O. Cucciati

COSMOS PHOTOMETRIC REDSHIFTS WITH 30-BANDS FOR 2-deg2

We present accurate photometric redshifts in the 2-deg2 COSMOS field. The redshifts are computed with 30 broad, intermediate, and narrow bands covering the UV (GALEX), Visible-NIR (Subaru, CFHT, UKIRT and NOAO) and mid-IR (Spitzer/IRAC). A chi2 template-fitting method (Le Phare) was used and calibrated with large spectroscopic samples from VLT-VIMOS and Keck-DEIMOS. We develop and implement a new method which accounts for the contributions from emission lines (OII, Hbeta, Halpha and Ly) to the spectral energy distributions (SEDs). The treatment of emission lines improves the photo-z accuracy by a factor of 2.5. Comparison of the derived photo-z with 4148 spectroscopic redshifts (i.e. Delta z = zs - zp) indicates a dispersion of sigma_{Delta z/(1+zs)}=0.007 at i<22.5, a factor of 2-6 times more accurate than earlier photo-z in the COSMOS, CFHTLS and COMBO-17 survey fields. At fainter magnitudes i<24 and z<1.25, the accuracy is sigma_{Delta z/(1+zs)}=0.012. The deep NIR and IRAC coverage enables the photo-z to be extended to z~2 albeit with a lower accuracy (sigma_{Delta z/(1+zs)}=0.06 at i~24). The redshift distribution of large magnitude-selected samples is derived and the median redshift is found to range from z=0.66 at 22

zCOSMOS: A large VLT/VIMOS redshift survey covering 0 < z < 3 in the COSMOS field

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) zCOSMOSbright, a magnitude-limited I-band I_(AB) < 22.5 sample of about 20,000 galaxies with 0.1 < z < 1.2 covering the whole 1.7 deg^2 COSMOS ACS field, for which the survey parameters at z ~ 0.7 are designed to be directly comparable to those of the 2dFGRS at z ~ 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^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 < 3.0 and of VIMOS to measure their redshifts using ultraviolet absorption lines.

A test of the nature of cosmic acceleration using galaxy redshift distortions

Observations of distant supernovae indicate that the Universe is now in a phase of accelerated expansion the physical cause of which is a mystery. Formally, this requires the inclusion of a term acting as a negative pressure in the equations of cosmic expansion, accounting for about 75 per cent of the total energy density in the Universe. The simplest option for this ‘dark energy’ corresponds to a ‘cosmological constant’, perhaps related to the quantum vacuum energy. Physically viable alternatives invoke either the presence of a scalar field with an evolving equation of state, or extensions of general relativity involving higher-order curvature terms or extra dimensions. Although they produce similar expansion rates, different models predict measurable differences in the growth rate of large-scale structure with cosmic time. A fingerprint of this growth is provided by coherent galaxy motions, which introduce a radial anisotropy in the clustering pattern reconstructed by galaxy redshift surveys. Here we report a measurement of this effect at a redshift of 0.8. Using a new survey of more than 10,000 faint galaxies, we measure the anisotropy parameter β = 0.70 ± 0.26, which corresponds to a growth rate of structure at that time of f = 0.91 ± 0.36. This is consistent with the standard cosmological-constant model with low matter density and flat geometry, although the error bars are still too large to distinguish among alternative origins for the accelerated expansion. The correct origin could be determined with a further factor-of-ten increase in the sampled volume at similar redshift.

THE zCOSMOS 10k-BRIGHT SPECTROSCOPIC SAMPLE*

We present spectroscopic redshifts of a large sample of galaxies with I_(AB) < 22.5 in the COSMOS field, measured from spectra of 10,644 objects that have been obtained in the first two years of observations in the zCOSMOS-bright redshift survey. These include a statistically complete subset of 10,109 objects. The average accuracy of individual redshifts is 110 km s^(–1), independent of redshift. The reliability of individual redshifts is described by a Confidence Class that has been empirically calibrated through repeat spectroscopic observations of over 600 galaxies. There is very good agreement between spectroscopic and photometric redshifts for the most secure Confidence Classes. For the less secure Confidence Classes, there is a good correspondence between the fraction of objects with a consistent photometric redshift and the spectroscopic repeatability, suggesting that the photometric redshifts can be used to indicate which of the less secure spectroscopic redshifts are likely right and which are probably wrong, and to give an indication of the nature of objects for which we failed to determine a redshift. Using this approach, we can construct a spectroscopic sample that is 99% reliable and which is 88% complete in the sample as a whole, and 95% complete in the redshift range 0.5 < z < 0.8. The luminosity and mass completeness levels of the zCOSMOS-bright sample of galaxies is also discussed.

