This paper presents both the result of a search for fossil systems within the XMM Cluster Survey ... more This paper presents both the result of a search for fossil systems within the XMM Cluster Survey and the Sloan Digital Sky Survey and the results of a study of the stellar mass assembly and stellar populations of their fossil galaxies. In total, 17 groups and clusters are identified at z < 0.25 with large magnitude gaps between the first and fourth brightest galaxies. All the information necessary to classify these systems as fossils is provided. For both groups and clusters, the total and fractional luminosity of the brightest galaxy are positively correlated with the magnitude gap. The brightest galaxies in fossil systems (called fossil galaxies) have stellar populations and star-formation histories which are similar to normal brightest cluster galaxies. However, at fixed group/cluster mass, the stellar masses of the fossil galaxies are larger compared to normal brightest cluster galaxies, a fact that holds true over a wide range of group/cluster masses. Moreover, the fossil galaxies are found to contain a significant fraction of the the total optical luminosity of the group/cluster within 0.5R 200 , as much as 85%, compared to the non-fossils, which can have as little as 10%. Our results suggest that fossil systems formed early and in the highest density regions of the Universe and that fossil galaxies represent the end products of galaxy mergers in groups and clusters.
We present deep J and K s band photometry of 20 high redshift galaxy clusters between z = 0.8−1.5... more We present deep J and K s band photometry of 20 high redshift galaxy clusters between z = 0.8−1.5, 19 of which are observed with the MOIRCS instrument on the Subaru Telescope. By using nearinfrared light as a proxy for stellar mass we find the surprising result that the average stellar mass of Brightest Cluster Galaxies (BCGs) has remained constant at ∼ 9 × 10 11 M ⊙ since z ∼ 1.5. We investigate the effect on this result of differing star formation histories generated by three well known and independent stellar population codes and find it to be robust for reasonable, physically motivated choices of age and metallicity. By performing Monte Carlo simulations we find that the result is unaffected by any correlation between BCG mass and cluster mass in either the observed or model clusters. The large stellar masses imply that the assemblage of these galaxies took place at the same time as the initial burst of star formation. This result leads us to conclude that dry merging has had little effect on the average stellar mass of BCGs over the last 9 − 10 Gyr in stark contrast to the predictions of semi-analytic models, based on the hierarchical merging of dark matter haloes, which predict a more protracted mass build up over a Hubble time. We discuss however that there is potential for reconciliation between observation and theory if there is a significant growth of material in the intracluster light over the same period.
We present a study of the morphological fractions and color-magnitude relation in the most distan... more We present a study of the morphological fractions and color-magnitude relation in the most distant X-ray selected galaxy cluster currently known, XMMXCS J2215.9 − 1738 at z = 1.46, using a combination of optical imaging data obtained with the Hubble Space Telescope Advanced Camera for Surveys, and infrared data from the Multi-Object Infrared Camera and Spectrograph, mounted on the 8.2m Subaru telescope. We find that the morphological mix of the cluster galaxy population is similar to clusters at z ∼ 1. Within the central 0.5 Mpc, approximately ∼ 62% of the galaxies identified as likely cluster members are ellipticals or S0s; and ∼ 38% are spirals or irregulars. Therefore early type galaxies were already entrenched as the dominant galaxy population in at least some clusters approximately ∼ 4.5 Gyr after the Big Bang. We measure the color-magnitude relations for the early type galaxies, finding that the slope in the z 850 − J relation is consistent with that measured in the Coma cluster, some ∼ 9 Gyr earlier, although the uncertainty is large. In contrast, the measured intrinsic scatter about the color-magnitude relation is more than three times the value measured in Coma, after conversion to rest frame U − V . From comparison with stellar population synthesis models, the intrinsic scatter measurements imply mean luminosity weighted ages for the early type galaxies in J2215.9 − 1738 of ≈ 3 Gyr, corresponding to the major epoch of star formation coming to an end at z f ≈ 3 − 5. We find that the cluster exhibits evidence of the 'downsizing' phenomenon: the fraction of faint cluster members on the red sequence expressed using the Dwarf-to-Giant Ratio (DGR) is 0.32 ± 0.18 within a radius of 0.5R 200 . This is consistent with extrapolation of the redshift evolution of the DGR seen in cluster samples at z < 1. In contrast to observations of some other z > 1 clusters, we find a lack of very bright galaxies within the cluster. J * -1.0 J * +0.0 J * +1.0 J * +2.0 J J * +0.0 J * +1.0 J * +2.0 J * +3.0 z 850 -J (AB)
Background: We present a systematic search for regulatory elements in a 3.5 Mb region on human ch... more Background: We present a systematic search for regulatory elements in a 3.5 Mb region on human chromosome 20q13.12, a region associated with a number of medical conditions such as type II diabetes and obesity.
