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Register a new userEvolution of the Color-Magnitude Relation in Galaxy Clusters at z ~1 from the ACS Intermediate Redshift Cluster Survey
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We apply detailed observations of the Color-Magnitude Relation (CMR) with the ACS/HST to study galaxy evolution in eight clusters at z~1. The early-type red sequence is well defined and elliptical and lenticular galaxies lie on similar CMRs. We analyze CMR parameters as a function of redshift, galaxy properties and cluster mass. For bright galaxies (M_B < -21mag), the CMR scatter of the elliptical population in cluster cores is smaller than that of the S0 population, although the two become similar at faint magnitudes. While the bright S0 population consistently shows larger scatter than the ellipticals, the scatter of the latter increases in the peripheral cluster regions. If we interpret these results as due to age differences, bright elliptical galaxies in cluster cores are on average older than S0 galaxies and peripheral elliptical galaxies (by about 0.5Gyr). CMR zero point, slope, and scatter in the (U-B)_z=0 rest-frame show no significant evolution out to redshift z~1.3 nor significant dependence on cluster mass. Two of our clusters display CMR zero points that are redder (by ~2sigma) than the average (U-B)_z=0 of our sample. We also analyze the fraction of morphological early-type and late-type galaxies on the red sequence. We find that, while in the majority of the clusters most (80% to 90%) of the CMR population is composed of early-type galaxies, in the highest redshift, low mass cluster of our sample, the CMR late-type/early-type fractions are similar (~50%), with most of the late-type population composed of galaxies classified as S0/a. This trend is not correlated with the clusters X-ray luminosity, nor with its velocity dispersion, and could be a real evolution with redshift.
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We investigate the color-magnitude relation for globular clusters (GCs) -- the so-called blue tilt -- detected in the ACS Fornax Cluster Survey and using the combined sample of GCs from the ACS Fornax and Virgo Cluster Surveys. We find a tilt of gamma_z=d(g-z)/dz=-0.0257 +- 0.0050 for the full GC sample of the Fornax Cluster Survey (~5800 GCs). This is slightly shallower than the value gamma_z=-0.0459 +- 0.0048 found for the Virgo Cluster Survey GC sample (~11100 GCs). The slope for the merged Fornax and Virgo datasets (~16900 GCs) is gamma_z=-0.0293 +- 0.0085, corresponding to a mass-metallicity relation of Z ~ M^0.43. We find that the blue tilt sets in at GC masses in excess of M ~ 2*10^5 M_sun. The tilt is stronger for GCs belonging to high-mass galaxies (M_* > 5 * 10^10 M_sun) than for those in low-mass galaxies (M_* < 5 * 10^10 M_sun). It is also more pronounced for GCs with smaller galactocentric distances. Our findings suggest a range of mass-metallicity relations Z_GC ~ M_GC^(0.3-0.7) which vary as a function of host galaxy mass/luminosity. We compare our observations to a recent model of star cluster self-enrichment with generally favorable results. We suggest that, within the context of this model, the proto-cluster clouds out of which the GCs formed may have had density profiles slightly steeper than isothermal and/or star formation efficiencies somewhat below 0.3. We caution, however, that the significantly different appearance of the CMDs defined by the GC systems associated with galaxies of similar mass and morphological type pose a challenge to any single mechanism that seeks to explain the blue tilt. We therefore suggest that the merger/accretion histories of individual galaxies have played a non-negligible role determining the distribution of GCs in the CMDs of individual GC systems.
We present a study of the color evolution of elliptical and S0 galaxies in six clusters of galaxies inside the redshift range 0.78 < z < 1.27. For each cluster, we used imaging from the Hubble Space Telescope to determine morphological types by both an automated technique and from visual inspection. We performed simulations to determine the accuracy of the automated classifications and found a success rate of ~75% at m(L*) or brighter magnitudes for most of our HST imaging data with the fraction of late--type galaxies identified as early--type galaxies to be ~10% at m(L*) to ~20% at m(L*)+2. From ground based optical and near-infrared imaging, we measured the zero-point and scatter in the color--magnitude relation of the early-type populations, which when combined with Stanford et al. (1998), yields a sample of cluster early--type galaxies that span a lookback time of 9 gigayears from the present. We see the colors of the early--type cluster members become bluer with increasing redshift. We fit a set of models to the change in the color as a function of redshift with the best fitting values ranging from a formation redshift of 3^+2_-1 to 5_-3. The large scatter in resulting formation epochs, which depends on the details of the models used, implies that we can conclude that the oldest stars in the elliptical galaxies appear to have formed at redshifts of z>3. We find possible evolution in the scatter of the colors, with some high redshift clusters showing scatter as small as the Coma cluster but others showing much larger scatter. Those clusters with a small scatter imply either a formation redshift of at least z ~ 3 or a smaller spread in the range of formation redshifts at lower redshifts, assuming a Gaussian distribution of star-formation around the mean epoch.
Color-magnitude relations have been derived in the clusters RX J0849+4452 and RX J0848+4453 at z~1.26. The color-magnitude relation was determined from Advanced Camera for Surveys imaging in the WFC F775W (i_775) and F850LP (z_850) filters combined with ground-based spectroscopy. Early-type cluster candidates have been identified according to the Postman et al. morphological classification. In both clusters the bright red early-type population defines a tight color-magnitude relation very similar in color, although the two clusters present different X-ray luminosities and shapes, with RX J0849+4452 being three times more X-ray luminous and more compact, and having a temperature two times higher. The elliptical galaxy color-magnitude relations (CMR) in RX J0849+4452 and RX J0848+4453 show an intrinsic (i_775-z_850) color scatter of 0.026 +/- 0.012 mag and 0.024 +/- 0.015 mag, respectively, within 2 arcminutes (~1Mpc at z=1.26) from the cluster X-ray emission centers. Simple modeling of the scatters using stellar population models from Bruzual and Charlot, gives a mean luminosity-weighted age t > 2.5 Gyr (z_f > 2.75). S0 galaxies follow the elliptical CMR; they show larger scatters about the CMR. The intrinsic scatter decreases and the CMR slopes are steeper at smaller radii, within both clusters. We conclude that old stellar populations in cluster elliptical galaxies are already in place at z=1.26, both in the more evolved cluster RX J0849+4452, and in its less evolved companion RX J0848+4453. Even at a lookback time of 9 Gyr, in the early merging and buildup of massive clusters, the bulk of the stellar content of the bright elliptical galaxy population was in place - apparently formed some 2.5~Gyr earlier at z~3
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: approximately ~62% of the galaxies identified as likely cluster members are ellipticals or S0s; and ~38% are spirals or irregulars. 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. 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.
We investigate the origin of the colour-magnitude relation (CMR) observed in cluster galaxies by using a combination of a cosmological N-body simulation of a cluster of galaxies and a semi-analytic model of galaxy formation. The departure of galaxies in the bright end of the CMR with respect to the trend denoted by less luminous galaxies could be explained by the influence of minor mergers
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