Context.The VIMOS VLT Deep Survey (VVDS) was undertaken to map the evolution of galaxies, large scale structures, and active galaxy nuclei from the redshift spectroscopic measurements of ~105 objects ...down to an apparent magnitude $I_{AB} = 24$, in combination with a multi-wavelength acquisition for radio, infrared, optical, ultraviolet, and X-rays data. Aims.We present the evolution of the comoving star formation rate (SFR) density in the redshift range $0 < z < 5$ using the first epoch data release of the VVDS, that is 11564 spectra over 2200 arcmin2 in two fields of view, the VVDS-0226-04 and the VVDS-CDFS-0332-27, and the cosmological parameters ($\Omega_\mathrm{M}$, $\Omega_{\Lambda}$, $h)=(0.3$, 0.7, 0.7). Methods.We study the multi-wavelength non dust-corrected luminosity densities at $0 < z < 2$ from the rest-frame far ultraviolet to the optical passbands, and the rest-frame 1500 Å luminosity functions and densities at $2.7 < z < 5$. Results.They evolve from $z=1.2$ to $z=0.05$ according to $(1+z)^{x}$ with $x = 2.05, 1.94, 1.92, 1.14, 0.73, 0.42$, and 0.30 in the FUV-1500, NUV-2800, U-3600, B-4400, V-5500, R-6500, and I-7900 passbands, respectively. From $z=1.2$ to $z=0.2$ the B-band density for the irregular-like galaxies decreases markedly by a factor 3.5 while it increases by a factor 1.7 for the elliptical-like galaxies. We identify several SFR periods; from $z=5$ to 3.4 the FUV-band density increases by at most 0.5 dex, from $z=3.4$ to 1.2 it decreases by 0.08 dex, from $z=1.2$ to $z=0.05$ it declines steadily by 0.6 dex. For the most luminous $M_{AB}(1500~\AA)<-21$ galaxies the FUV-band density drops by 2 dex from $z=3.9$ to $z=1.2$, and for the intermediate $-21<M_{AB}(1500~\AA)<-20$ galaxies it drops by 2 dex from $z=0.2$ to $z=0$. Comparing with dust corrected surveys, at $0.4 \la z \la 2$ the FUV seems obscured by a constant factor of ${\sim}1.8$–2 mag, while at $z<0.5$ it seems progressively less obscured by up to ${\sim}0.9$–1 mag when the dust-deficient early-type population is increasingly dominating the B-band density. Conclusions.The VVDS results agree with a downsizing picture where the most luminous sources cease to efficiently produce new stars 12 Gyrs ago (at $z\simeq 4$), while intermediate luminosity sources keep producing stars until 2.5 Gyrs ago (at $z\simeq 0.2$). A modest contribution of dry mergers and morphologies evolving towards early-type galaxies might contribute to increase the number density of the bright early types at $z<1.5$. Our observed SFR density is not in agreement with a continuous smooth decrease since $z\sim4$.
Context. Active galactic nuclei (AGN) are thought to play an important role in galaxy evolution. It has been suggested that AGN feedback could be partly responsible for quenching star-formation in ...the hosts, leading to transition from the blue cloud to the red sequence. The transition seems to occur faster for the most massive galaxies, where traces of AGN activity have been found as early as at z < 0.1. The correlation betweenAGN activity, aging of the stellar populations, and stellar mass still needs to be fully understood, especially at high redshifts. Aims. Our aim is to investigate the link between AGN activity, star-formation, and stellar mass of the host galaxy at 0 < z < 1, looking for spectroscopic traces of AGN and aging of the host. This work provides an extension of the existing studies at z < 0.1 and contributes to shed light on galaxy evolution at intermediate redshifts. Methods. We used the zCOSMOS 20k data to create a sample of galaxies at z < 1. We divided the sample into several mass-redshift bins to obtain stacked galaxy spectra with an improved signal-to-noise ratio (S/N). We exploited emission-line diagnostic diagrams to separate AGN from star-forming galaxies. Results. We found an indication of a role for the total galaxy stellar mass in leading galaxy classification. Stacked spectra show AGN signatures above the log M∗/M⊙ > 10.2 threshold. Moreover, the stellar populations of AGN hosts are found to be older than star-forming and composite galaxies. This could be due to the the tendency of AGN to reside in massive hosts. Conclusions. The dependence of the AGN classification on the stellar mass agrees with what has been found in previous research. Together with the evidence of older stellar populations inhabiting the AGN-like galaxies, it is consistent with the downsizing scenario. In particular, our evidence points to an evolutionary scenario where the AGN-feedback is capable of quenching the star formation in the most massive galaxies. Therefore, the AGN-feedback is the best candidate for initiating the passive evolutionary phase of galaxies.
