5 contaminants. The position of object
HUDF-JD2, overlapping with the solid black line in the middle
part of the diagram, does not allow us to give any further constraint on its nature.
6. The comoving stellar mass density at 3.5 < z < 5
Recent studies (Drory et al. 2005; Fontana et al. 2006; Stark et al.
2007; Verma et al. 2007; Eyles et al. 2007; Yan et al. 2006) have
extended measurements of the total stellar mass density up to
z ∼ 4 − 5, using deep multicolour data from optical and near-IR
selected samples. One of the goals of this paper is to determine
how our IRAC-4.5 selected sample contributes to the comoving
stellar mass density at z ≥ 3.5. In fact, as already discussed,
observations of the high redshift Universe are generally biased
against red galaxies with large mass-to-light ratios, potentially
missing galaxies that could potentially make a strong contribution to the total mass density (see Fontana et al. 2006). Here, we
emphasize that we can only determine a lower limit to the comoving stellar mass density in the redshift interval 3.5 ≤ z ≤ 5,
for several reasons. First, the combination of our IRAC 4.5 µm
magnitude limit and the 1.6 µm peak SED criterion leads to
missing objects with lower masses at these redshifts, such as
the fainter, bluer Lyman break galaxies, as we have discussed
in Sect. 5. More generally, a magnitude-limited sample (selected
at any wavelength) is not the same as a mass–limited sample, as
we will discuss further in the next section.
6.1. Incompleteness effects and mass selection criteria
In estimating the comoving stellar mass density we must take
into account that our sample could suffer from incompleteness.
As several other authors (Fontana et al. 2004; Drory et al. 2004;
Labbé et al. 2005) have noted, IR-selected samples are still not
equivalent to mass-selected samples. In fact, at any redshift, the
galaxies detected above the sample magnitude limit can have a
fairly large range of possible M⋆ /L ratios, depending on their
spectral properties, ages, dust extinction, metallicities, etc. The
effect is that magnitude-limited samples, at higher redshifts, are
progressively biased against objects with lower masses and high
M⋆ /L ratios, such as galaxies that are not currently forming
stars, or which are highly extincted.
We have attempted to evaluate the consequences and minimize the effects of incompleteness by determining the threshold
mass limit as a function of redshift for a variety of galaxy models spanning a range of ages and degrees of extinction. For this
purpose we used the MA05 template library with the Chabrier
IMF, the same as was used in the preceding sections to derive
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
the galaxy stellar masses. We consider both the simplest case of
SSP models with different ages (0.1, 0.5 and 1 Gyr), and a set
of models with a constant star formation rate, an age of 0.5 Gyr,
and different amounts of extinction (EB−V = 0.0, 0.3, 0.5, applied
using the Calzetti attenuation law; Calzetti et al. 2000). The observed 4.5 µm flux as a function of redshift was derived from
these models, and from this we translate our 4.5 µm magnitude
limit (m4.5 ≤ 23) into a limiting stellar mass at each redshift.
The results are shown in Fig. 10. For passively evolving galaxies
(i.e., the SSPs) with ages less than 0.5 Gyr, and for star-forming
galaxies with EB−V < 0.3, our sample should be fairly complete
at M > 5 × 1010 M⊙ out to z < 5. This is another advantage of
our 4.5µm sample selection from the very deep GOODS IRAC
data, which is complete to significantly lower mass thresholds
at these very high redshifts compared to K–selected samples(see
Fontana et al. 2006).
Assuming that the redshift estimate for the objects in our
high-z sample are correct, we derive their contribution to the comoving number density and to the total stellar mass density, both
for the total sample (MIPS-u+MIPS-d) and for the MIPS-u sample only, in the redshift interval 3.5 ≤ z ≤ 5 as described in the
following sections. The comoving volume in this redshift range
over the total GOODS-N solid angle (∼ 165 arcmin2 , coincident
with the ACS z-band area) is Vcom ∼ 0.725 × 106 Mpc3 .
Fig. 10. The redshift dependence of the stellar mass limit for our IRAC
magnitude-limited sample (m4.5 ≤ 23), derived from MA05 stellar population synthesis models. The red curves represent SSP models with
different ages (0.1, 0.5 and 1 Gyr). The green curves are models with
constant SFR, an age of 0.5 Gyr, and dust extinction, with three different values of the colour excess E B−V (0.0, 0.3 and 0.5). The points
indicate the objects of our z ≥ 3.5 sample, with error bars indicating the
68% confidence ranges on their stellar masses.
