We investigate how the O ii properties and the morphologies of galaxies in clusters and groups at image depend on projected local galaxy density, and compare with the field at similar redshifts and ...clusters at low z. In both nearby and distant clusters, higher density regions contain proportionally fewer star-forming galaxies, and the average O ii equivalent width of star-forming galaxies is independent of local density. However, in distant clusters the average current star formation rate (SFR) in star- forming galaxies seems to peak at densities similar to 15-40 galaxies Mpc super(-2). At odds with low-z results, at high z the relation between star-forming fraction and local density varies from high- to low-mass clusters. Overall, our results suggest that at high z the current star formation (SF) activity in star-forming galaxies does not depend strongly on global or local environment, though the possible SFR peak seems at odds with this conclusion. We find that the cluster SFR normalized by cluster mass anticorrelates with mass and correlates with the star-forming fraction. These trends can be understood given (1) that the average star-forming galaxy forms about 1 M sub(image) yr super(-1) (uncorrected for dust) in all clusters; (2) that the total number of galaxies scales with cluster mass; and (3) the dependence of star-forming fraction on cluster mass. We present the morphology-density (MD) relation for our image clusters, and uncover that the decline of the spiral fraction with density is entirely driven by galaxies of type Sc or later. For galaxies of a given Hubble type, we see no evidence that SF properties depend on local environment. In contrast with recent findings at low z, in our distant clusters the SF-density relation and the MD relation are equivalent, suggesting that neither of the two is more fundamental than the other.
We study how the proportion of star-forming galaxies evolves between z = 0.8 and 0 as a function of galaxy environment, using the OII line in emission as a signature of ongoing star formation. Our ...high-z data set comprises 16 clusters, 10 groups, and another 250 galaxies in poorer groups and the field at z = 0.4-0.8 from the ESO Distant Cluster Survey, plus another 9 massive clusters at similar redshifts. As a local comparison, we use galaxy systems selected from the Sloan Digital Sky Survey (SDSS) at 0.04 < z < 0.08. At high z most systems follow a broad anticorrelation between the fraction of star-forming galaxies and the system velocity dispersion. At face value, this suggests that at z = 0.4-0.8 the mass of the system largely determines the proportion of galaxies with ongoing star formation. At these redshifts the strength of star formation (as measured by the O II equivalent width) in star-forming galaxies is also found to vary systematically with environment. SDSS clusters have much lower fractions of star-forming galaxies than clusters at z = 0.4-0.8 and, in contrast with the distant clusters, show a plateau for velocity dispersions .550 km s super(-1), where the fraction of galaxies with O II emission does not vary systematically with velocity dispersion. We quantify the evolution of the proportion of star-forming galaxies as a function of the system velocity dispersion and find that it is strongest in intermediate-mass systems (s 6 500-600 km s super(-1) at z = 0). To understand the origin of the observed trends, we use the Press-Schechter formalism and the Millennium Simulation and show that galaxy star formation histories may be closely related to the growth history of clusters and groups. If the scenario we propose is roughly correct, the link between galaxy properties and environment is extremely simple to predict purely from a knowledge of the growth of dark matter structures.
We present new deep spectroscopic observations of 0.05–0.5 L* galaxies in one cluster (cl1232.5-1250) drawn from the ESO (European Southern Observatory) Distant Cluster Survey (EDisCS) sample, at z= ...0.54. The new data extend the spectroscopy already available for this cluster by about 1 mag. The cluster has a large fraction of passive galaxies and exhibits a well-defined and relatively tight colour–magnitude relation. Among spectroscopic members, only six galaxies are classified as ‘post-starburst’ (k+a). For another EDisCS cluster at similar redshift and with as deep spectroscopy, no member is found to have a k+a spectrum. The low measured numbers of post-starburst systems appear to be inadequate to explain the observed increase of faint red galaxies at lower redshift, even when accounting for the infall of new galaxies on to the cluster. Post-starburst galaxies represent a possible channel to move galaxies from the blue star-forming cloud to the red sequence, but the available data suggest this is not the dominant channel in galaxy clusters. If the galaxies at the faint end of the red sequence in nearby clusters originate from the blue galaxies observed in distant galaxy clusters, the transformation must have occurred primarily through physical processes that do not lead to a post-starburst phase. In addition, our data exclude a large population of k+a galaxies at faint magnitudes, as found in the Coma cluster.