Aims. We present a homogeneous and complete catalogue of optical galaxy groups identified in the purely flux-limited (17.5 ≤ IAB ≤ 24.0) VIMOS-VLT deep redshift Survey (VVDS). Methods. We use mock ...catalogues extracted from the Millennium Simulation, to correct for potential systematics that might affect the overall distribution as well as the individual properties of the identified systems. Simulated samples allow us to forecast the number and properties of groups that can be potentially found in a survey with VVDS-like selection functions. We use them to correct for the expected incompleteness and, to asses in addition, how well galaxy redshifts trace the line-of-sight velocity dispersion of the underlying mass overdensity. In particular, on these mock catalogues we train the adopted group-finding technique i.e., the Voronoi-Delaunay Method (VDM). The goal is to fine-tune its free parameters, recover in a robust and unbiased way the redshift and velocity dispersion distributions of groups (n(z) and n(σ), respectively), and maximize, at the same time, the level of completeness and purity of the group catalogue. Results. We identify 318 VVDS groups with at least 2 members in the range 0.2 ≤ z ≤ 1.0, among which 144 (/30) with at least 3 (/5) members. The sample has an overall completeness of ~60% and a purity of ~50%. Nearly 45% of the groups with at least 3 members are still recovered if we run the algorithm with a particular parameter set that maximizes the purity (~75%) of the resulting catalogue. We use the group sample to explore the redshift evolution of the fraction fb of blue galaxies (U–B ≤ 1) in the redshift range 0.2 ≤ z ≤ 1. We find that the fraction of blue galaxies is significantly lower in groups than in the global population (i.e. in the whole ensemble of galaxies irrespective of their environment). Both of these quantities increase with redshift, the fraction of blue galaxies in groups exhibiting a marginally significant steeper increase. We also investigate the dependence of fb on group richness: not only we confirm that, at any redshift, the blue fraction decreases in systems with increasing richness, but we find that this result continues to hold towards fainter luminosities.
Aims. Our aim is to investigate the history of mass assembly for galaxies of different stellar masses and types. Methods. We selected a mass-limited sample of 4048 objects from the VIMOS VLT Deep ...Survey (VVDS) in the redshift interval 0.5 le z le 1.3. We then used an empirical criterion, based on the amplitude of the 4000 AaBalmer break (D_{\rm n}4000), to separate the galaxy population into spectroscopically early- and late-type systems. The equivalent width of the OII3727 line is used as proxy for the star formation activity. We also derived a type-dependent stellar mass function in three redshift bins. Results. We discuss to what extent stellar mass drives galaxy evolution, showing for the first time the interplay between stellar ages and stellar masses over the past 8 Gyr. Low-mass galaxies have small D_{\rm n}4000 and at increasing stellar mass, the galaxy distribution moves to higher D_{\rm n}4000 values as observed in the local Universe. As cosmic time goes by, we witness an increasing abundance of massive spectroscopically early-type systems at the expense of the late-type systems. This spectral transformation of late-type systems into old massive galaxies at lower redshift is a process started at early epochs (z > 1.3) and continuing efficiently down to the local Universe. This is also confirmed by the evolution of our type-dependent stellar mass function. The underlying stellar ages of late-type galaxies apparently do not show evolution, most likely as a result of a continuous and efficient formation of new stars. All star formation activity indicators consistently point towards a star formation history peaked in the past for massive galaxies, with little or no residual star formation taking place in the most recent epochs. In contrast, most of the low-mass systems show just the opposite characteristics, with significant star formation present at all epochs. The activity and efficiency of forming stars are mechanisms that depend on galaxy stellar mass, and the stellar mass assembly becomes progressively less efficient in massive systems as time elapses. The concepts of star formation downsizing and mass assembly downsizing describe a single scenario that has a top-down evolutionary pattern in how the star formation is quenched, as well as how the stellar mass is grown. The role of (dry) merging events seems to be only marginal at z < 1.3, as our estimated efficiency in stellar mass assembly can possibly account for the progressive accumulation of observed passively evolving galaxies.