Monthly Notices of the Royal Astronomical Society, 2009
We forecast the constraints on the values of σ 8 , Ω m , and cluster scaling relation parameters ... more We forecast the constraints on the values of σ 8 , Ω m , and cluster scaling relation parameters which we expect to obtain from the XMM Cluster Survey (XCS). We assume a flat ΛCDM Universe and perform a Monte Carlo Markov Chain analysis of the evolution of the number density of galaxy clusters that takes into account a detailed simulated selection function. Comparing our current observed number of clusters shows good agreement with predictions. We determine the expected degradation of the constraints as a result of self-calibrating the luminosity-temperature relation (with scatter), including temperature measurement errors, and relying on photometric methods for the estimation of galaxy cluster redshifts. We examine the effects of systematic errors in scaling relation and measurement error assumptions. Using only (T, z) self-calibration, we expect to measure Ω m to ±0.03 (and Ω Λ to the same accuracy assuming flatness), and σ 8 to ±0.05, also constraining the normalization and slope of the luminosity-temperature relation to ±6 and ±13 per cent (at 1σ) respectively in the process. Self-calibration fails to jointly constrain the scatter and redshift evolution of the luminositytemperature relation significantly. Additional archival and/or follow-up data will improve on this. We do not expect measurement errors or imperfect knowledge of their distribution to degrade constraints significantly. Scaling-relation systematics can easily lead to cosmological constraints 2σ or more away from the fiducial model. Our treatment is the first exact treatment to this level of detail, and introduces a new 'smoothed ML' estimate of expected constraints.
Monthly Notices of the Royal Astronomical Society, 2012
We measure the evolution of the X-ray luminosity-temperature (L X −T ) relation since z ∼ 1.5 usi... more We measure the evolution of the X-ray luminosity-temperature (L X −T ) relation since z ∼ 1.5 using a sample of 211 serendipitously detected galaxy clusters with spectroscopic redshifts drawn from the XMM Cluster Survey first data release (XCS-DR1). This is the first study spanning this redshift range using a single, large, homogeneous cluster sample. Using an orthogonal regression technique, we find no evidence for evolution in the slope or intrinsic scatter of the relation since z ∼ 1.5, finding both to be consistent with previous measurements at z ∼ 0.1. However, the normalisation is seen to evolve negatively with respect to the selfsimilar expectation: we find E −1 (z) L X = 10 44.67±0.09 (T /5) 3.04±0.16 (1 + z) −1.5±0.5 , which is within 2σ of the zero evolution case. We see milder, but still negative, evolution with respect to self-similar when using a bisector regression technique. We compare our results to numerical simulations, where we fit simulated cluster samples using the same methods used on the XCS data. Our data favour models in which the majority of the excess entropy required to explain the slope of the L X − T relation is injected at high redshift. Simulations in which AGN feedback is implemented using prescriptions from current semi-analytic galaxy formation models predict positive evolution of the normalisation, and differ from our data at more than 5σ. This suggests that more efficient feedback at high redshift may be needed in these models.
