The goal of this work is to study the evolution of high-redshift (z≥ 1.4) quiescent galaxies over an effective area of ∼1.7 deg2 in the COSMOS field. Galaxies have been divided according to their ...star formation activity and the evolution of the different populations, in particular of the quiescent galaxies, has been investigated in detail. We have studied an IRAC (mag 3.6 μm < 22.0) selected sample of ∼18 000 galaxies at z≥ 1.4 in the COSMOS field with multiwavelength coverage extending from the U band to the Spitzer 24 μm one. We have derived accurate photometric redshifts (
) through a SED-fitting procedure. Other important physical parameters masses, ages and star formation rates (SFR) of the galaxies have been obtained using Maraston models. We have divided our sample into actively star-forming, intermediate and quiescent galaxies depending on their specific star formation rate (SSFR = SFR/M). We have computed the galaxy stellar mass function (GSMF) of the total sample and the different populations at z= 1.4-3.0. We have studied the properties of high-redshift quiescent galaxies finding that they are old (1-4 Gyr), massive (〈M〉∼ 1010.65 M⊙), weakly star-forming stellar populations with low dust extinction E(B−V) ≤ 0.15 and small e-folding time-scales (τ∼ 0.1-0.3 Gyr). We observe a significant evolution of the quiescent stellar mass function from 2.5 < z < 3.0 to 1.4 < z < 1.6, increasing by ∼1 dex in this redshift interval. We find that z∼ 1.5 is an epoch of transition of the GSMF: while the GSMF at z≳ 1.5 is dominated by the star-forming galaxies at all stellar masses, at z≲ 1.5 the contribution to the total GSMF of the quiescent galaxies is significant and becomes higher than that of the star-forming population for M≥ 1010.75 M⊙. The fraction of star-forming galaxies decreases from 60-20 per cent from z∼ 2.5-3.0 to 1.4-1.6 for M∼ 1011.0 M⊙, while the quiescent population increases from 10-50 per cent at the same redshift and mass intervals. We compare the fraction of quiescent galaxies derived with that predicted by theoretical models and find that the Kitzbichler & White model, implemented on the Millennium Simulation, is the one that better reproduces the shape of the data. Finally, we calculate the stellar mass density of the star-forming and quiescent populations as a function of redshift and find that there is already a significant number of quiescent galaxies at z > 2.5 (logρM⊙ Mpc−3∼ 6), meaning that efficient star formation had to take place before that time.
We study the spatial clustering of 538 X-ray selected AGN in the 2 deg2 XMM-COSMOS field that are spectroscopically identified with I_AB<23 and span the redshift range z=0.2-3.0. The median redshift ...and X-ray luminosity of the sample are z = 0.98 and L0.5-10=6.3× 1043 erg s-1, respectively. A strong clustering signal is detected at 18σ level, which is the most significant measurement obtained to date for clustering of X-ray selected AGN. By fitting the projected correlation function w(r_p) with a power law on scales of r_p=0.3-40 h-1 Mpc, we derive a best-fit comoving correlation length of r0 = 8.6±0.5 h-1 Mpc and slope of γ=1.88±0.07 (Poissonian errors; bootstrap errors are about a factor of 2 larger). An excess signal is observed in the range r_p˜5-15 h-1 Mpc, which is due to a large-scale structure at z˜ 0.36 containing about 40 AGN, a feature which is evident over many wavelengths in the COSMOS field. When removing the z˜ 0.36 structure or computing w(r_p) in a narrower range around the peak of the redshift distribution (e.g. z=0.4-1.6), the correlation length decreases to r0 ˜ 5-6 h-1 Mpc, which is consistent with what is observed for bright optical QSOs at the same redshift. We investigate the clustering properties of obscured and unobscured AGN separately, adopting different definitions for the source obscuration. For the first time, we are able to provide a significant measurement for the spatial clustering of obscured AGN at z˜ 1. Within the statistical uncertainties, we do not find evidence that AGN with broad optical lines (BLAGN) cluster differently from AGN without broad optical lines (non-BLAGN). Based on these results, which are limited by object statistics, however, obscured and unobscured AGN are consistent with inhabiting similar environments. The evolution of AGN clustering with redshift is also investigated. No significant difference is found between the clustering properties of XMM-COSMOS AGN at redshifts below or above z=1. The correlation length measured for XMM-COSMOS AGN at z˜ 1 is similar to that of massive galaxies (stellar mass M_star⪆ 3× 1010 Mȯ) at the same redshift. This suggests that AGN at z˜ 1 are preferentially hosted by massive galaxies, as observed both in the local and in the distant (z˜ 2) Universe. According to a simple clustering evolution scenario, we find that the relics of AGN are expected to have a correlation length as large as r0 ˜ 8 h-1 Mpc by z=0, and hence to be hosted by local bright (L˜ L_star) ellipticals. We make use of dark matter halo catalogs from the Millennium simulation to determine the typical halo hosting moderately luminous z˜ 1 AGN. We find that XMM-COSMOS AGN live in halos with masses M⪆ 2.5× 1012 Mȯ h-1. By combining the number density of XMM-COSMOS AGN to that of the hosting dark matter halos we estimate the AGN duty cycle and lifetimes. We find lifetimes approximately of 1 Gyr for AGN at z˜ 1, which are longer than those estimated for optically bright QSOs at the same redshift. These longer lifetimes mainly reflect the higher number density of AGN selected by X-ray samples.