Aims. We present a measurement of the dependence of galaxy clustering on galaxy stellar mass at redshift z\sim0.9, based on the first-epoch data from the VVDS-Deep survey. Methods. Concentrating on ...the redshift interval 0.5< z <1.2, we measured the projected correlation function, w_{\rm p}(r_{\rm p}), within mass-selected sub-samples covering the range similar to 10 super(9) and similar to 10 similar to M_\odot. We explored and quantify in detail the observational selection biases due to the flux-limited nature of the survey, both from the data themselves and with a suite of realistic mock samples constructed by coupling the Millennium Simulation to semi-analytic models. We identify the range of masses within which our main conclusions are robust against these effects. Serious incompleteness in mass is present below log\,(M/M_\odot) =9.5, with about two thirds of the galaxies in the range 9<\log\,(M/M_\odot) <9.5 that are lost due to their low luminosity and high mass-to-light ratio. However, the sample is expected to be 100% complete in mass above log\,(M/M_\odot) =10. Results. We present the first direct evidence for a dependence of clustering on the galaxy stellar mass at a redshift as high as z\sim0.85. We quantify this by fitting the projected function w_{\rm p}(r_{\rm p}) with a power-law model. The clustering length increases from r sub(0) =2.76 sub(-0.15) super(+0.17) h super(-1) Mpc for galaxies with mass 10 {9} similar to M_\odot$--> M>10 {9} similar to M_\odot to r sub(0) =4.28 sub(-0.45) super(+0.43) h super(-1) Mpc when only the most massive (10 similar to M_\odot$--> M>10 similar to M_\odot) are considered. At the same time, we observe a significant increase in the slope, which over the same range of masses, changes from gamma=1.67_to gamma=2.28_. Comparison to the SDSS measurements at z\sim0.15 shows that the evolution of w_{\rm p}(r_{\rm p}) is significant for samples of galaxies with M<10 similar to M_\odot, while it is negligible for more massive objects. Considering the growth of structure, this implies that the linear bias b_{\rm L} of the most massive galaxies evolves more rapidly between these two cosmic epochs. We quantify this effect by computing the value of b_{\rm L} from the SDSS and VVDS clustering amplitudes and find that b_{\rm L} decreases from 1.5\pm0.2 at z\sim0.85 to 1.33\pm0.03 at z\sim0.15, for the most massive galaxies, while it remains virtually constant (b_{\rm L}\sim1.3) for the remaining population. Qualitatively, this is the kind of scenario expected for the clustering of dark-matter halos as a function of their total mass and redshift. Our result therefore seems to indicate that galaxies with the highest stellar mass today were originally central objects of the most massive dark-matter halos at earlier times, whose distribution was strongly biased with respect to the overall mass density field.