Improved constraints on the expansion rate of the Universe up to z ∼ 1.1 from the spectroscopic evolution of cosmic chronometers

We present new improved constraints on the Hubble parameter H(z) in the redshift range 0.15 \textless z \textless 1.1, obtained from the differential spectroscopic evolution of early-type galaxies as a function of redshift. We extract a large sample of early-type galaxies ( 11000) from several spectroscopic surveys, spanning almost 8 billion years of cosmic lookback time (0.15 \textless z \textless 1.42). We select the most massive, red elliptical galaxies, passively evolving and without signature of ongoing star formation. Those galaxies can be used as standard cosmic chronometers, as firstly proposed by Jimenez & Loeb (2002), whose (life! Nit age evolution as a function of cosmic time directly probes H (z). We analyze the 4000 angstrom break (D4000) as a function of redshift, use stellar population synthesis models to theoretically calibrate the dependence of the differential age evolution on the differential D4000, and estimate the Hubble parameter taking into account both statistical and systematical errors. We provide 8 new measurements of H(z) (see table 4), and determine its change in H(z) to a precision of 5-12% mapping homogeneously the redshift range up to z 1.1; for the first time, we place a constraint on 11(z) at z not equal 0 with a precision comparable with the one achieved for the Hubble constant (about 5-6% at z similar to 0.2), and covered a redshift range (0.5 \textless z \textless 0.8) which is crucial to distinguish many different quintessence cosmologies. These measurements have been tested to best match a ACDM model, clearly providing a statistically robust indication that the Universe is undergoing an accelerated expansion. This method shows the potentiality to open a new avenue in constrain a variety of alternative cosmologies, especially when future surveys (e.g. Euclid) will open the possibility to extend it up to z similar to 2.

ONGOING AND CO-EVOLVING STAR FORMATION IN zCOSMOS GALAXIES HOSTING ACTIVE GALACTIC NUCLEI

We present a study of the host galaxies of AGN selected from the zCOSMOS survey to establish if accretion onto supermassive black holes and star formation are explicitly linked up to z~1. We identify 152 galaxies that harbor AGN, based on XMM observations of 7543 galaxies (i<22.5). Star formation rates (SFRs), including those weighted by stellar mass, are determined using the [OII]3727 emission-line, corrected for an AGN contribution. We find that the majority of AGN hosts have significant levels of star formation with a distribution spanning ~1-100 Msun yr^-1. The close association between AGN activity and star formation is further substantiated by an increase in the AGN fraction with the youthfulness of their stars as indicated by the rest-frame color (U-V) and spectral index Dn(4000); we demonstrate that mass-selection alleviates an artifical peak falling in the transition region when using luminosity-limited samples. We also find that the SFRs of AGN hosts evolve with cosmic time in a manner that closely mirrors the overall galaxy population and explains the low SFRs in AGNs (z<0.3) from the SDSS. We conclude that the conditions most conducive for AGN activity are a massive host galaxy and a large reservoir of gas. Furthermore, a direct correlation between mass accretion rate onto SMBHs and SFR is shown to be weak although the average ratio is constant with redshift, effectively shifting the evidence for a co-evolution scenario in a statistical manner to smaller physical scales. Our findings illustrate an intermittent scenario with an AGN lifetime substantially shorter than that of star formation and underlying complexities regarding fueling over vastly different physical scales yet to be determined [Abridged].

The VIMOS Public Extragalactic Redshift Survey (VIPERS) - Galaxy clustering and redshift-space distortions at z ≃ 0.8 in the first data release

We present in this paper the general real- and redshift-space clustering properties of galaxies as measured in the first data release of the VIPERS survey. VIPERS is a large redshift survey designed to probe the distant Universe and its large-scale structure at 0.5 < z < 1.2. We describe in this analysis the global properties of the sample and discuss the survey completeness and associated corrections. This sample allows us to measure the galaxy clustering with an unprecedented accuracy at these redshifts. From the redshift-space distortions observed in the galaxy clustering pattern we provide a first measurement of the growth rate of structure at z = 0.8: f\sigma_8 = 0.47 +/- 0.08. This is completely consistent with the predictions of standard cosmological models based on Einstein gravity, although this measurement alone does not discriminate between different gravity models.

