A. Cimatti

Submillimeter galaxies at z similar to 2: Evidence for major mergers and constraints on lifetimes, IMF, and CO-H(2) conversion factor

We report subarcsecond resolution IRAM PdBI millimeter CO interferometry of four z ~ 2 submillimeter galaxies (SMGs), and sensitive CO(3-2) flux limits toward three z ~ 2 UV/optically selected star-forming galaxies. The new data reveal for the first time spatially resolved CO gas kinematics in the observed SMGs. Two of the SMGs show double or multiple morphologies, with complex, disturbed gas motions. The other two SMGs exhibit CO velocity gradients of ~500 km s^−1 across ≤0.2 (1.6 kpc) diameter regions, suggesting that the star-forming gas is in compact, rotating disks. Our data provide compelling evidence that these SMGs represent extreme, short-lived maximum star-forming events in highly dissipative mergers of gas-rich galaxies. The resulting high-mass surface and volume densities of SMGs are similar to those of compact quiescent galaxies in the same redshift range and much higher than those in local spheroids. From the ratio of the comoving volume densities of SMGs and quiescent galaxies in the same mass and redshift ranges, and from the comparison of gas exhaustion timescales and stellar ages, we estimate that the SMG phase duration is about 100 Myr. Our analysis of SMGs and optically/UV selected high-redshift star-forming galaxies supports a universal Chabrier IMF as being valid over the star-forming history of these galaxies. We find that the ^(12)CO luminosity to total gas mass conversion factors at z ~ 2-3 are probably similar to those assumed at z ~ 0. The implied gas fractions in our sample galaxies range from 20% to 50%.

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.

PACS Evolutionary Probe (PEP) - A Herschel Key Program

Deep far-infrared photometric surveys studying galaxy evolution and the nature of the cosmic infrared background are a key strength of the Herschel mission. We describe the scientific motivation for the PACS Evolutionary Probe (PEP) guaranteed time key program and its role within the entire set of Herschel surveys, and the field selection that includes popular multiwavelength fields such as GOODS, COSMOS, Lockman Hole, ECDFS, and EGS. We provide an account of the observing strategies and data reduction methods used. An overview of first science results illustrates the potential of PEP in providing calorimetric star formation rates for high-redshift galaxy populations, thus testing and superseding previous extrapolations from other wavelengths, and enabling a wide range of galaxy evolution studies.

Near‐infrared template spectra of normal galaxies: k‐corrections, galaxy models and stellar populations

We have observed 28 local galaxies in the wavelength range between 1 and 2.4xa0μm in order to define template spectra of the normal galaxies along the Hubble sequence. Five galaxies per morphological type were observed in most cases, and the resulting rms spread of the normalized spectra of each class, including both intrinsic differences and observational uncertainties, is about 1 per cent in K, 2 per cent in H and 3 per cent in J. Many absorption features can be accurately measured. The target galaxies and the spectroscopic aperture (7×53xa0arcsec2) were chosen to be similar to those used by Kinney et al. to define template UV and optical spectra. The two data sets are matched in order to build representative spectra between 0.1 and 2.4xa0μm. The continuum shape of the optical spectra and the relative normalization of the near-IR ones were set to fit the average effective colours of the galaxies of the various Hubble classes. The resulting spectra are used to compute the k-corrections of the normal galaxies in the near-IR bands, and to check the predictions of various spectral synthesis models: while the shape of the continuum is generally well predicted, large discrepancies are found in the absorption lines. Among the other possible applications, here we also show how these spectra can be used to place constraints on the dominant stellar population in local galaxies. Spectra and k-corrections are publicly available and can be downloaded from the web site http://www.arcetri.astro.it/~filippo/spectra.