6.2. Contribution to the stellar mass density from our total
sample in the redshift bin z = 3.5 − 5
Based on the preceding analysis, we assume a limiting stellar
mass M∼ 5 × 1010 M⊙ as the completeness threshold for our
magnitude-limited sample. For quiescent galaxies with intermediate ages (∼ 0.5 Gyr), we should be complete above this mass
threshold out to z ∼ 5. However, for z > 4, we may be incomplete near this mass limit for the oldest possible passive stellar
populations, or for heavily dust-obscured galaxies.
15
Given this potential incompleteness, and the fact that we are
undoubtedly missing a large percentage of bluer, lower–mass
LBGs in the same redshift range (as discussed in section 5.2),
we must consider our estimate for the stellar mass density at
3.5 ≤ z ≤ 5 to be a lower limit. Fig. 10 shows the derived stellar masses versus redshift for the objects in our IRAC-selected
sample. We count objects and sum their stellar masses for galaxies above our mass threshold (M∼ 5 × 1010 M⊙ ). The resulting
comoving number density and the stellar mass density for this
sample are 2.6 × 10−5 Mpc−3 and (2.9 ± 1.5) × 106 M⊙ Mpc−3 ,
respectively.
In Fig. 11 we show our lower limit to the comoving stellar
mass density (ρ∗ ) at 3.5 < z < 5, and compare it to another
estimate (Fontana et al. 2006, F06) at 3 < z < 4 for galaxies
in a similar mass range (M⋆ = 3 × 1010 − 3 × 1011 M⊙ ). We
take the F06 value from the compilation published by Wilkins
et al. (2008), and have corrected it for the M/L differences between the Salpeter and Chabrier IMFs (log M(Chabrier)=log
M(Salpeter)-0.23), as well as by a factor of 1.4 to account for the
difference between the BC03 used by F06 and the MA05 models
used in our study (Sect. 5.1). We also show a curve derived from
the Millennium Simulation models for the redshift evolution of
the stellar mass density, computed for galaxy masses above the
threshold limit of ≥ 5 × 1010 M⊙ . Assuming that the photometric
redshifts for all the selected high-z candidates are correct, our
results are in good agreement with the Millennium Simulation
predictions.
Fig. 11. Comparison between our estimate of the lower limit of the stellar mass density (ρ⋆ ) at 3.5 < z < 5 (red triangle) and the stellar mass
density estimated by Fontana et al. (2006, F06) at 3 < z < 4, for galaxies in the mass range M⋆ ∼ 3 × 1010 − 3 × 1011 M⊙ . The curve shows
the evolution of stellar mass density with redshift from the Millennium
Simulation models, computed for galaxies above our threshold limit of
≥ 5 × 1010 M⊙ .
6.3. Stellar mass density for the MIPS-u candidates in the
redshift bin z = 3.5 − 5
As described above, our sample should be largely complete for
‘quiescent’, non-star-forming galaxies with ages ≤ 0.5 Gyr and
masses M > 5 × 1010 M⊙ . However, at these large redshifts,
the flux limit at 24 µm, even for very deep data from GOODS,
corresponds to quite large star formation rates > 1000 M⊙ /yr.
Therefore, we cannot affirm that the MIPS-undetected (MIPS-
16
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
u) objects in are sample are truly ‘quiescent’, and thus we can
only derive an upper limit for the number and stellar mass densities of quiescent galaxies. For the MIPS-u subsample with stellar masses ≥ 5 × 1010 M⊙ at z = 3.5 − 5, we find a comoving
number density of 0.97 × 10−5 Mpc−3 , and a stellar mass density
of (1.15 ± 0.7) × 106 M⊙ Mpc−3 (MA05 templates and Chabrier
IMF).
