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Register a new userThe red-sequence of 72 WINGS local galaxy clusters
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Bianca Poggianti dr
Publication date
2011
fields
Physics
and research's language is
English
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We study the color-magnitude red sequence and blue fraction of 72 X-ray selected galaxy clusters at z=0.04-0.07 from the WINGS survey, searching for correlations between the characteristics of the red sequence and the environment. We consider the slope and scatter of the red sequence, the number ratio of red luminous-to-faint galaxies, the blue fraction and the fractions of ellipticals, S0s and spirals that compose the red sequence. None of these quantities correlate with the cluster velocity dispersion, X-ray luminosity, number of cluster substructures, BCG prevalence over next brightest galaxies and spatial concentration of ellipticals. Instead, the properties of the red sequence depend strongly on local galaxy density. Higher density regions have a lower RS scatter, a higher luminous-to-faint ratio, a lower blue fraction, and a lower spiral fraction on the RS. Our results highlight the prominent effect of the local density in setting the epoch when galaxies become passive and join the red sequence, as opposed to the mass of the galaxy host structure.
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We investigate the origin of the color-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 defined by less luminous galaxies could be explained by the influence of minor mergers.
We investigate the evolution of the optical and near-infrared colour-magnitude relation in an homogeneous sample of massive clusters from z = 1 to the present epoch. By comparing deep Hubble Space Telescope ACS imaging of X-ray selected MACS survey clusters at z = 0.5 to the similarly selected LARCS sample at z = 0.1 we find that the rest-frame d(U -V)/dV slope of the colour-magnitude relation evolves with redshift which we attribute to the build up of the red sequence over time. This rest frame slope evolution is not adequately reproduced by that predicted from semi-analytic models based on the Millennium Simulation despite a prescription for the build up of the red sequence by in-falling galaxies, strangulation. We observe no strong correlation between this slope and the cluster environment at a given redshift demonstrating that the observed evolution is not due to a secondary correlation. Also presented are near-infrared UKIRT WFCAM observations of the LARCS clusters which confirm and improve on the the result from Stott et al. (2007) finding that there has been a two-fold increase in faint MV > -20 galaxies on the red sequence since z = 0.5 to a significance of 5sigma.
Massive quiescent galaxies at z>1 have been found to have small physical sizes, hence to be superdense. Several mechanisms, including minor mergers, have been proposed for increasing galaxy sizes from high- to low-z. We search for superdense massive galaxies in the WIde-field Nearby Galaxy-cluster Survey (WINGS) of X-ray selected galaxy clusters at 0.04<z<0.07. We discover a significant population of superdense massive galaxies with masses and sizes comparable to those observed at high redshift. They approximately represent 22% of all cluster galaxies more massive than 3x10^10Msol, are mostly S0 galaxies, have a median effective radius <Re> =1.61+/-0.29kpc, a median Sersic index <n> = 3.0+/-0.6, and very old stellar populations with a median mass-weighted age of 12.1+/-1.3Gyr. We calculate a number density of 2.9x10^-2Mpc^-3 for superdense galaxies in local clusters, and a hard lower limit of 1.3x10^-5Mpc^-3 in the whole comoving volume between z = 0.04 and z = 0.07. We find a relation between mass, effective radius and luminosity-weighted age in our cluster galaxies, which can mimic the claimed evolution of the radius with redshift, if not properly taken into account. We compare our data with spectroscopic high-z surveys and find that -when stellar masses are considered- there is consistency with the local WINGS galaxy sizes out to z~2, while a discrepancy of a factor of 3 exists with the only spectroscopic z>2 study. In contrast, there is strong evidence for a large evolution in radius for the most massive galaxies with M*>4x10^11Msol compared to similarly massive galaxies in WINGS, i.e. the BCGs.
Using samples drawn from the Sloan Digital Sky Survey, we study the relationship between local galaxy density and the properties of galaxies on the red sequence. After removing the mean dependence of average overdensity (or environment) on color and luminosity, we find that there remains a strong residual trend between luminosity-weighted mean stellar age and environment, such that galaxies with older stellar populations favor regions of higher overdensity relative to galaxies of like color and luminosity (and hence of like stellar mass). Even when excluding galaxies with recent star-formation activity (i.e., younger mean stellar ages) from the sample, we still find a highly significant correlation between stellar age and environment at fixed stellar mass. This residual age-density relation provides direct evidence for an assembly bias on the red sequence such that galaxies in higher-density regions formed earlier than galaxies of similar mass in lower-density environments. We discuss these results in the context of the age-metallicity degeneracy and in comparison to previous studies at low and intermediate redshift. Finally, we consider the potential role of assembly bias in explaining recent results regarding the evolution of post-starburst (or post-quenching) galaxies and the environmental dependence of the type Ia supernova rate.
We present a study of local post-starburst galaxies (PSGs) using the photometric and spectroscopic observations from the Sloan Digital Sky Survey (SDSS) and the results from the Galaxy Zoo project. We find that the majority of our local PSG population have neither early- nor late- type morphologies but occupy a well-defined space within the colour-stellar mass diagram, most notably, the low-mass end of the green valley below the transition mass thought to be the mass division between low-mass star-forming galaxies and high-mass passively-evolving bulge-dominated galaxies. Our analysis suggests that it is likely that a local PSG will quickly transform into red, low-mass early-type galaxies as the stellar morphologies of the green PSGs largely resemble that of the early-type galaxies within the same mass range. We propose that the current population of PSGs represents a population of galaxies which is rapidly transitioning between the star-forming and the passively-evolving phases. Subsequently, these PSGs will contribute towards the build-up of the low-mass end of the red sequence once the current population of young stars fade and stars are no longer being formed. These results are consistent with the idea of downsizing where the build-up of smaller galaxies occurs at later epochs.
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