Monthly Notices of the Royal Astronomical Society, 2012
Using a sample of 123 X-ray clusters and groups drawn from the XMM Cluster Survey first data rele... more Using a sample of 123 X-ray clusters and groups drawn from the XMM Cluster Survey first data release, we investigate the interplay between the brightest cluster galaxy (BCG), its black hole and the intracluster/group medium (ICM). It appears that for groups and clusters with a BCG likely to host significant active galactic nuclei (AGN) feedback, gas cooling dominates in those with T X > 2 keV while AGN feedback dominates below. This may be understood through the subunity exponent found in the scaling relation we derive between the BCG mass and cluster mass over the halo mass range 10 13 < M 500 < 10 15 M and the lack of correlation between radio luminosity and cluster mass, such that BCG AGN in groups can have relatively more energetic influence on the ICM. The L X -T X relation for systems with the most massive BCGs, or those with BCGs co-located with the peak of the ICM emission, is steeper than that for those with the least massive and most offset, which instead follows self-similarity. This is evidence that a combination of central gas cooling and powerful, well fuelled AGN causes the departure of the ICM from pure gravitational heating, with the steepened relation crossing self-similarity at T X = 2 keV. Importantly, regardless of their black hole mass, BCGs are more likely to host radio-loud AGN if they are in a massive cluster (T X 2 keV) and again co-located with an effective fuel supply of dense, cooling gas. This demonstrates that the most massive black holes appear to know more about their host cluster than they do about their host galaxy. The results lead us to propose a physically motivated, empirical definition of 'cluster' and 'group', delineated at 2 keV.
ABSTRACT The XMM Cluster Survey (XCS) is a serendipitous search for galaxy clusters using all pub... more ABSTRACT The XMM Cluster Survey (XCS) is a serendipitous search for galaxy clusters using all publicly available data in the XMM-Newton Science Archive. Our recent first data release (XCS-DR1) contains 503 optically confirmed groups and clusters, among which 256 new to the literature and 357 whose X-ray emission was detected for the first time. We discuss their properties and provide an update on the work being done. As examples of the applications of XCS-DR1, we mention the 17 fossil groups/clusters identified with the help of the Sloan Digital Sky Survey. Their brightest galaxies have stellar populations and star-formation histories which are similar to normal brightest cluster galaxies, but their stellar masses are significantly larger and correspond to a much bigger fraction of the total group/cluster optical luminosity. We also highlight the 15 clusters expected to be also detected by the Planck satellite, and characterize the expected overlap between the final XCS and Planck cluster catalogues.
This paper presents both the result of a search for fossil systems within the XMM Cluster Survey ... more This paper presents both the result of a search for fossil systems within the XMM Cluster Survey and the Sloan Digital Sky Survey and the results of a study of the stellar mass assembly and stellar populations of their fossil galaxies. In total, 17 groups and clusters are identified at z < 0.25 with large magnitude gaps between the first and fourth brightest galaxies. All the information necessary to classify these systems as fossils is provided. For both groups and clusters, the total and fractional luminosity of the brightest galaxy are positively correlated with the magnitude gap. The brightest galaxies in fossil systems (called fossil galaxies) have stellar populations and star-formation histories which are similar to normal brightest cluster galaxies. However, at fixed group/cluster mass, the stellar masses of the fossil galaxies are larger compared to normal brightest cluster galaxies, a fact that holds true over a wide range of group/cluster masses. Moreover, the fossil galaxies are found to contain a significant fraction of the the total optical luminosity of the group/cluster within 0.5R 200 , as much as 85%, compared to the non-fossils, which can have as little as 10%. Our results suggest that fossil systems formed early and in the highest density regions of the Universe and that fossil galaxies represent the end products of galaxy mergers in groups and clusters.
We present deep J and K s band photometry of 20 high redshift galaxy clusters between z = 0.8−1.5... more We present deep J and K s band photometry of 20 high redshift galaxy clusters between z = 0.8−1.5, 19 of which are observed with the MOIRCS instrument on the Subaru Telescope. By using nearinfrared light as a proxy for stellar mass we find the surprising result that the average stellar mass of Brightest Cluster Galaxies (BCGs) has remained constant at ∼ 9 × 10 11 M ⊙ since z ∼ 1.5. We investigate the effect on this result of differing star formation histories generated by three well known and independent stellar population codes and find it to be robust for reasonable, physically motivated choices of age and metallicity. By performing Monte Carlo simulations we find that the result is unaffected by any correlation between BCG mass and cluster mass in either the observed or model clusters. The large stellar masses imply that the assemblage of these galaxies took place at the same time as the initial burst of star formation. This result leads us to conclude that dry merging has had little effect on the average stellar mass of BCGs over the last 9 − 10 Gyr in stark contrast to the predictions of semi-analytic models, based on the hierarchical merging of dark matter haloes, which predict a more protracted mass build up over a Hubble time. We discuss however that there is potential for reconciliation between observation and theory if there is a significant growth of material in the intracluster light over the same period.