In Fig. 12 we compare our results for the MIPS-u sample
with estimates of the stellar mass density for quiescent galaxies at other redshifts from the literature. To facilitate this comparison, here we have scaled all values to the Chabrier IMF
and MA05 models by means of the M/L conversion relations
given above (see Sect.6.2). We have also computed the comoving stellar mass density of the MIPS-u galaxies considering the
unlikely case that they were all truly “passive”, consistent with
single burst stellar populations (SSPs) with no dust. The masses
were derived using the ratio between the observed IRAC-4.5 µm
flux for each galaxy and the expected flux for an MA05 SSP
model at the same redshift, assuming an age of ∼ 0.5 Gyr and
mass normalized to 1 M⊙ . In this case, the number density of
MIPS-u candidates above the mass threshold would be larger
(1.52 × 10−5 Mpc−3 ). The corresponding stellar mass density
would be (1.46 ± 0.7) × 106 M⊙ Mpc−3 . This value is also represented in Fig. 12 by the red dashed line. From this exercise
we concluded that even if all the MIPS-u galaxies were considered as old SSPs, the number density of ‘quiescent’ galaxies at
z = 3.5 − 5 would remain very small in comparison with that
found at z ≤ 2 − 2.5 by previous studies.
The stellar mass density we found for the MIPS-u galaxies in the redshift range z = 3.5 − 5 is ∼ 2 orders of magnitude lower than the local value (black asterisk; Baldry et al.
2004), and more than 10 times lower than the values found at
z ∼ 1.7 by Daddi et al. (2005, ; black filled rhombus) for the
passive-BzK galaxies in the Hubble Ultra Deep Field (HUDF)
and at z ∼ 2.5 by Labbé et al. (2005, black open square) for
Distant Red Galaxies (DRGs) in the Hubble Deep Field South.
Moreover our result is also in good agreement with the mass
density estimated by Wiklind et al. (2008) for z ∼ 5.5 Balmerbreak-selected galaxies without MIPS detections in the GOODS
South Field at redshift z ∼ 5.5 (open rhombus in the figure).
Considering that our estimate is an upper limit to the comoving
stellar mass density of massive and quiescent galaxies, we conclude that there is evidence of strong evolution beyond z ∼ 2
(see also Daddi et al. 2005). The decreasing number density of
quiescent objects at high redshifts could mean that most massive
galaxies at early epochs were experiencing strong star formation
activity.
7. Summary and conclusions
In recent years, a substantial number of massive galaxies have
been found at redshifts as large as z ∼ 5. However, although the
range of stellar populations and observed colors among galaxies at all redshifts studied to date has proven to be quite heterogeneous, most surveys at z > 3.5 to date have primarily selected samples of star-forming blue galaxies, and have been biased against red massive objects that are faint or completely undetected in optical and near-IR bands.
In this work we have studied galaxies selected at IRAC4.5 µm in the GOODS-N field, in a manner that is complementary with respect to optical or near-IR selection. This allows us
to pick out massive, evolved galaxies at high redshift, for which
the optical rest frame emission (which is most nearly related to
the stellar mass) is sampled by the redshifted, rest-frame near-
Fig. 12. Comparison between the upper limit to stellar mass density (ρ∗ )
found in this paper for ‘quiescent’ galaxies in the redshift bin z = 3.5−5
and results from the literature at other redshifts. All points have been
scaled to Chabrier IMF and MA05 masses (see Sect. 6.2). Our estimate
is represented as a red bold triangle. The horizontal red dashed line
represent the stellar mass density we derive by considering all MIPSu galaxies to be SSPs with age of 0.5 Gyr. The local mass density in
passive galaxies is shown by the black asterisk (Baldry et al. 2004). The
black open square is for Distant Red Galaxies (DRGs) at z = 2 − 3
in the Hubble Deep Field South (Labbé et al. 2005). The black filled
rhombus is from Daddi et al. (2005) for passive-BzK galaxies in the
Hubble Ultra Deep Field (HUDF) at z = 1.39− 2, and the open rhombus
is for Balmer-break MIPS undetected galaxies at z = 5–7 in GOODSSouth from Wiklind et al. (2008).
infrared portion of the spectrum. On the other hand, this selection may also be sensitive to ‘active’ obscured galaxies at
high redshift, for which IR emission could be reprocessed radiation from absorbing material surrounding an AGN, or from
dust heated by star formation, like that seen in dusty starburst
galaxies such as local ULIRGs.