The radial and azimuthal profiles of Mg II absorption around 0,5 <z <0,9 zCosmos galaxies of different colors, masses, and environments

We map the radial and azimuthal distribution of Mg II gas within ~ 200 kpc (physical) of ~ 4000 galaxies at redshifts 0.5 1. We investigate the variation of Mg II rest-frame equivalent width (EW) as a function of the radial impact parameter for different subsets of foreground galaxies selected in terms of their rest-frame colors and masses. Blue galaxies have a significantly higher average Mg II EW at close galactocentric radii as compared to the red galaxies. Among the blue galaxies, there is a correlation between Mg II EW and galactic stellar mass of the host galaxy. We also find that the distribution of Mg II absorption around group galaxies is more extended than that for non-group galaxies, and that groups as a whole have more extended radial profiles than individual galaxies. Interestingly, these effects can be satisfactorily modeled by a simple superposition of the absorption profiles of individual member galaxies, assuming that these are the same as those of non-group galaxies, suggesting that the group environment may not significantly enhance or diminish the Mg II absorption of individual galaxies. We show that there is a strong azimuthal dependence of the Mg II absorption within 50 kpc of inclined disk-dominated galaxies, indicating the presence of a strongly bipolar outflow aligned along the disk rotation axis. There is no significant dependence of Mg II absorption on the apparent inclination angle of disk-dominated galaxies.

The VIMOS VLT Deep Survey - Evolution of the major merger rate since z ~ 1 from spectroscopically confirmed galaxy pairs

Context: The rate at which galaxies grow via successive mergers is a key element in understanding the main phases of galaxy evolution. Aims: We measure the evolution of the fraction of galaxies in pairs and the merging rate since redshift z 1 assuming a (H0 = 70 km s-1 Mpc-1, ΩM = 0.3 and ΩΛ = 0.7) cosmology. Methods: From the VIMOS VLT Deep Survey we use a sample of 6464 galaxies with I_AB ≤ 24 to identify 314 pairs of galaxies, each member with a secure spectroscopic redshift, which are close in both projected separation and in velocity. Results: We estimate that at z 0.9, 10.9 ± 3.2% of galaxies with MB(z) ≤ -18-Qz (Q = 1.11) are in pairs with separations Δ rp ≤ 20 h-1 kpc, Δ v≤ 500 km s-1, and with Δ MB ≤ 1.5, significantly larger than 3.8 ± 1.7% at z 0.5; thus, the pair fraction evolves as (1 + z)m with m = 4.73 ± 2.01. For bright galaxies with MB(z = 0) ≤ -18.77, the pair fraction is higher and its evolution with redshift is flatter with m = 1.50 ± 0.76, a property also observed for galaxies with increasing stellar masses. Early-type pairs (dry mergers) increase their relative fraction from 3% at z 0.9 to 12% at z 0.5. The star formation rate traced by the rest-frame [OII] EW increases by 26 ± 4% for pairs with the smallest separation rp ≤ 20 h-1 kpc. Following published prescriptions to derive merger timescales, we find that the merger rate of MB(z) ≤ -18-Qz galaxies evolves as N_mg = (4.96 ± 2.07)×10-4×(1 + z)2.20 ± 0.77 mergers Mpc-3 Gyr-1. Conclusions: The merger rate of galaxies with MB(z) ≤ -18-Qz has significantly evolved since z 1 and is strongly dependent on the luminosity or stellar mass of galaxies. The major merger rate increases more rapidly with redshift for galaxies with fainter luminosities or stellar mass, while the evolution of the merger rate for bright or massive galaxies is slower, indicating that the slow evolution reported for the brightest galaxies is not universal. The merger rate is also strongly dependent on the spectral type of galaxies involved. Late-type mergers were more frequent in the past, while early-type mergers are more frequent today, contributing to the rise in the local density of early-type galaxies. About 20% of the stellar mass in present day galaxies with log(M/M{ȯ}) ≥ 9.5 has been accreted through major merging events since z = 1. This indicates that major mergers have contributed significantly to the growth in stellar mass density of bright galaxies over the last half of the life of the Universe. based on observations obtained with the European Southern Observatory Telescopes at the Paranal Observatory, under programs 072.A-0586 and 073.A-0647.

THE DENSITY FIELD OF THE 10k zCOSMOS GALAXIES

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 present the new algorithm 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. We present a number of tests on mock catalogs used to justify this approach. The ZADE algorithm 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 h^(–1) Mpc transverse direction of the COSMOS field and extending up to Δz ~ 0.05 in redshift. We present the three-dimensional overdensity maps and compare the reconstructed overdensity field to the independently identified virialized 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 virialized structures. A comparison of the large-scale structures in the zCOSMOS data and in the mock catalogs 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, although in the data these overdense regions are in generally larger contiguous structures.