The impact of environment on active galactic nucleus (AGN) activity up to z ~ 1 is assessed by utilizing a mass-selected sample of galaxies from the 10k catalog of the zCOSMOS spectroscopic redshift ...survey. We identify 147 AGN by their X-ray emission as detected by XMM-Newton from a parent sample of 7234 galaxies. We measure the fraction of galaxies with stellar mass M * > 2.5 X 1010 M that host an AGN as a function of local overdensity using the 5th, 10th, and 20th nearest neighbors that cover a range of physical scales (~1-4 Mpc). Overall, we find that AGNs prefer to reside in environments equivalent to massive galaxies with substantial levels of star formation. Specifically, AGNs with host masses between 0.25 and 1 X 1011 M span the full range of environments (i.e., field to group) exhibited by galaxies of the same mass and rest-frame color or specific star formation rate. Host galaxies having M * > 1011 M clearly illustrate the association with star formation since they are predominantly bluer than the underlying galaxy population and exhibit a preference for lower-density regions analogous to Sloan Digital Sky Survey studies of narrow-line AGN. To probe the environment on smaller physical scales, we determine the fraction of galaxies (M * > 2.5 X 1010 M ) hosting AGNs inside optically selected groups, and find no significant difference with field galaxies. We interpret our results as evidence that AGN activity requires a sufficient fuel supply; the probability of a massive galaxy to have retained some sufficient amount of gas, as evidence by its ongoing star formation, is higher in underdense regions where disruptive processes (i.e., galaxy harassment, tidal stripping) are lessened.
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 $F_{\rm blue}$, 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 $F_{\rm blue}$) 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., $\log ({\cal M}_{*}/{\cal M}_{\odot}) \leq 10.6 $, 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 $\log ({\cal M}_{*}/{\cal M}_{\odot}) \geq 10.8$ 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.
The Vimos VLT deep survey Garilli, B.; Le Fèvre, O.; Guzzo, L. ...
Astronomy and astrophysics (Berlin),
08/2008, Letnik:
486, Številka:
3
Journal Article
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Context. The VVDS-Wide survey has been designed to trace the large-scale distribution of galaxies at z similar to 1 on comoving scales reaching similar to 100 h super(-1) Mpc, while providing a good ...control of cosmic variance over areas as large as a few square degrees. This is achieved by measuring redshifts with VIMOS at the ESO VLT to a limiting magnitude I AB = 22.5, targeting four independent fields with sizes of up to 4 deg super(2) each. Aims. We discuss the survey strategy which covers 8.6 deg super(2) and present the general properties of the current redshift sample. This includes 32 734 spectra in the four regions, covering a total area of 6.1 deg super(2) with a sampling rate of 22 to 24%. This paper accompanies the public release of the first 18 143 redshifts of the VVDS-Wide survey from the 4 deg super(2) contiguous area of the F22 field at RA = 22\rm phi . Methods. We have devised and tested an objective method to assess the quality of each spectrum, providing a compact figure-of-merit. This is particularly effective in the case of long-lasting spectroscopic surveys with varying observing conditions. Our figure of merit is a measure of the robustness of the redshift measurement and, most importantly, can be used to select galaxies with uniform high-quality spectra to carry out reliable measurements of spectral features. We also use the data available over the four independent regions to directly measure the variance in galaxy counts. We compare it with general predictions from the observed galaxy two-point correlation function at different redshifts and with that measured in mock galaxy surveys built from the Millennium simulation. Results. The purely magnitude-limited VVDS Wide sample includes 19 977 galaxies, 304 type I AGNs, and 9913 stars. The redshift success rate is above 90% independent of magnitude. A cone diagram of the galaxy spatial distribution provides us with the current largest overview of large-scale structure up to z similar to 1, showing a rich texture of over- and under-dense regions. We give the mean N (z) distribution averaged over 6.1 deg super(2) for a sample limited in magnitude to I AB = 22.5. Comparing galaxy densities from the four fields shows that in a redshift bin Delta z = 0.1 at z similar to 1 one still has factor-of-two variations over areas as large as similar to 0.25 deg super(2). This level of cosmic variance agrees with that obtained by integrating the galaxy two-point correlation function estimated from the F22 field alone. It is also in fairly good statistical agreement with that predicted by the Millennium simulations. Conclusions. The VVDS WIDE survey currently provides the largest area coverage among redshift surveys reaching z similar to 1. The variance estimated over the survey fields shows explicitly how clustering results from deep surveys of even 1 deg super(2) size should be interpreted with caution. The survey data represent a rich data base to select complete sub-samples of high-quality spectra and to study galaxy ensemble properties and galaxy clustering over unprecedented scales at these redshifts. The redshift catalog of the 4 deg super(2) F22 field is publicly available at http://cencosw.oamp.fr.
