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We present the analysis of the U-V rest-frame color distribution and some spectral features as a function of mass and environment for two sample of early-type galaxies up to z=1 extracted from the zCOSMOS spectroscopic survey. The first sample (red galaxies) is defined with a photometric classification, while the second (ETGs) by combining morphological, photometric, and spectroscopic properties to obtain a more reliable sample. We find that the color distribution of red galaxies is not strongly dependent on environment for all mass bins, with galaxies in overdense regions redder than galaxies in underdense regions with a difference of 0.027pm0.008 mag. The dependence on mass is far more significant, with average colors of massive galaxies redder by 0.093pm0.007 mag than low-mass galaxies throughout the entire redshift range. We study the color-mass relation, finding a mean slope 0.12pm0.005, while the color-environment relation is flatter, with a slope always smaller than 0.04. The spectral analysis that we perform on our ETGs sample is in good agreement with our photometric results: we find for D4000 a dependence on mass between high and low-mass galaxies, and a much weaker dependence on environment (respectively a difference of of 0.11pm0.02 and of 0.05pm0.02); for the equivalent width of H{delta}we measure a difference of 0.28pm0.08 {AA}across the same mass range and no significant dependence on environment.By analyzing the lookback time of early-type galaxies, we support the possibility of a downsizing scenario, in which massive galaxies with a stronger D4000 and an almost constant equivalent width of $Hdelta$ formed their mass at higher redshift than lower mass ones. We also conclude that the main driver of galaxy evolution is the galaxy mass, the environment playing a subdominant role.
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We present the Galaxy Stellar Mass Function (MF) up to z~1 from the zCOSMOS-bright 10k spectroscopic sample. We investigate the total MF and the contribution of ETGs and LTGs, defined by different criteria (SED, morphology or star formation). We unveil a galaxy bimodality in the global MF, better represented by 2 Schechter functions dominated by ETGs and LTGs, respectively. For the global population we confirm that low-mass galaxies number density increases later and faster than for massive galaxies. We find that the MF evolution at intermediate-low values of Mstar (logM<10.6) is mostly explained by the growth in stellar mass driven by smoothly decreasing star formation activities. The low residual evolution is consistent with ~0.16 merger per galaxy per Gyr (of which fewer than 0.1 are major). We find that ETGs increase in number density with cosmic time faster for decreasing Mstar, with a median building redshift increasing with mass, in contrast with hierarchical models. For LTGs we find that the number density of blue or spiral galaxies remains almost constant from z~1. Instead, the most extreme population of active star forming galaxies is rapidly decreasing in number density. We suggest a transformation from blue active spirals of intermediate mass into blue quiescent and successively (1-2 Gyr after) into red passive types. The complete morphological transformation into red spheroidals, required longer time-scales or follows after 1-2 Gyr. A continuous replacement of blue galaxies is expected by low-mass active spirals growing in stellar mass. We estimate that on average ~25% of blue galaxies is transforming into red per Gyr for logM<11. We conclude that the build-up of galaxies and ETGs follows the same downsizing trend with mass as the formation of their stars, converse to the trend predicted by current SAMs. We expect a negligible evolution of the global Galaxy Baryonic MF.
In our earlier 2019 paper, we evaluated the reliability of Schwarzschilds orbit-superposition dynamical modelling method in estimating the internal mass distribution, intrinsic stellar shapes and orbit distributions of early-type galaxies (ETGs) taken from the Illustris cosmological simulation. We now apply the same techniques to galaxies taken from the integral-field survey Mapping Nearby Galaxies with APO (MaNGA), using a sample of 149 ETGs in the mass range of $10^{9.90}sim10^{11.80} M_{odot}$ and made up of 105 central and 44 satellite galaxies. We find that low-mass ETGs with $log(M_*/M_{odot})<11.1$ have an average dark matter fraction of $sim0.2$ within one effective radius $R_{rm e}$, tend to be oblate-like, and are dominated by rotation about their minor axis. High-mass ETGs with $log(M_*/M_{odot})>11.1$ have an average dark matter fraction of $sim0.4$ within one effective radius $R_{rm e}$, tend to be prolate-like, and are dominated by rotation about their major axis and by centrophilic orbits. The changes of internal structures within one $R_{rm e}$ are dominated by the total stellar mass of the individual galaxies. We find no differences of internal structures between central and satellite ETGs for the same stellar masses. However, for similar stellar mass and colour distributions, we find that ETGs more prolate-like, or with more hot orbits, tend to have higher close neighbour counts at $r_psim40$ kpc.
We study the evolution of galaxies inside and outside of the group environment since z=1 using a large well defined set of groups and galaxies from the zCOSMOS-bright redshift survey in the COSMOS field. The fraction of galaxies with early-type morphologies increases monotonically with M_B luminosity and stellar mass and with cosmic epoch. It is higher in the groups than elsewhere, especially at later epochs. The emerging environmental effect is superposed on a strong global mass-driven evolution, and at z~0.5 and log(M*/Msol)~10.2, the effect of group environment is equivalent to (only) about 0.2 dex in stellar mass or 2 Gyr in time. The stellar mass function of galaxies in groups is enriched in massive galaxies. We directly determine the transformation rates from late to early morphologies, and for transformations involving colour and star formation indicators. The transformation rates are systematically about twice as high in the groups as outside, or up to 3-4 times higher correcting for infall and the appearance of new groups. The rates reach values, for masses around the crossing mass 10^10.5 Msol, as high as (0.3-0.7)/Gyr in the groups, implying transformation timescales of 1.4-3 Gyr, compared with less than 0.2/Gyr, i.e. timescales >5 Gyr, outside of groups. All three transformation rates decrease at higher stellar masses, and must decrease also at the lower masses below 10^10 Msol which we cannot well probe. The rates involving colour and star formation are consistently higher than those for morphology, by a factor of about 50%. Our conclusion is that the transformations which drive the evolution of the overall galaxy population since z~1 must occur at a rate 2-4 times higher in groups than outside of them.
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
We use the current sample of ~10,000 zCOSMOS spectra of sources selected with I(AB) < 22.5 to define the density field out to z~1, with much greater resolution in the radial dimension than has been possible with either photometric redshifts or weak lensing. We apply new algorithms that we have developed (ZADE) to incorporate objects not yet observed spectroscopically by modifying their photometric redshift probability distributions using the spectroscopic redshifts of nearby galaxies. This strategy allows us to probe a broader range of galaxy environments and reduce the Poisson noise in the density field. The reconstructed overdensity field of the 10k zCOSMOS galaxies consists of cluster-like patterns surrounded by void-like regions, extending up to z~1. Some of these structures are very large, spanning the ~50 Mpc/h transverse direction of the COSMOS field and extending up to Delta z~0.05 in redshift. We present the three dimensional overdensity maps and compare the reconstructed overdensity field to the independently identified virialised groups of galaxies and clusters detected in the visible and in X-rays. The distribution of the overdense structures is in general well traced by these virialised structures. A comparison of the large scale structures in the zCOSMOS data and in the mock catalogues reveals an excellent agreement between the fractions of the volume enclosed in structures of all sizes above a given overdensity between the data and the mocks in 0.2<z<1.
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