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We present results from a low-resolution spectroscopic survey for 21 galaxy clusters at 0.4 < z < 0.8 selected from the ESO Distant Cluster Survey. We measured spectra using the low-dispersion prism ...in IMACS on the Magellan Baade telescope and calculate redshifts with an accuracy of z = 0.007. We find 1763 galaxies that are brighter than R = 22.9 in the large-scale cluster environs. We identify the galaxies expected to be accreted by the clusters as they evolve to z = 0 using spherical infall models and find that ∼30%-70% of the z = 0 cluster population lies outside the virial radius at z ∼ 0.6. For analogous clusters at z = 0, we calculate that the ratio of galaxies that have fallen into the clusters since z ∼ 0.6 to those that were already in the core at that redshift is typically between ∼0.3 and 1.5. This wide range of ratios is due to intrinsic scatter and is not a function of velocity dispersion, so a variety of infall histories is to be expected for clusters with current velocity dispersions of 300 km s−1 1200 km s−1. Within the infall regions of z ∼ 0.6 clusters, we find a larger red fraction of galaxies than in the field and greater clustering among red galaxies than blue. We interpret these findings as evidence of "preprocessing," where galaxies in denser local environments have their star formation rates affected prior to their aggregation into massive clusters, although the possibility of backsplash galaxies complicates the interpretation.
Abstract
A major question in galaxy formation is how the gas supply that fuels activity in galaxies is modulated by their environment. We use spectroscopy of a set of well-characterized clusters and ...groups at 0.4 <
z
< 0.8 from the ESO Distant Cluster Survey and compare it to identically selected field galaxies. Our spectroscopy allows us to isolate galaxies that are dominated by old stellar populations. Here we study a stellar-mass-limited sample (
) of these old galaxies with weak O
ii
emission. We use line ratios and compare to studies of local early-type galaxies to conclude that this gas is likely excited by post-AGB stars and hence represents a diffuse gas component in the galaxies. For cluster and group galaxies the fraction with EW(O
ii
) > 5 Å is
f
O
II
=
and
f
O
II
=
, respectively. For field galaxies we find
f
O
II
=
, representing a 2.8
σ
difference between the O
ii
fractions for old galaxies between the different environments. We conclude that a population of old galaxies in all environments has ionized gas that likely stems from stellar mass loss. In the field galaxies also experience gas accretion from the cosmic web, and in groups and clusters these galaxies have had their gas accretion shut off by their environment. Additionally, galaxies with emission preferentially avoid the virialized region of the cluster in position–velocity space. We discuss the implications of our results, among which is that gas accretion shutoff is likely effective at group halo masses (log
/
> 12.8) and that there are likely multiple gas removal processes happening in dense environments.
We study the evolution of spectral early-type galaxies in clusters, groups, and the field up to redshift 0.9 using the ESO Distant Cluster Survey (EDisCS) dataset. We measure structural parameters ...(circularized half-luminosity radii Re, surface brightness Ie, and velocity dispersions sigma) for 154 cluster and 68 field galaxies. On average, we achieve precisions of 10% in Re, 0.1 dex in log Ie, and 10% in sigma. We sample 20% of cluster and 10% of field spectral early-type galaxies to an I band magnitude in a 1 arcsec radius aperture as faint as I1 = 22. We study the evolution of the zero point of the fundamental plane (FP) and confirm results in the literature, but now also for the low cluster velocity dispersion regime. Taken at face value, the mass-to-light ratio varies as Delta log M/LB = (-0.54 ± 0.01) z = (-1.61 ± 0.01) log (1+z) in clusters, independent of their velocity dispersion. The evolution is stronger (Delta log M/LB = (-0.76 ± 0.01) z = (-2.27 ± 0.03) log (1+z)) for field galaxies. A somewhat milder evolution is derived if a correction for incompleteness is applied. A rotation in the FP with redshift is detected