Recently, other studies based on near-IR selection have detected red, massive galaxy candidates at z phot ≥ 4, but have not
obtained complete samples. In most of these studies, the samples
have been selected in the K-band, with the possibility of missing
K-undetected objects visible in the longer-wavelength IRAC filters (e.g. Mobasher et al. 2005; Wiklind et al. 2008). Rodighiero
et al. (2007) used IRAC-3.6 µm selection (m3.6 < 23.26) to identify a sample of high-z galaxies missed by optical and K-band
selection. However they assumed as additional conditions the
non-detection of their candidates in optical bands, and a detection close to the sky threshold limit in the K band (K > 23, AB
system), which make their sample a priori incomplete.
Here, we attempt to select a sample of massive high redshift red galaxies that is as complete as possible, by means of
IRAC selection, that could allow us to recover massive objects
that may have been missed by previous studies. The most important results of this paper are summarized in the following items:
• By selecting at IRAC-4.5 µm (m4.5 ≤ 23), we extracted a
sample of 4 142 objects in the ∼ 165 arcmin2 of the GOODSN field. We performed an SED fitting analysis to estimate
photometric redshifts for those objects, and found fifty-three
candidates at z ≥ 3.5, also requiring that peak of the fν stellar
spectral energy distribution (at 1.6 µm in the rest-frame) falls
in the IRAC-8.0 µm band. We excluded unobscured AGN
from our final sample, identified by their hard and/or soft-X
ray emission. Almost 81% of our candidates are completely
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
missed by the B- and V-band Lyman break dropout techniques designed to identify UV-bright, star-forming galaxies
at similar redshifts. For each object, we evaluate the reliability and the confidence limits of our photometric redshift
estimates with a χ2 test. Two objects have spectroscopically
confirmed redshifts (GN20 and GN20.2; Daddi et al. 2008)
in good agreement with our estimates.
• We divide our final sample in two sub-samples of (32) MIPSdetected and (21) MIPS-undetected objects (the MIPS-d and
MIPS-u samples, respectively).
MIPS-d sample: In the subsample detected at 24µm, we
found 18 galaxies with unambiguous high photometric redshift solutions and 14 with degenerate solutions. If the high
redshift solution is correct, the 24 µm emission could indicate the presence of a heavily obscured AGN (perhaps
Compton thick due to the absence of X ray emission), or perhaps strong star formation activity in a dusty, hyperluminous
starburst. We cannot firmly exclude the possibility for these
galaxies to be dusty starburst galaxies at lower redshifts, for
which the 24 µm flux could be explained with PAH emission
at z ∼ 2 − 3.
MIPS-u sample: 21 of our z ≥ 3.5 candidates are undetected at 24µm. The lack of 24 µm emission is a necessary
but not sufficient condition to indicate that these are quiescent galaxy. The upper limit to the SFR set by MIPS detection limit ( f24 < 20 µJy) corresponds to a star formation
rate < 1500 M⊙ yr−1 , assuming bolometric corrections derived from spectral templates for local ultraluminous infrared
galaxies. For this reason we can use this sub-sample only to
give an upper limit to the total stellar mass of ‘quiescent’
galaxies at z ≥ 3.5.
• 14 galaxies from our z ≥ 3.5 sample are detected at submillimeter and millimeter wavelengths with SCUBA (Pope et al.
2006), MAMBO (Greve et al. 2008) and AzTEC (Perera
et al. 2008). All but two of are also detected at 24µm.
Two objects, GN20 and GN20.2, have spectroscopically confirmed redshifts at z ∼ 4 Daddi et al. (2008). By comparing
their colours with other SMGs galaxies in the S850 /S24 vs
S24 /S8 diagram, we find that ∼ 78% of the sub-mm detected
objects among our candidates have properties that are fully
consistent with those of starburst galaxies at z ≥ 3.5. The
remaining ∼ 22% are more likely at lower redshift.
• We have computed the contribution to the number density
and to the stellar mass density by the total (MIPS-d+MIPSu) sample and by the subsample that is undetected at 24µm
(MIPS-u) for galaxies with stellar masses above a limit of
5 × 1010 M⊙ , chosen to minimize incompleteness at z < 5.
For the redshift range 3.5 < z < 5, we find a comoving number density 2.6 × 10−5 Mpc−3 for average masses
of ∼ 1011 M⊙ . The corresponding stellar mass density is
(2.9 ± 1.5) × 106 M⊙ Mpc−3 . Our results are in good agreement with previous determinations in the literature, and with
the prediction from the Millennium simulations.
