M. Bolzonella

We studied the chronology of galactic bulge and disc formation by analysing the relative contributions of these components to the B-band rest-frame luminosity density at different epochs. We present the first estimate of the evolution of the fraction of rest-frame B-band light in galactic bulges and discs since redshift z ~ 0.8. We performed a bulge-to-disc decomposition of HST/ACS images of 3266 galaxies in the zCOSMOS-bright survey with spectroscopic redshifts in the range 0.7 ≤ z ≤ 0.9. We find that the fraction of B-band light in bulges and discs is (26 ± 4)% and (74 ± 4)%, respectively. When compared with rest-frame B-band measurements of galaxies in the local Universe in the same mass range (), we find that the B-band light in discs decreases by ~30% from z ~ 0.7−0.9 to z ~ 0, while the light from the bulge increases by ~30% over the same period of time. We interpret this evolution as the consequence of star formation and mass assembly processes, as well as morphological transformation, which gradually shift stars formed at half the age of the Universe from star-forming late-type/irregular galaxies to earlier types and ultimately into spheroids.

We present a group-galaxy cross-correlation analysis using a group catalog produced from the 16,500 spectra from the optical zCOSMOS galaxy survey. Our aim is to perform a consistency test in the redshift range 0.2 ≤ z ≤ 0.8 between the clustering strength of the groups and mass estimates that are based on the richness of the groups. We measure the linear bias of the groups by means of a group-galaxy cross-correlation analysis and convert it into mass using the bias-mass relation for a given cosmology, checking the systematic errors using realistic group and galaxy mock catalogs. The measured bias for the zCOSMOS groups increases with group richness as expected by the theory of cosmic structure formation and yields masses that are reasonably consistent with the masses estimated from the richness directly, considering the scatter that is obtained from the 24 mock catalogs. Some exceptions are the richest groups at high redshift (estimated to be more massive than 10^13.5 M _☉), for which the measured bias is significantly larger than for any of the 24 mock catalogs (corresponding to a 3σ effect), which is attributed to the extremely large structure that is present in the COSMOS field at z ~ 0.7. Our results are in general agreement with previous studies that reported unusually strong clustering in the COSMOS field.

We use the zCOSMOS galaxy overdensity field to study the biasing of galaxies in the COSMOS field. By comparing the probability distribution function of the galaxy density contrast δ _g to the lognormal approximation of the mass density contrast δ, we obtain the mean biasing function b(δ, z, R) between the galaxy and matter overdensity fields and its second moments and . Over the redshift interval 0.4 < z < 1 the conditional mean function δ _g |δ = b(δ, z, R)δ is of a characteristic shape, requiring nonlinear biasing in the most overdense and underdense regions. Taking into account the uncertainties due to cosmic variance, we do not detect any significant evolution in the δ _g |δ function, but we do detect a significant redshift evolution in the linear biasing parameter from 1.23 ± 0.11 at z ~ 0.55 to 1.62 ± 0.14 at z ~ 0.75, for a luminosity-complete sample of M_B < –20 – z galaxies. The parameter does not change significantly with smoothing scale between 8 and 12 h ^–1 Mpc, but increases systematically with luminosity (at 2σ-3σ significance between the M_B < –20.5 – z and M_B < –20 – z samples). The nonlinearity parameter is offset from unity by at most 2%, with an uncertainty of the same order. The parameter does not show any significant redshift evolution, dependence on the smoothing scale or on the luminosity. By matching the linear bias of galaxies to the halo bias, we infer that the M_B < –20 – z galaxies reside in dark matter halos with a characteristic mass of about (2.6 – 5.6) × 10¹² M _☉ with a small dependence on the adopted bias-mass relation. Our detailed error analysis and comparison with previous studies lead us to conclude that cosmic variance is the main contributor to the differences in the linear bias measured from different surveys. While our results support the general picture of biased galaxy formation up to z ~ 1, the fine-tuning of the galaxy formation models is still limited by the restrictions of the current spectroscopic surveys at these redshifts.