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 spatial clustering of distant, z ~ 1, early-type galaxies

We examine the spatial clustering of extremely red objects (EROs) found in a relatively large survey of 700 arcmin 2 , containing 400 galaxies with R Ks > 5t oK s=1 9:2. A comoving correlation length r0 = 12 3 h 1 Mpc is derived, under the assumption that the selection function is described by a passively evolving early-type galaxy population, with an eective redshift of z 1:2. This correlation length is very similar to that of local L elliptical galaxies implying, at face value, no signicant clustering evolution in comoving coordinates of early-type galaxies to the limiting depth of our sample, z 1:5. A rapidly evolving clustering bias can be designed to reproduce a null result; however, our data do not show the corresponding strong reduction in the average population density expected for consistency with underlying growth of the mass-function. We discuss our data in the context of recent ideas regarding bias evolution. The uncertainty we quote on r0 accounts for the spikey redshift distribution expected along relatively narrow sightlines, which we quantify with detailed simulations. This is an improvement over the standard use of Limbers equation which, because of its implicit assumption of a smooth selection function, underestimates the true noise by a factor of3 for the parameters of our survey. We propose a general recipe for the analysis of angular clustering, suggesting that any measurement of the angular clustering amplitude, A, has an intrinsic additional uncertainty of A=A = p AC ,w hereAC is the appropriate integral constraint.

The K20 survey II. The dierent spatial clustering of z 1 old and dusty star-forming EROs ?

We compare the 3D clustering of old passively-evolving and dusty star-forming z 1E ROs from the K20 survey. With detailed simulations of clustering, the comoving correlation length of dusty star-forming EROs is constrained to be less than r0 2:5 h 1 Mpc. In contrast, the old EROs are much more positively correlated, with 5:5 < r0=(h 1 Mpc) < 16, consistent with previous claims for z 1 eld early-type galaxies based on analyses of ERO angular clustering. The low level of clustering of dusty star-forming EROs does not support these to be major mergers building up an elliptical galaxy, or typical counterparts of SCUBA sources, but it is instead consistent with the weak clustering of high redshift blue galaxies and of luminous local IRAS galaxies. Current hierarchical merging models can explain the large r0 for z 1 eld early-type galaxies, but fail in matching their high number density and overall old ages.

The Colors of Central and Satellite Galaxies in zCOSMOS Out to z ~= 0.8 and Implications for Quenching

We examine the red fraction of central and satellite galaxies in the large zCOSMOS group catalog out to z ~= 0.8, correcting for both the incompleteness in stellar mass and for the less than perfect purities of the central and satellite samples. We show that at all masses and at all redshifts, the fraction of satellite galaxies that have been quenched, i.e., that are red, is systematically higher than that of centrals, as seen locally in the Sloan Digital Sky Survey (SDSS). The satellite quenching efficiency, which is the probability that a satellite is quenched because it is a satellite rather than a central, is, as locally, independent of stellar mass. Furthermore, the average value is about 0.5, which is also very similar to that seen in the SDSS. We also construct the mass functions of blue and red centrals and satellites and show that these broadly follow the predictions of the Peng et al. analysis of the SDSS groups. Together, these results indicate that the effect of the group environment in quenching satellite galaxies was very similar to what it is today when the universe was about half its present age. European Southern Observatory (ESO), Large Program 175.A-0839.