For the MIPS-u sample, we determined a number density of
0.97 × 10−5 Mpc−3 and a corresponding stellar mass density of (1.15 ± 0.7) × 106 M⊙ Mpc−3 . We also considered
the extreme case that all of the MIPS-undetected galaxies
were purely quiescent, with ages ∼ 0.5 Gyr. This results in
larger upper limits for their number and stellar mass densities, 1.52 × 10−5 Mpc−3 and (1.46 ± 0.7) × 106 M⊙ Mpc−3
respectively. Our MIPS-u sample of quiescent candidates accounts for ∼ 5 − 6% of the mass density that was in place at
z ∼ 2 and only for ∼ 1% of local stellar mass density (Baldry
et al. 2004). This provides evidence for strong growth in the
17
number of massive galaxies between redshift z = 4 − 5 and
z = 2.
Acknowledgements. We thank Kyoung-Soo Lee for her reductions of the
KPNO-4m FLAMINGOS imaging data of the GOODS-North field,and for information about the details of this data set. CM and ED acknowledge support
from the French ANR grant numbers ANR-07-BLAN-0228 and ANR-08-JCJC0008.
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
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a function of redshift (right panels) in 90% confidence intervals. For each candidate the most probable redshifts (first and second redshift solutions
: z phot and z phot2 ) and the χ2ν values with the respective probability are also labeled. See also Tab. 4.
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wavelength [A%]
106
z_phot =7.9
# 13129
104
105
wavelength [A%]
# 13129
106
25
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
AB mag
AB mag
AB mag
AB mag
AB mag
AB mag
26
18
20
22
24
26
28
30
103
18
20
22
24
26
28
30
103
18
20
22
24
26
28
30
103
18
20
22
24
26
28
30
103
18
20
22
24
26
28
30
103
18
20
22
24
26
28
30
103
z_phot =3.7
# 13857
# 13857
104
105
wavelength [A%]
106
z_phot =3.5
# 14505
# 14505
104
105
wavelength [A%]
106
z_phot =4.6
# 14668
# 14668
104
105
wavelength [A%]
106
z_phot =4.1
# 14722
# 14722
104
105
wavelength [A%]
106
z_phot =4.4
# 14793
# 14793
104
105
wavelength [A%]
106
z_phot =3.9
# 15541
# 15541
104
105
wavelength [A%]
106
AB mag
AB mag
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
18
20
22
24
26
28
30
103
18
20
22
24
26
28
30
103
z_phot =4.2
# 15761
# 15761
104
105
wavelength [A%]
106
z_phot =3.5
# 15771
# 15771
104
105
wavelength [A%]
106
27
28
Fig. 15. Multi-wavelength images of the fifty-three galaxies of the final sample (MIPS-d+MIPS-u).
The cut-outs have a size of 10′′ × 10′′ . North is up, East is at left.
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
29
30
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
31
32
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
33
34
Table 6. Multi-band photometry for the final sample of 44 candidates at z ≥ 3.5. The coordinates reported here are from the Spitzer data-set.
U
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
> 26.7
B
> 26.3
27.774
−−−
26.968
> 26.3
27.262
−−−
> 26.3
> 26.3
> 26.3
27.283
> 26.3
> 26.3
> 26.3
> 26.3
> 26.3
27.265
27.241
> 26.3
27.209
27.438
> 26.3
