ABSTRACT
Using the wide multiband photometry available in the Cosmic Evolution Survey (COSMOS) field, we explore the host galaxy properties of a large sample of active galactic nuclei (AGNs; ∼1700 ...objects) with Lbol ranging from 1043 to 1047 erg s−1, obtained by combining X‐ray and optical spectroscopic selections. Based on a careful study of their spectral energy distributions, which have been parametrized using a two‐component (AGN+galaxy) model fit, we have derived dust‐corrected rest‐frame magnitudes, colours and stellar masses of the obscured and unobscured AGN hosts up to high redshift (z≲3). Moreover, for the sample of obscured AGNs, we have also derived reliable star formation rates (SFRs). We find that AGN hosts span a large range of stellar masses and SFRs. No colour‐bimodality is seen at any redshift in the AGN hosts, which are found to be mainly massive, red galaxies. Once we have accounted for the colour–mass degeneracy in well‐defined mass‐matched samples, we find a residual (marginal) enhancement of the incidence of AGNs in redder galaxies with lower specific SFRs. We argue that this result might emerge because of our ability to properly account for AGN light contamination and dust extinction, compared to surveys with a more limited multiwavelength coverage. However, because these colour shifts are relatively small, systematic effects could still be considered responsible for some of the observed trends. Interestingly, we find that the probability for a galaxy to host a black hole that is growing at any given ‘specific accretion rate’ (i.e. the ratio of X‐ray luminosity to the host stellar mass) is almost independent of the host galaxy mass, while it decreases as a power law with LX/M*. By analysing the normalization of such a probability distribution, we show how the incidence of AGNs increases with redshift as rapidly as (1 + z)4, which closely resembles the overall evolution of the specific SFR of the entire galaxy population. We provide analytical fitting formulae that describe the probability of a galaxy of any mass (above the completeness limit of the COSMOS) to host an AGN of any given specific accretion rate as a function of redshift. These can be useful tools for theoretical studies of the growing population of black holes within galaxy evolution models. Although AGN activity and star formation in galaxies do appear to have a common triggering mechanism, at least in a statistical sense, within the COSMOS sample, we do not find any conclusive evidence to suggest that AGNs have a powerful influence on the star‐forming properties of their host galaxies.
We present new improved constraints on the Hubble parameter H(z) in the red-shift range 0.15 < z < 1.1, obtained from the differential spectroscopic evolution of early-type galaxies as a function of ...redshift. 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 H(z) at z not = to 0 with a precision comparable with the one achieved for the Hubble constant (about 5-6% at z ~ 0.2), and covered a redshift range (0.5 < z < 0.8) which is crucial to distinguish many different quintessence cosmologies. 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 ~ 2.
ABSTRACT The role of major mergers in galaxy and black hole formation is not well-constrained. To help address this, we develop an automated method to identify late-stage galaxy mergers before ...coalescence of the galactic cores. The resulting sample of mergers is distinct from those obtained using pair-finding and morphological indicators. Our method relies on median-filtering of high-resolution images to distinguish two concentrated galaxy nuclei at small separations. This method does not rely on low surface brightness features to identify mergers, and is therefore reliable to high redshift. Using mock images, we derive statistical contamination and incompleteness corrections for the fraction of late-stage mergers. The mock images show that our method returns an uncontaminated ( ) sample of mergers with projected separations between 2.2 and 8 out to . We apply our new method to a magnitude-limited ( ) sample of 44,164 galaxies from the COSMOS HST/ACS catalog. Using a mass-complete sample with and , we find of systems are late-stage mergers. Correcting for incompleteness and contamination, the fractional merger rate increases strongly with redshift as , in agreement both with earlier studies and with dark matter halo merger rates. Separating the sample into star-forming and quiescent galaxies shows that the merger rate for star-forming galaxies increases strongly with redshift, , while the merger rate for quiescent galaxies is consistent with no evolution, . The merger rate also becomes steeper with decreasing stellar mass. Limiting our sample to galaxies with spectroscopic redshifts from zCOSMOS, we find that the star formation rates and X-ray selected active galactic nucleus (AGN) activity in likely late-stage mergers are higher by factors of relative to those of a control sample. Combining our sample with more widely separated pairs, we find that of star formation and of AGN activity are triggered by close encounters ( ) or mergers, providing additional evidence that major mergers are not the only channels for star formation and black hole growth.