Proto-groups at 1.8 < z < 3 in the zCOSMOS-deep Sample

We identify 42 candidate groups lying between 1.8 < z < 3.0 from a sample of 3502 galaxies with spectroscopic redshifts in the zCOSMOS-deep redshift survey within this same redshift interval. These systems contain three to five spectroscopic galaxies that lie within 500 kpc in projected distance (in physical space) and within 700 km s^(–1) in velocity. Based on extensive analysis of mock catalogs that have been generated from the Millennium simulation, we examine the likely nature of these systems at the time of observation, and what they will evolve into down to the present epoch. Although few of the member galaxies are likely to reside in the same halo at the epoch we observe them, 50% of the systems will have, by the present epoch, all of the member galaxies in the same halo, and almost all (93%) will have at least some of the potential members in the same halo. Most of the candidate groups can therefore be described as proto-groups. A crude estimate of the overdensities of these structures is also consistent with the idea that these systems are being seen as they assemble. We also examine present-day halos and ask whether their progenitors would have been seen among our candidate groups. For present-day halos between 10^(14) and 10^(15) M_☉ h^(–1), 35% should have appeared among our candidate groups, and this would have risen to 70% if our survey had been fully sampled, so we can conclude that our sample can be taken as representative of a large fraction of such systems. There is a clear excess of massive galaxies above 10^(10) M_☉ around the locations of the candidate groups in a large independent COSMOS photo-z sample, but we see no evidence in this latter data for any color differentiation with respect to the field. This is, however, consistent with the idea that such differentiation arises in satellite galaxies, as indicated at z < 1, if the candidate groups are indeed only starting to be assembled.

Spot the difference: Impact of different selection criteria on observed properties of passive galaxies in zCOSMOS-20k* sample

Aims. We present the analysis of photometric, spectroscopic, and morphological properties for differently selected samples of passive galaxies up to z = 1 extracted from the zCOSMOS-20k spectroscopic survey. This analysis intends toexplore the dependence of galaxy properties on the selection criterion adopted, study the degree of contamination due to star-forming outliers, and provide a comparison between different commonly used selection criteria. This work is a first step to fully investigating the selection effects of passive galaxies for future massive surveys such as Euclid. nMethods. We extracted from the zCOSMOS-20k catalog six different samples of passive galaxies, based on morphology (3336 “morphological” early-type galaxies), optical colors (4889 “red-sequence” galaxies and 4882 “red UVJ” galaxies), specific star-formation rate (2937 “quiescent” galaxies), a best fit to the observed spectral energy distribution (2603 “red SED” galaxies), and a criterion that combines morphological, spectroscopic, and photometric information (1530 “red & passive early-type galaxies”). For all the samples, we studied optical and infrared colors, morphological properties, specific star-formation rates (SFRs), and the equivalent widths of the residual emission lines; this analysis was performed as a function of redshift and stellar mass to inspect further possible dependencies. nResults. We find that each passive galaxy sample displays a certain level of contamination due to blue/star-forming/nonpassive outliers. The morphological sample is the one that presents the higher percentage of contamination, with ~12−65% (depending on the mass range) of galaxies not located in the red sequence, ~25−80% of galaxies with a specific SFR up to ~25 times higher than the adopted definition of passive, and significant emission lines found in the median stacked spectra, at least for log (M/M_⊙) 10.25, very limited tails in sSFR, a median value ~20% higher than the chosen passive cut, and equivalent widths of emission lines mostly compatible with no star-formation activity. However, it is also the less economic criterion in terms of information used. Among the other criteria, we found that the best performing are the red SED and the quiescent ones, providing a percentage of contamination only slightly higher than the red & passive ETGs criterion (on average of a factor of ~2) but with absolute values of the properties of contaminants still compatible with a red, passively evolving population. We also find a strong dependence of the contamination on the stellar mass and conclude that, almost irrespective of the adopted selection criteria, a cut at log (M/M_⊙) > 10.75 provides a significantly purer sample in terms of star-forming contaminants. By studying the restframe color-mass and color−color diagrams, we provided two revised definitions of passive galaxies based on these criteria that better reproduce the observed bimodality in the properties of zCOSMOS-20k galaxies. The analysis of the number densities of the various samples shows evidences of mass-assembly “downsizing”, with galaxies at 10.25 < logu2009 (M/M_⊙) < 10.75 increasing their number by a factor ~2−4 from z = 0.6 to z = 0.2, by a factor ~2−3 from z = 1 to z = 0.2 at 10.75 < logu2009 (M/M_⊙) < 11, and by only ~10−50% from z = 1 to z = 0.2 at 11 < logu2009 (M/M_⊙) < 11.5.