We present a study of the morphological fractions and color-magnitude relation in the most distan... more We present a study of the morphological fractions and color-magnitude relation in the most distant X-ray selected galaxy cluster currently known, XMMXCS J2215.9 − 1738 at z = 1.46, using a combination of optical imaging data obtained with the Hubble Space Telescope Advanced Camera for Surveys, and infrared data from the Multi-Object Infrared Camera and Spectrograph, mounted on the 8.2m Subaru telescope. We find that the morphological mix of the cluster galaxy population is similar to clusters at z ∼ 1. Within the central 0.5 Mpc, approximately ∼ 62% of the galaxies identified as likely cluster members are ellipticals or S0s; and ∼ 38% are spirals or irregulars. Therefore early type galaxies were already entrenched as the dominant galaxy population in at least some clusters approximately ∼ 4.5 Gyr after the Big Bang. We measure the color-magnitude relations for the early type galaxies, finding that the slope in the z 850 − J relation is consistent with that measured in the Coma cluster, some ∼ 9 Gyr earlier, although the uncertainty is large. In contrast, the measured intrinsic scatter about the color-magnitude relation is more than three times the value measured in Coma, after conversion to rest frame U − V . From comparison with stellar population synthesis models, the intrinsic scatter measurements imply mean luminosity weighted ages for the early type galaxies in J2215.9 − 1738 of ≈ 3 Gyr, corresponding to the major epoch of star formation coming to an end at z f ≈ 3 − 5. We find that the cluster exhibits evidence of the 'downsizing' phenomenon: the fraction of faint cluster members on the red sequence expressed using the Dwarf-to-Giant Ratio (DGR) is 0.32 ± 0.18 within a radius of 0.5R 200 . This is consistent with extrapolation of the redshift evolution of the DGR seen in cluster samples at z < 1. In contrast to observations of some other z > 1 clusters, we find a lack of very bright galaxies within the cluster. J * -1.0 J * +0.0 J * +1.0 J * +2.0 J J * +0.0 J * +1.0 J * +2.0 J * +3.0 z 850 -J (AB)
Background: We present a systematic search for regulatory elements in a 3.5 Mb region on human ch... more Background: We present a systematic search for regulatory elements in a 3.5 Mb region on human chromosome 20q13.12, a region associated with a number of medical conditions such as type II diabetes and obesity.
Monthly Notices of the Royal Astronomical Society, 2009
We forecast the constraints on the values of σ 8 , Ω m , and cluster scaling relation parameters ... more We forecast the constraints on the values of σ 8 , Ω m , and cluster scaling relation parameters which we expect to obtain from the XMM Cluster Survey (XCS). We assume a flat ΛCDM Universe and perform a Monte Carlo Markov Chain analysis of the evolution of the number density of galaxy clusters that takes into account a detailed simulated selection function. Comparing our current observed number of clusters shows good agreement with predictions. We determine the expected degradation of the constraints as a result of self-calibrating the luminosity-temperature relation (with scatter), including temperature measurement errors, and relying on photometric methods for the estimation of galaxy cluster redshifts. We examine the effects of systematic errors in scaling relation and measurement error assumptions. Using only (T, z) self-calibration, we expect to measure Ω m to ±0.03 (and Ω Λ to the same accuracy assuming flatness), and σ 8 to ±0.05, also constraining the normalization and slope of the luminosity-temperature relation to ±6 and ±13 per cent (at 1σ) respectively in the process. Self-calibration fails to jointly constrain the scatter and redshift evolution of the luminositytemperature relation significantly. Additional archival and/or follow-up data will improve on this. We do not expect measurement errors or imperfect knowledge of their distribution to degrade constraints significantly. Scaling-relation systematics can easily lead to cosmological constraints 2σ or more away from the fiducial model. Our treatment is the first exact treatment to this level of detail, and introduces a new 'smoothed ML' estimate of expected constraints.