with low statistical significance. The alpha and beta FP coefficients decrease with redshift, or, equivalently, the FP residuals correlate with galaxy mass and become progressively negative at low masses. The effect is visible at z >= 0.7 for cluster galaxies and at lower redshifts z >= 0.5 for field galaxies. We investigate the size evolution of our galaxy sample. In agreement with previous results, we find that the half-luminosity radius for a galaxy with a dynamical or stellar mass of 2 × 1011 M_ varies as (1+z)-1.0±0.3 for both cluster and field galaxies. At the same time, stellar velocity dispersions grow with redshift, as (1+z)0.59±0.10 at constant dynamical mass, and as (1+z)0.34±0.14 at constant stellar mass. The measured size evolution reduces to Re ∝ (1+z)-0.5±0.2 and sigma ∝ (1+z)0.41±0.08, at fixed dynamical masses, and Re ∝ (1+z)-0.68±0.4 and sigma ∝ (1+z)0.19±0.10, at fixed stellar masses, when the progenitor bias (PB, galaxies that locally are of spectroscopic early-type, but are not very old, disappear progressively from the EDisCS high-redshift sample; often these galaxies happen to be large in size) is taken into account. Taken together, the variations in size and velocity dispersion imply that the luminosity evolution with redshift derived from the zero point of the FP is somewhat milder than that derived without taking these variations into account. When considering dynamical masses, the effects of size and velocity dispersion variations almost cancel out. For stellar masses, the luminosity evolution is reduced to LB ∝ (1+z)1.0 for cluster galaxies and LB ∝ (1+z)1.67 for field galaxies. Using simple stellar population models to translate the observed luminosity evolution into a formation age, we find that massive (>1011 M_ ) cluster galaxies are old (with a formation redshift zf > 1.5) and lower mass galaxies are 3-4 Gyr younger, in agreement with previous EDisCS results from color and line index analyses. This confirms the picture of a progressive build-up of the red sequence in clusters with time. Field galaxies follow the same trend, but are 1 Gyr younger at a given redshift and mass. Taking into account the size and velocity dispersion evolution quoted above pushes all formation ages upwards by 1 to 4 Gyr.
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zCOSMOS is a large-redshift survey that is being undertaken in the COSMOS field using 600 hr of observation with the VIMOS spectrograph on the 8 m VLT. The survey is designed to characterize the ...environments of COSMOS galaxies from the 100 kpc scales of galaxy groups up to the 100 Mpc scale of the cosmic web and to produce diagnostic Information on galaxies and active galactic nuclei. The zCOSMOS survey consists of two parts: (1) zCOSMOS-bright, a magnitude-limited I-band ha < 22.5 sample of about 20,000 galaxies with 0.1 < z < 1.2 covering the whole 1.7 deg super(2) COSMOS ACS field, for which the survey parameters at z similar to 0.7 are designed to be directly comparable to those of the 2dFGRS at z similar to 0.1; and (2) zCOSMOS-deep, a survey of approximately 10,000 galaxies selected through color-selection criteria to have 1.4 < z < 3.0, within the central 1 deg super(2). This paper describes the survey design and the construction of the target catalogs and briefly outlines the observational program and the data pipeline. In the first observing season, spectra of 1303 zCOSMOS-bright targets and 977 zCOSMOS-deep targets have been obtained. These are briefly analyzed to demonstrate the characteristics that may be expected from zCOSMOS, and particularly zCOSMOS-bright, when it is finally completed between 2008 and 2009. The power of combining spectroscopic and photometric redshifts is demonstrated, especially in correctly identifying the emission line in single-line spectra and in determining which of the less reliable spectroscopic redshifts are correct and which are incorrect. These techniques bring the overall success rate in the zCOSMOS-bright so far to almost 90% and to above 97% in the 0.5 < z < 0.8 redshift range. Our zCOSMOS-deep spectra demonstrate the power of our selection techniques to Isolate high-redshift galaxies at 1.4 < z < 3, C and of VIMOS to measure their redshifts using ultraviolet absorption lines.