> 26.3
> 26.3
> 26.3
27.443
27.718
27.099
> 26.3
26.790
> 26.3
> 26.3
> 26.3
> 26.3
> 26.3
26.988
> 26.3
27.318
> 26.3
−−−
26.925
> 26.3
> 26.3
−−−
> 26.3
> 26.3
> 26.3
> 26.3
−−−
> 26.3
> 26.3
> 26.3
> 26.3
V
26.485
27.187
> 26.9
25.998
26.397
25.325
> 26.9
26.722
> 26.9
26.926
25.263
27.155
> 26.9
> 26.9
> 26.9
26.382
> 26.9
27.154
25.754
25.492
> 26.9
> 26.9
> 26.9
26.322
25.418
25.467
25.498
24.698
25.844
25.090
25.750
> 26.9
> 26.9
26.557
26.310
26.705
> 26.9
27.906
26.464
> 26.9
25.271
> 26.9
25.150
> 26.9
25.352
> 26.9
> 26.9
25.010
26.436
> 26.9
25.185
26.783
26.170
i
26.980
26.210
> 26.3
24.907
25.778
24.536
> 26.3
26.810
> 26.3
26.256
24.600
26.106
26.751
> 26.3
> 26.3
25.038
27.550
26.063
24.742
24.543
> 26.3
25.766
> 26.3
26.938
24.665
26.915
24.823
23.850
24.729
24.743
25.303
> 26.3
> 26.3
25.606
25.492
> 26.3
> 26.3
> 26.3
25.416
26.574
24.616
> 26.3
24.638
25.899
24.183
> 26.3
25.723
23.929
25.824
25.710
24.487
25.670
26.087
z
> 25.5
> 25.5
> 25.5
24.317
26.033
24.243
> 25.5
26.070
> 25.5
27.044
24.272
25.940
> 25.5
> 25.5
> 25.5
25.135
> 25.5
25.954
24.932
23.806
> 25.5
25.270
> 25.5
> 25.5
24.685
24.953
24.915
23.618
24.927
24.539
25.209
> 25.5
> 25.5
25.600
26.253
26.493
> 25.5
> 25.5
25.536
> 25.5
24.460
> 25.5
24.404
26.511
23.941
> 25.5
25.684
23.658
25.501
> 25.5
24.155
25.788
25.386
J
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
24.240
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
24.163
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
24.288
> 23.3
25.240
23.792
24.971
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
> 23.3
23.892
> 23.3
> 23.3
> 23.3
23.682
> 23.3
> 23.3
23.133
> 23.3
> 23.3
> 23.3
24.071
> 23.3
H
> 23.0
> 23.0
> 23.0
23.242
> 23.0
23.187
> 23.0
> 23.0
> 23.0
> 23.0
> 23.0
> 23.0
23.329
> 23.0
> 23.0
23.058
> 23.0
> 23.0
24.431
22.865
> 23.0
24.283
> 23.0
> 23.0
> 23.0
24.464
24.377
23.265
22.877
23.162
> 23.0
23.802
> 23.0
> 23.0
> 23.0
> 23.0
> 23.0
> 23.0
> 23.0
> 23.0
24.640
24.368
24.107
> 23.0
24.668
> 23.0
> 23.0
23.167
> 23.0
> 23.0
> 23.0
> 23.0
23.687
Ks
> 23.1
> 23.1
> 23.1
23.127
23.021
22.482
23.222
23.778
22.404
> 23.1
> 23.1
> 23.1
> 23.1
23.202
23.330
23.489
23.846
> 23.1
24.036
22.320
> 23.1
24.065
23.287
> 23.1
23.463
22.810
23.068
23.026
22.841
22.617
23.721
> 23.1
22.973
22.824
23.056
23.298
> 23.1
> 23.1
24.296
24.705
22.082
> 23.1
> 23.1
23.387
22.678
> 23.1
23.497
23.125
23.757
> 23.1
22.753
22.707
22.170
m3.6
23.288
23.379
−−−
22.422
22.954
21.360
21.999
22.958
21.869
23.399
22.859
22.834
22.423
22.073
−−−
22.107
21.479
22.729
23.019
21.429
22.129
22.791
22.875
23.042
23.224
22.519
23.031
22.584
22.198
21.867
22.399
22.924
22.360
22.133
−−−
22.203
22.428
21.918
22.702
22.645
22.107
22.963
22.605
23.048
22.552
22.924
22.936
22.352
21.952
22.790
21.874
21.688
21.418
m4.5
22.895
22.960
22.964
22.465
22.611
21.118
21.587
22.559
21.528
22.997
22.638
22.638
22.064
21.579
22.069
21.831
21.102
22.373
22.691
21.444
21.678
22.688
22.426
22.801
22.992
22.265
22.875
22.616
21.971
21.739
22.033