Monthly Notices of the Royal Astronomical Society, 2012
We measure the evolution of the X-ray luminosity-temperature (L X −T ) relation since z ∼ 1.5 usi... more We measure the evolution of the X-ray luminosity-temperature (L X −T ) relation since z ∼ 1.5 using a sample of 211 serendipitously detected galaxy clusters with spectroscopic redshifts drawn from the XMM Cluster Survey first data release (XCS-DR1). This is the first study spanning this redshift range using a single, large, homogeneous cluster sample. Using an orthogonal regression technique, we find no evidence for evolution in the slope or intrinsic scatter of the relation since z ∼ 1.5, finding both to be consistent with previous measurements at z ∼ 0.1. However, the normalisation is seen to evolve negatively with respect to the selfsimilar expectation: we find E −1 (z) L X = 10 44.67±0.09 (T /5) 3.04±0.16 (1 + z) −1.5±0.5 , which is within 2σ of the zero evolution case. We see milder, but still negative, evolution with respect to self-similar when using a bisector regression technique. We compare our results to numerical simulations, where we fit simulated cluster samples using the same methods used on the XCS data. Our data favour models in which the majority of the excess entropy required to explain the slope of the L X − T relation is injected at high redshift. Simulations in which AGN feedback is implemented using prescriptions from current semi-analytic galaxy formation models predict positive evolution of the normalisation, and differ from our data at more than 5σ. This suggests that more efficient feedback at high redshift may be needed in these models.
Monthly Notices of the Royal Astronomical Society, 2012
Using a sample of 123 X-ray clusters and groups drawn from the XMM Cluster Survey first data rele... more Using a sample of 123 X-ray clusters and groups drawn from the XMM Cluster Survey first data release, we investigate the interplay between the brightest cluster galaxy (BCG), its black hole and the intracluster/group medium (ICM). It appears that for groups and clusters with a BCG likely to host significant active galactic nuclei (AGN) feedback, gas cooling dominates in those with T X > 2 keV while AGN feedback dominates below. This may be understood through the subunity exponent found in the scaling relation we derive between the BCG mass and cluster mass over the halo mass range 10 13 < M 500 < 10 15 M and the lack of correlation between radio luminosity and cluster mass, such that BCG AGN in groups can have relatively more energetic influence on the ICM. The L X -T X relation for systems with the most massive BCGs, or those with BCGs co-located with the peak of the ICM emission, is steeper than that for those with the least massive and most offset, which instead follows self-similarity. This is evidence that a combination of central gas cooling and powerful, well fuelled AGN causes the departure of the ICM from pure gravitational heating, with the steepened relation crossing self-similarity at T X = 2 keV. Importantly, regardless of their black hole mass, BCGs are more likely to host radio-loud AGN if they are in a massive cluster (T X 2 keV) and again co-located with an effective fuel supply of dense, cooling gas. This demonstrates that the most massive black holes appear to know more about their host cluster than they do about their host galaxy. The results lead us to propose a physically motivated, empirical definition of 'cluster' and 'group', delineated at 2 keV.
ABSTRACT The XMM Cluster Survey (XCS) is a serendipitous search for galaxy clusters using all pub... more ABSTRACT The XMM Cluster Survey (XCS) is a serendipitous search for galaxy clusters using all publicly available data in the XMM-Newton Science Archive. Our recent first data release (XCS-DR1) contains 503 optically confirmed groups and clusters, among which 256 new to the literature and 357 whose X-ray emission was detected for the first time. We discuss their properties and provide an update on the work being done. As examples of the applications of XCS-DR1, we mention the 17 fossil groups/clusters identified with the help of the Sloan Digital Sky Survey. Their brightest galaxies have stellar populations and star-formation histories which are similar to normal brightest cluster galaxies, but their stellar masses are significantly larger and correspond to a much bigger fraction of the total group/cluster optical luminosity. We also highlight the 15 clusters expected to be also detected by the Planck satellite, and characterize the expected overlap between the final XCS and Planck cluster catalogues.
Uploads
Papers by M. Sahlén