Post-starburst (E+A or k+a) spectra, characterized by their exceptionally strong Balmer lines in absorption and the lack of emission lines, belong to galaxies in which the star formation (SF) ...activity ended abruptly sometime during the past Gyr. We perform a spectral analysis of galaxies in clusters, groups, poor groups, and the field at z = 0.4-0.8 based on the ESO Distant Cluster Survey. We find that the incidence of k+a galaxies at these redshifts depends strongly on environment. K+a's reside preferentially in clusters and, unexpectedly, in a subset of the s = 200-400 km s-1 groups, those that have a low fraction of O II emitters. In these environments, 20%-30% of the star-forming galaxies have had their SF activity recently truncated. In contrast, there are proportionally fewer k+a galaxies in the field, the poor groups, and groups with a high O II fraction. An important result is that the incidence of k+a galaxies correlates with the cluster velocity dispersion: more massive clusters have higher proportions of k+a's. Spectra of dusty starburst candidates, with strong Balmer absorption and emission lines, present a very different environmental dependence from k+a's. They are numerous in all environments at z = 0.4-0.8, but they are especially numerous in all types of groups, favoring the hypothesis of triggering by a merger. We present the morphological type, stellar mass, luminosity, mass-to-light ratio, local galaxy density, and clustercentric distance distributions of galaxies of different spectral types. These properties are consistent with previous suggestions that cluster k+a galaxies are observed in a transition phase, at the moment they are rather massive S0 and Sa galaxies, evolving from star-forming, recently infallen later types to passively evolving cluster early-type galaxies. The correlation between k+a fraction and cluster velocity dispersion supports the hypothesis that k+a galaxies in clusters originate from processes related to the intracluster medium, while several possibilities are discussed for the origin of the puzzling k+a frequency in low-O II groups.
Context. There is some disagreement about the abundance of faint galaxies in high-redshift clusters, with contradictory results in the literature arising from studies of the optical galaxy luminosity ...function (GLF) for small cluster samples. Aims. We compute GLFs for one of the largest medium-to-high-redshift (0.4 ≤ z < 0.9) cluster samples to date in order to probe the abundance of faint galaxies in clusters. We also study how the GLF depends on cluster redshift, mass, and substructure and compare the GLFs of clusters with those of the field. We separately investigate the GLFs of blue and red-sequence (RS) galaxies to understand the evolution of different cluster populations. Methods. We calculated the GLFs for 31 clusters taken from the DAFT/FADA survey in the B,V,R, and I rest-frame bands. We used photometric redshifts computed from BVRIZJ images to constrain galaxy cluster membership. We carried out a detailed estimate of the completeness of our data. We distinguished the red-sequence and blue galaxies using a V − I versus I colour−magnitude diagram. We studied the evolution of these two populations with redshift. We fitted Schechter functions to our stacked GLFs to determine average cluster characteristics. Results. We find that the shapes of our GLFs are similar for the B,V,R, and I bands with a drop at the red GLF faint ends that is more pronounced at high redshift: αred ~ −0.5 at 0.40 ≤ z < 0.65 and αred > 0.1 at 0.65 ≤ z < 0.90. The blue GLFs have a steeper faint end (αblue ~ −1.6) than the red GLFs, which appears to be independent of redshift. For the full cluster sample, blue and red GLFs meet at MV = −20, MR = −20.5, and MI = −20.3. A study of how galaxy types evolve with redshift shows that late-type galaxies appear to become early types between z ~ 0.9 and today. Finally, the faint ends of the red GLFs of more massive clusters appear to be richer than less massive clusters, which is more typical of the lower redshift behaviour. Conclusions. Our results indicate that these clusters form at redshifts higher than z = 0.9 from galaxy structures that already have an established red sequence. Late-type galaxies then appear to evolve into early types, enriching the red sequence between this redshift and today. This effect is consistent with the evolution of the faint-end slope of the red sequence and the galaxy type evolution that we find. Finally, faint galaxies accreted from the field environment at all redshifts might have replaced the blue late-type galaxies that converted into early types, explaining the lack of evolution in the faint-end slopes of the blue GLFs.
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Using galaxy clusters from the ESO Distant Cluster Survey, we study how the distribution of galaxies along the colour–magnitude relation has evolved since z∼ 0.8. While red-sequence galaxies in all ...these clusters are well described by an old, passively evolving population, we confirm our previous finding of a significant evolution in their luminosity distribution as a function of redshift. When compared to galaxy clusters in the local Universe, the high-redshift EDisCS clusters exhibit a significant deficit of faint red galaxies. Combining clusters in three different redshift bins, and defining as ‘faint’ all galaxies in the range 0.4 ≳L/L*≳ 0.1, we find a clear decrease in the luminous-to-faint ratio of red galaxies from z∼ 0.8 to ∼0.4. The amount of such a decrease appears to be in qualitative agreement with predictions of a model where the blue bright galaxies that populate the colour–magnitude diagram of high-redshift clusters, have their star formation suppressed by the hostile cluster environment. Although model results need to be interpreted with caution, our findings clearly indicate that the red-sequence population of high-redshift clusters does not contain all progenitors of nearby red-sequence cluster galaxies. A significant fraction of these must have moved on to the red sequence below z∼ 0.8.
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