Aims. We investigate the relationships between three main optical galaxy observables (spectral properties, colors, and morphology), exploiting the data set provided by the COSMOS/zCOSMOS survey. The ...purpose of this paper is to define a simple galaxy classification cube, with a carefully selected sample of ≈1000 galaxies. Methods. Using medium resolution spectra of the first $1k$ zCOSMOS-bright sample, optical photometry from the Subaru/COSMOS observations, and morphological measurements derived from ACS imaging, we analyze the properties of the galaxy population out to z ~ 1. Applying three straightforward classification schemes (spectral, photometric, and morphological), we identify two main galaxy types, which appear to be linked to the bimodality of galaxy population. The three parametric classifications constitute the axes of a “classification cube”. Results. A very good agreement exists between the classification from spectral data (quiescent/star-forming galaxies) and the one based on colors (red/blue galaxies). The third parameter (morphology) is not as well correlated with the first two; in fact, a good correlation between the spectral classification and the classification based on morphological analysis (early-/late-type galaxies) is achieved only after partially complementing the morphological classification with additional color information. Finally, analyzing the 3D-distribution of all galaxies in the sample, we find that about 85% of the galaxies show a fully concordant classification, being either quiescent, red, bulge-dominated galaxies (~20%) or star-forming, blue, disk-dominated galaxies (~65%). These results imply that the galaxy bimodality is a consistent behavior both in morphology, color, and dominant stellar population, at least out to z ~ 1.
Measuring the build-up of stellar mass is one of the main objectives of studies of galaxy evolution. Traditionally, the mass in stars and the star formation rates have been measured by different ...indicators, such as photometric colours, emission lines, and the UV and IR emission. We intend to show that it is possible to derive the physical parameters of galaxies from their broad-band spectral energy distribution out to a redshift of 1.2. This method has the potential to yield the physical parameters of all galaxies in a single field in a homogeneous way, thus overcoming problems with the sample size that particularly plague methods relying on spectroscopy. We use an extensive dataset, assembled in the context of the VVDS survey, which reaches from the UV to the IR and covers a sample of 84 073 galaxies over an area of 0.89 deg2. We also use a library of 100 000 model galaxies with a wide variety of star formation histories (in particular including late bursts of star formation). We find that we can determine the physical parameters stellar mass, age, and star formation rate with good confidence. We validate the star formation rate determination in particular by comparing it to a sample of spectroscopically observed galaxies with an emission-line measurement. While the attenuation in the galaxies shows more scatter, the mean over the sample is unbiased. Metallicity, however, cannot be measured from rest-frame optical photometry alone. As a first application we use our sample to build the number density function of galaxies as a function of stellar mass, specific star formation rate, and redshift. We are then able to study whether the stellar mass function at a later time can be predicted from the stellar mass function and star formation rate distribution at an earlier time. We find that, between redshifts of 1.02 and 0.47, the predicted growth in stellar mass from star formation agrees with the observed one. However, the predicted stellar mass density for massive galaxies is lower than observed, while the mass density of intermediate mass galaxies is overpredicted. This apparent discrepancy can be explained by major and minor mergers. Indeed, when comparing with a direct measurement of the major merger rate from the VVDS survey, we find that major mergers can account for about half of the mass build-up at the massive end. Minor mergers are very likely to contribute the missing fraction.