22.484
21.967
21.873
22.686
21.487
21.882
21.373
22.682
22.189
21.959
22.364
22.465
22.900
22.680
22.388
22.749
22.714
21.566
21.887
21.573
21.299
21.117
m5.8
22.517
22.274
22.857
22.065
22.354
20.898
21.227
22.510
21.167
22.608
−−−
22.146
21.615
21.125
21.817
21.602
20.764
21.895
22.781
21.219
21.269
22.433
21.948
22.468
23.337
21.941
23.306
22.637
22.106
21.504
21.749
22.332
21.735
21.650
22.597
21.199
21.439
20.939
22.455
22.091
21.996
22.029
21.913
22.846
22.814
22.140
22.956
−−−
20.716
21.316
21.010
20.985
20.795
m8.0
21.583
21.836
22.254
21.807
22.218
20.928
21.137
22.183
21.227
22.521
22.480
22.169
21.376
21.066
21.405
21.165
20.856
21.745
22.567
20.221
21.209
22.222
21.903
22.386
23.059
22.033
22.830
22.473
21.684
21.488
21.528
22.270
21.555
21.485
22.463
21.046
21.237
20.617
22.507
21.367
21.895
21.515
21.578
22.596
22.647
21.573
22.385
22.414
20.492
21.071
20.595
20.931
20.743
m24
> 20.6
20.218
20.081
> 20.6
20.161
19.156
18.527
> 20.6
18.029
20.496
> 20.6
> 20.6
19.581
20.037
> 20.6
> 20.6
18.519
20.189
> 20.6
18.084
18.305
19.809
19.177
> 20.6
> 20.6
> 20.6
> 20.6
> 20.6
> 20.6
20.144
19.433
20.255
19.661
> 20.6
> 20.6
18.889
20.034
19.959
> 20.6
> 20.6
19.942
20.186
20.637
> 20.6
> 20.6
20.211
20.610
19.934
18.787
> 20.6
19.359
19.140
19.659
RA
12:36:27.2
12:36:32.7
12:36:15.7
12:36:36.8
12:36:51.0
12:36:13.3
12:35:53.0
12:36:04.7
12:36:03.1
12:37:10.0
12:36:44.4
12:35:59.9
12:37:19.5
12:37:12.5
12:36:53.7
12:36:36.1
12:36:37.5
12:37:18.9
12:36:31.3
12:36:37.1
12:36:08.6
12:37:20.6
12:37:02.5
12:36:30.4
12:37:25.5
12:37:09.5
12:36:41.6
12:36:55.9
12:37:03.8
12:36:45.1
12:37:34.7
12:38:03.6
12:37:33.8
12:36:39.4
12:37:52.7
12:37:39.5
12:36:22.0
12:36:31.9
12:37:39.2
12:38:05.4
12:37:24.0
12:37:28.1
12:37:08.8
12:37:36.3
12:37:13.0
12:36:44.0
12:37:12.3
12:37:11.5
12:36:58.9
12:37:09.6
12:37:11.9
12:36:55.3
12:37:01.5
Dec
62:06:06.1
62:06:21.8
62:06:43.4
62:07:14.0
62:08:29.8
62:09:02.0
62:09:30.2
62:09:25.5
62:09:47.5
62:09:56.4
62:10:03.9
62:10:08.4
62:10:21.4
62:10:36.1
62:11:12.9
62:11:54.7
62:11:57.2
62:12:17.9
62:12:18.3
62:12:31.1
62:12:51.2
62:13:00.2
62:13:02.6
62:13:33.0
62:13:39.8
62:13:50.3
62:13:50.1
62:14:12.5
62:14:51.6
62:14:48.8
62:15:15.8
62:15:30.5
62:15:32.1
62:15:42.8
62:15:49.3
62:15:59.0
62:16:16.2
62:17:14.8
62:17:36.8
62:18:16.4
62:18:33.7
62:19:20.5
62:22:02.2
62:21:25.3
62:21:11.4
62:19:38.8
62:23:03.4
62:21:55.8
62:22:15.3
62:22:02.5
62:22:12.4
62:21:07.9
62:20:25.3
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
ID
# 522
# 588
# 702
# 1098
# 1720
# 2172
# 2560
# 2574
# 2796
# 2894
# 3081
# 3302
# 3393
# 3850
# 4008
# 5073
# 5157
# 5367
# 5460
# 5511
# 6099
# 6245
# 6463
# 6876
# 7042
# 7286
# 7309
# 7785
# 8403
# 8520
# 9007
# 9301
# 9306
# 9537
# 9561
# 9782
# 10176
# 11304
# 11682
# 12327
# 12699
# 13129
# 13857
# 14130
# 14260
# 14505
# 14668
# 14722
# 14793
# 15268
# 15541
# 15761
# 15771
Mancini C. et al.: Searching for massive galaxies at z ≥ 3.5 in GOODS-North
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