We study the impact of the environment on the evolution of galaxies in the zCOSMOS 10 k sample in the redshift range 0.1 ≤ z ≤ 1.0 over an area of ~1.5 deg2. The considered sample of secure spectroscopic redshifts contains about 8500 galaxies, with their stellar masses estimated by SED fitting of the multiwavelength optical to near-infrared (NIR) photometry. The evolution of the galaxy stellar mass function (GSMF) in high and low density regions provides a tool to study the mass assembly evolution in different environments; moreover, the contributions to the GSMF from different galaxy types, as defined by their SEDs and their morphologies, can be quantified. At redshift z ~ 1, the GSMF is only slightly dependent on environment, but at lower redshifts the shapes of the GSMFs in high- and low-density environments become extremely different, with high density regions exhibiting a marked bimodality, not reproducible by a single Schechter function. As a result of this analysis, we infer that galaxy evolution depends on both the stellar mass and the environment, the latter setting the probability of a galaxy to have a given mass: all the galaxy properties related to the stellar mass show a dependence on environment, reflecting the difference observed in the mass functions. The shapes of the GSMFs of early- and late-type galaxies are almost identical for the extremes of the density contrast we consider, ranging from isolated galaxies to rich group members. The evolution toward z = 0 of the transition mass ℳcross, i.e., the mass at which the early- and late-type GSMFs match each other, is more rapid in high density environments, because of a difference in the evolution of the normalisation of GSMFs compared to the total one in the considered environment. The same result is found by studying the relative contributions of different galaxy types, implying that there is a more rapid evolution in overdense regions, in particular for intermediate stellar masses. The rate of evolution is different for sets of galaxy types divided on the basis of their SEDs or their morphologies, tentatively suggesting that the migration from the blue cloud to the red sequence occurs on a shorter timescale than the transformation from disc-like morphologies to ellipticals. Our analysis suggests that environmental mechanisms of galaxy transformation start to be more effective at z < 1. The comparison of the observed GSMFs to the same quantities derived from a set of mock catalogues based on semi-analytical models shows disagreement, in both low and high density environments: in particular, blue galaxies in sparse environments are overproduced in the semi-analytical models at intermediate and high masses, because of a deficit of star formation suppression, while at z < 0.5 an excess of red galaxies is present in dense environments at intermediate and low masses, because of the overquenching of satellites.

We study the impact of the environment on the evolution of galaxies in the zCOSMOS 10k sample in the redshift range 0.1<z<1.0 over an area of ~1.5 deg2. The considered sample of secure spectroscopic redshifts contains about 8500 galaxies, with their stellar masses estimated by SED fitting of the multiwavelength optical to NIR photometry. The evolution of the galaxy stellar mass function (GSMF) in high and low density regions provides a tool to study the mass assembly evolution in different environments; moreover, the contributions to the GSMF from different galaxy types, as defined by their SEDs and their morphologies, can be quantified. At redshift z~1, the GSMF is only slightly dependent on environment, but at lower redshifts the shapes of the GSMFs in high- and low-density environments become extremely different, with high density regions exhibiting a marked bimodality. As a result, we infer that galaxy evolution depends on both the stellar mass and the environment, the latter setting the probability of a galaxy to have a given mass: all the galaxy properties related to the stellar mass show a dependence on environment, reflecting the difference observed in the mass functions. The shapes of the GSMFs of early- and late-type galaxies are almost identical for the extremes of the density contrast we consider. The evolution toward z=0 of the mass at which the early- and late-type GSMFs match each other is more rapid in high density environments. The comparison of the observed GSMFs to the same quantities derived from a set of mock catalogues shows that blue galaxies in sparse environments are overproduced in the semi-analytical models at intermediate and high masses, because of a deficit of star formation suppression, while at z<0.5 an excess of red galaxies is present in dense environments at intermediate and low masses, because of the overquenching of satellites. ABRIDGED

Context. Groups of galaxies are a common environment, bridging the gap between starforming field galaxies and quiescent cluster galaxies. Within groups secular processes could be at play, contributing to the observed strong decrease of star formation with cosmic time in the global galaxy population.Aims. We took advantage of the wealth of information provided by the first ~10 000 galaxies of the zCOSMOS-bright survey and its group catalogue to study in detail the complex interplay between group environment and galaxy properties. Methods. The classical indicator , i.e., the fraction of blue galaxies, proved to be a simple but powerful diagnostic tool. We studied its variation for different luminosity and mass selected galaxy samples, divided as to define groups/field/isolated galaxy subsamples.Results. Using rest-frame evolving B-band volume-limited samples, the groups galaxy population exhibits significant blueing as redshift increases, but maintains a systematic difference (a lower ) with respect to the global galaxy population, and an even larger difference with respect to the isolated galaxy population. However moving to mass selected samples it becomes apparent that such differences are largely due to the biased view imposed by the B-band luminosity selection, being driven by the population of lower mass, bright blue galaxies for which we miss the redder, equally low mass, counterparts. By carefully focusing the analysis on narrow mass bins such that mass segregation becomes negligible we find that only for the lowest mass bin explored, i.e., , does a significant residual difference in color remain as a function of environment, while this difference becomes negligible toward higher masses.Conclusions. Our results indicate that red galaxies of mass are already in place at z ~ 1 and do not exhibit any strong environmental dependence, possibly originating from so-called nature or internal mechanisms. In contrast, for lower galaxy masses and redshifts lower than z ~ 1, we observe the emergence in groups of a population of nurture red galaxies: slightly deviating from the trend of the downsizing scenario followed by the global galaxy population, and more so with cosmic time. These galaxies exhibit signatures of group-related secular physical mechanisms directly influencing galaxy evolution. Our analysis implies that these mechanisms begin to significantly influence galaxy evolution after z ~ 1, a redshift corresponding to the emergence of structures in which these mechanisms take place.