We analyze the environments and galactic properties (morphologies and star formation histories) of a sample of 153 close kinematic pairs in the redshift range 0.2 < z < 1 identified in the ...zCOSMOS-bright 10k spectroscopic sample of galaxies. We then examine the morphologies and stellar populations of galaxies in the pairs, comparing them to control samples that are carefully matched in environment so as to remove as much of the well-known effects of environment on the properties of the parent population of galaxies as possible. Once the environment is properly taken into account in this way, we find that the early-late morphology mix is the same as for the parent population, but that the fraction of irregular galaxies is boosted by 50%-75%, with a disproportionate increase in the number of irregular-irregular pairs (factor of 4-8 times), due to the disturbance of disk galaxies.
Aims. We study the dependence of galaxy clustering on luminosity and stellar mass at redshifts $z\sim$ 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 $\xi(r_{\rm p},\pi)$ and $\xi(s)$ and the projected function, $w_{\rm p}(r_{\rm p})$ 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 ($0.5<z<0.8$), where $w_{\rm p}(r_{\rm p})$ shows strong excess power on scales >1 h-1 Mpc. This is reflected in the shape of the full $\xi(r_{\rm p},\pi)$ that we interpret as produced by dominating structures almost perpendicular to the line of sight in the survey volume. Comparing to $z\sim 0$ 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 $0.5<z<1$ for galaxies with $\log(M/M_\odot)\ge 10$ 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 $z\sim 1$ Universe to appropriately characterise the level of structure at this epoch.
We derive the mass-metallicity relation of star-forming galaxies up to $z\sim0.9$, using data from the VIMOS VLT Deep Survey. Automatic measurement of emission-line fluxes and equivalent widths have ...been performed on the full spectroscopic sample. This sample is divided into two sub-samples depending on the apparent magnitude selection: wide ($I_{\mathrm{AB}}<22.5$) and deep $I_{\mathrm{AB}}<24$). These two samples span two different ranges of stellar masses. Emission-line galaxies have been separated into star-forming galaxies and active galactic nuclei using emission line ratios. For the star-forming galaxies the emission line ratios have also been used to estimate gas-phase oxygen abundance, using empirical calibrations renormalized in order to give consistent results at low and high redshifts. The stellar masses have been estimated by fitting the whole spectral energy distributions with a set of stellar population synthesis models. We assume at first order that the shape of the mass-metallicity relation remains constant with redshift. Then we find a stronger metallicity evolution in the wide sample as compared to the deep sample. We thus conclude that the mass-metallicity relation is flatter at higher redshift. The observed flattening of the mass-metallicity relation at high redshift is analyzed as an evidence in favor of the open-closed model.