We model the evolution of the mean galaxy occupation of dark matter haloes over the range 0.1 < z < 1.3, using the data from the VIMOS-VLT Deep Survey. The galaxy projected correlation function ...wp(rp) was computed for a set of luminosity-limited subsamples and fits to its shape were obtained using two variants of halo occupation distribution (HOD) models. These provide us with a set of best-fitting parameters, from which we obtain the average mass of a halo and average number of galaxies per halo. We find that after accounting for the evolution in luminosity and assuming that we are largely following the same population, the underlying dark matter halo shows a growth in mass with decreasing redshift as expected in a hierarchical structure formation scenario. Using two different HOD models, we see that the halo mass grows by 90 per cent over the redshift interval z=0.5, 1.0. This is the first time the evolution in halo mass at high redshifts has been obtained from a single data survey and it follows the simple form seen in N-body simulations with M(z) =M0 e−βz, and β= 1.3 ± 0.30. This provides evidence for a rapid accretion phase of massive haloes having a present-day mass M0∼ 1013.5 h−1 M⊙, with a m > 0.1 M0 merger event occurring between redshifts of 0.5 and 1.0. Furthermore, we find that more luminous galaxies are found to occupy more massive haloes irrespective of the redshift. Finally, the average number of galaxies per halo shows little increase from redshift z∼ 1.0 to ∼0.5, with a sharp increase by a factor of ∼3 from z∼ 0.5 to ∼0.1, likely due to the dynamical friction of subhaloes within their host haloes.
We present the spectrophotometric properties of a sample of 141 emission-line galaxies at redshifts in the range 0.2 < z < 1.0 with a peak around z 0.2, 0.4. The analysis is based on medium ...resolution (Rs = 500-600), optical spectra obtained at VLT and Keck. The targets are mostly "Canada-France Redshift Survey" emission-line galaxies, with the addition of field galaxies randomly selected behind lensing clusters. We complement this sample with galaxy spectra from the "Gemini Deep Deep Survey" public data release. We have computed absolute magnitudes of the galaxies and measured the line fluxes and equivalent widths of the main emission/absorption lines. The last two have been measured after careful subtraction of the fitted stellar continuum using the platefit software originally developed for the SDSS and adapted to our data. We present a careful comparison of this software with the results of manual measurements. The pipeline has also been tested on lower resolution spectra, typical of the "VIMOS/VLT Deep Survey" (Rs = 250), by resampling our medium resolution spectra. We show that we can successfully deblend the most important strong emission lines. These data are primarily used to perform a spectral classification of the galaxies in order to distinguish star-forming galaxies from AGNs. Among the initial sample of 141 emission-line galaxies, we find 7 Seyfert 2 (narrow-line AGN), 115 star-forming galaxies and 16 "candidate" star-forming galaxies. Scientific analysis of these data, in terms of chemical abundances, stellar populations, etc., will be presented in subsequent papers of this serie.
Aims. In this paper we discuss the mix of star-forming and passive galaxies up to z similar to 2, based on the first epoch VIMOS-VLT Deep Survey (VVDS) data. Methods. We compute rest-frame magnitudes ...and colors and analyse the color-magnitude relation and the color distributions. We also use the multi-band VVDS photometric data and spectral templates fitting to derive multi-color galaxy types. Using our spectroscopic dataset we separate galaxies based on a star-formation activity indicator derived combining the equivalent width of the OII emission line and the strength of the D sub( )n(4000) continuum break. Results. In agreement with previous works we find that the global galaxy rest-frame color distribution follows a bimodal distribution at z \le 1, and we establish that this bimodality holds up to at least z =1.5. The details of the rest-frame color distribution depend however on redshift and on galaxy luminosity, with faint galaxies being bluer than the luminous ones over the whole redshift range covered by our data, and with galaxies becoming bluer as redshift increases. This latter blueing trend does not depend, to a first approximation, on galaxy luminosity. The comparison between the spectral classification and the rest-frame colors shows that about 35-40% of the red objects are in fact star forming galaxies. Hence we conclude that the red sequence cannot be used to effectively isolate a sample of purely passively evolving objects within a cosmological survey. We show how multi-color galaxy types have a slightly higher efficiency than rest-frame color in isolating the passive, non star- forming galaxies within the VVDS sample. Connected to these results is also the finding that the color-magnitude relations derived for the color and for the spectroscopically selected early-type galaxies have remarkably similar properties, with the contaminating star-forming galaxies within the red sequence objects introducing no significant offset in the rest frame colors. Therefore the average color of the red objects does not appear to be a very sensitive indicator for measuring the evolution of the early-type galaxy population.
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