Aims. The identities of the main processes triggering and quenching star-formation in galaxies remain unclear. A key stage in evolution, however, appears to be represented by post-starburst galaxies. To investigate the prevalence of these galaxies and their impact on galaxy evolution in general, we initiated a multiwavelength study of galaxies with k+a spectral features in the well-studied COSMOS field.Methods. We examine a mass-selected sample of k+a galaxies in the COSMOS field at z = 0.48-1.2 using the spectroscopic zCOSMOS sample. To classify galaxies in their k+a phase, we use a spectroscopic criterion, based on the amplitude of the Hδ absorption line and the absence of the [OII] emission line. We develop our analysis for a well-defined sample by imposing stringent confidence levels on the spectroscopic redshifts and spectral measurements. We compare our results for two mass-selected samples of star-forming and quiescent galaxies selected using a purely spectral classification scheme from the 10 000 zCOSMOS catalogue (i.e. , based on measurements of 4000 Å break and EW[OII]).Results. In our mass-limited sample, k+a galaxies occupy the brightest tail of the luminosity distribution. They are as massive as quiescent galaxies and populate the green valley in the colour versus luminosity (or stellar mass) distribution. A small percentage ( <8%) of these galaxies have radio and/or X-ray counterparts (implying an upper limit to the SFR of ~8 yr-1). Over the entire redshift range explored, the class of post-starburst galaxies is morphologically a heterogeneous population with a similar incidence of bulge-dominated and disky galaxies. This distribution does not vary with the strength of the Hδ absorption line but instead with stellar mass in a way reminiscent of the well-known mass-morphology relation. The results about the incidence of asymmetries and the concentration of the light distribution derived from HST/ACS images imply that this galaxy population possibly represents an intermediate stage of galaxy evolution. Although k+a galaxies are also found in underdense regions, they appear to reside typically in a similarly rich environment as quiescent galaxies on a physical scale of ~2–8 Mpc, and in groups they show a morphological early-to-late type ratio similar to the quiescent galaxy class. With the current data set, we do not find evidence of statistical significant evolution in either the number/mass density of k+a galaxies at intermediate redshift with respect to the local values, or the spectral properties, although more solid results on this and other aspects will be obtained following the completion of the survey.Conclusions. Several mechanisms related and unrelated to the environment are at work in quenching star-formation activity in galaxies on short timescales ( <1 Gyr). Those galaxies, which are affected by a sudden quenching of their star-formation activity, may increase the stellar mass of the red-sequence by up to a non-negligible level of ~10%.

Aims. We study the dependence of galaxy clustering on luminosity and stellar mass at redshifts [0.2–1], using the first 10K redshifts from the zCOSMOS spectroscopic survey of the COSMOS field. Methods. We measured the redshift-space correlation functions and and the projected function, for subsamples covering different luminosity, mass, and redshift ranges. We explored and quantified in detail the observational selection biases from the flux-limited nature of the survey, using ensembles of realistic semi-analytic mock samples built from the Millennium simulation. We used the same mock data sets to carefully check our covariance and error estimate techniques, comparing the performances of methods based on the scatter in the mocks and on bootstrapping schemes. We finally compared our measurements to the cosmological model predictions from the mock surveys. Results. At odds with other measurements at similar redshift and in the local Universe, we find a weak dependence of galaxy clustering on luminosity in all three redshift bins explored. A mild dependence on stellar mass is instead observed, in particular on small scales, which becomes particularly evident in the central redshift bin (), where shows strong excess power on scales >1 h-1 Mpc. This is reflected in the shape of the full that we interpret as produced by dominating structures almost perpendicular to the line of sight in the survey volume. Comparing to measurements, we do not see any significant evolution with redshift of the amplitude of clustering for bright and/or massive galaxies. Conclusions. This is consistent with previous results and the standard picture in which the bias evolves more rapidly for the most massive haloes, which in turn host the highest-stellar-mass galaxies. At the same time, however, the clustering measured in the zCOSMOS 10K data at for galaxies with is only marginally consistent with the predictions from the mock surveys. On scales larger than ~2 h-1 Mpc, the observed clustering amplitude is compatible only with ~1% of the mocks. Thus, if the power spectrum of matter is ΛCDM with standard normalisation and the bias has no “unnatural” scale-dependence, this result indicates that COSMOS has picked up a particularly rare, ~2–3σ positive fluctuation in a volume of ~106 h-1 Mpc3. These findings underline the need for larger surveys of the Universe to appropriately characterise the level of structure at this epoch.

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