Aims. An unbiased and detailed characterization of the galaxy luminosity function (LF) is a basic requirement in many astrophysical issues: it is of particular interest in assessing the role of the ...environment in the evolution of the LF of different galaxy types. Methods. We studied the evolution in the B band LF to redshift $z\sim 1$ in the zCOSMOS 10k sample, for which both accurate galaxy classifications (spectrophotometric and morphological) and a detailed description of the local density field are available. Results. The global B band LF exhibits a brightening of ~0.7 mag in M* from $z\sim 0.2 $ to $z\sim 0.9$. At low redshifts ($z<0.35$), spectrophotometric late types dominate at faint magnitudes ($M_{B_{AB}} > -20$), while the bright end is populated mainly by spectrophotometric early types. At higher redshift, spectrophotometric late-type galaxies evolve significantly and, at redshift $z\sim 1$,the contribution from the various types to the bright end of the LF is comparable. The evolution for spectrophotometric early-type galaxies is in both luminosity and normalization: M* brightens by ~0.6 mag but $\phi^*$ decreases by a factor ~1.7 between the first and the last redshift bin. A similar behaviour is exhibited by spectrophotometric late-type galaxies, but with an opposite trend for the normalization: a brightening of ~0.5 mag is present in M*, while $\phi^*$ increases by a factor ~1.8. Studying the role of the environment, we find that the global LF of galaxies in overdense regions has always a brighter M* and a flatter slope. In low density environments, the main contribution to the LF is from blue galaxies, while for high density environments there is an important contribution from red galaxies to the bright end. The differences between the global LF in the two environments are not due to only a difference in the relative numbers of red and blue galaxies, but also to their relative luminosity distributions: the value of M* for both types in underdense regions is always fainter than in overdense environments. These results indicate that galaxies of the same type in different environments have different properties. We also detect a differential evolution in blue galaxies in different environments: the evolution in their LF is similar in underdense and overdense regions between $z\sim 0.25$ and $z\sim 0.55$, and is mainly in luminosity. In contrast, between $z\sim 0.55$ and $z\sim 0.85$ there is little luminosity evolution but there is significant evolution in $\phi^*$, that is, however, different between the two environments: in overdense regions $\phi^*$ increases by a factor ~1.6, while in underdense regions this increase reaches a factor ~2.8. Analyzing the blue galaxy population in more detail, we find that this evolution is driven mainly by the bluest types. Conclusions. The “specular” evolution of late- and early-type galaxies is consistent with a scenario where a part of blue galaxies is transformed in red galaxies with increasing cosmic time, without significant changes in the fraction of intermediate-type galaxies. The bulk of this tranformation in overdense regions probably happened before $z\sim 1$, while it is still ongoing at lower redshifts in underdense environments.
Aims. With the first ~10 000 spectra of the flux limited zCOSMOS sample (IAB ≤ 22.5) we want to study the evolution of environmental effects on galaxy properties since z ~ 1.0, and to disentangle ...the dependence among galaxy colour, stellar mass and local density. Methods. We use our previously derived 3D local density contrast δ, computed with the 5th nearest neighbour approach, to study the evolution with z of the environmental effects on galaxy U-B colour, D4000 Å break and OIIλ3727 equivalent width (EWOII). We also analyze the implications due to the use of different galaxy selections, using luminosity or stellar mass, and we disentangle the relations among colour, stellar mass and δ studying the colour-density relation in narrow mass bins. Results. We confirm that within a luminosity-limited sample (MB ≤ −20.5 − z) the fraction of red (U − B ≥ 1) galaxies depends on δ at least up to z ~ 1, with red galaxies residing mainly in high densities. This trend becomes weaker for increasing redshifts, and it is mirrored by the behaviour of the fraction of galaxies with D4000 Å break ≥ 1.4. We also find that up to z ~ 1 the fraction of galaxies with log (EW OII ) ≥ 1.15 is higher for lower δ, and also this dependence weakens for increasing z. Given the triple dependence among galaxy colours, stellar mass and δ, the colour-δ relation that we find in the luminosity-selected sample can be due to the broad range of stellar masses embedded in the sample. Thus, we study the colour-δ relation in narrow mass bins within mass complete subsamples, defining red galaxies with a colour threshold roughly parallel to the red sequence in the colour-mass plane. We find that once mass is fixed the colour-δ relation is globally flat up to z ~ 1 for galaxies with log (M/M⊙) ≳ 10.7. This means that for these masses any colour-δ relation found within a luminosity-selected sample is the result of the combined colour-mass and mass-δ relations. On the contrary, even at fixed mass we observe that within 0.1 ≤ z ≤ 0.5 the fraction of red galaxies with log (M/M⊙) ≲ 10.7 depends on δ. For these mass and redshift ranges, environment affects directly also galaxy colours. Conclusions. We suggest a scenario in which the colour depends primarily on stellar mass, but for an intermediate mass regime (10.2 ≲ log (M/M⊙) ≲ 10.7) the local density modulates this dependence. These relatively low mass galaxies formed more recently, in an epoch when more evolved structures were already in place, and their longer SFH allowed environment-driven physical processes to operate during longer periods of time.
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