We map the radial and azimuthal distribution of Mg II gas within ~ 200 kpc (physical) of ~ 4000 galaxies at redshifts 0.5 < z < 0.9 using co-added spectra of more than 5000 background galaxies at z > ...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.
We present the Galaxy Stellar Mass Function (GSMF) 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 broad-band ...SED, morphology, spectral properties or star formation activities. We unveil a galaxy bimodality in the global MF, at least up to the z~0.55, 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 Mstar (logM<10.6) is mostly explained by a growth in stellar mass driven by smooth and decreasing SFHs. The low/negligible evolution at higher Mstar sets a limit of 30-15%, decreasing with redshift, to the fraction of major merging. 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 model predictions. For LTGs we find that the number density of blue or spiral galaxies remains almost constant with cosmic time from z~1. Instead, the most extreme population of active star forming galaxies is rapidly decreasing in number density. We suggest, firstly, a transformation from blue active spiral galaxies of intermediate mass into blue quiescent and successively (1-2 Gyr after) into red passive types. The complete morphological transformation into red spheroidal galaxies, 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 expect a negligible evolution of the global Galaxy Baryonic MF. ABRIDGED
Close encounters between galaxies are expected to be a viable mechanism, as predicted by numerical simulations, by which accretion onto supermassive black holes can be initiated. To test this ...scenario, we construct a sample of 562 galaxies (M * > 2.5 X 1010 M ) in kinematic pairs over the redshift range 0.25 < z < 1.05 that are more likely to be interacting than a well-matched control sample of 2726 galaxies not identified as being in a pair, both from the zCOSMOS 20k spectroscopic catalog. Galaxies that harbor an active galactic nucleus (AGN) are identified on the basis of their X-ray emission (L 0.5-10 keV > 2 X 1042 erg s--1) detected by Chandra. We find a higher fraction of an AGN in galaxies in pairs relative to isolated galaxies of similar stellar mass. Our result is primarily due to an enhancement of AGN activity, by a factor of 1.9 (observed) and 2.6 (intrinsic), for galaxies in pairs of projected separation less than 75 kpc and line-of-sight velocity offset less than 500 km s--1. This study demonstrates that close kinematic pairs are conducive environments for black hole growth, either indicating a causal physical connection or an inherent relation, such as, to enhanced star formation. In the Appendix, we describe a method for estimating the intrinsic fractions of galaxies (either in pairs or the field) hosting an AGN with confidence intervals, and an excess fraction in pairs. We estimate that 17.8+8.4 --7.4% of all moderate-luminosity AGN activity takes place within galaxies undergoing early stages of interaction that leaves open the question as to what physical processes are responsible for fueling the remaining ~80% that may include late-stage mergers.
Aims. The role of galaxy mergers in massive galaxy evolution, and in particular to mass assembly and size growth, remains an open question. In this paper we measure the merger fraction and rate, both ...minor and major, of massive early-type galaxies (M ⋆ ≥ 1011 M⊙) in the COSMOS field, and study their role in mass and size evolution. Methods. We used the 30-band photometric catalogue in COSMOS, complemented with the spectroscopy of the zCOSMOS survey, to define close pairs with a separation on the sky plane 10 h-1 kpc ≤ rp ≤ 30 h-1 kpc and a relative velocity Δv ≤ 500 km s-1 in redshift space. We measured both major (stellar mass ratio μ ≡ M ⋆ ,2/M ⋆ ,1 ≥ 1/4) and minor (1/10 ≤ μ < 1/4) merger fractions of massive galaxies, and studied their dependence on redshift and on morphology (early types vs. late types). Results. The merger fraction and rate of massive galaxies evolves as a power-law (1 + z)n, with major mergers increasing with redshift, nMM = 1.4, and minor mergers showing little evolution, nmm ~ 0. When split by their morphology, the minor merger fraction for early-type galaxies (ETGs) is higher by a factor of three than that for late-type galaxies (LTGs), and both are nearly constant with redshift. The fraction of major mergers for massive LTGs evolves faster (nMMLT ~ 4 ) than for ETGs (nMMET= 1.8). Conclusions. Our results show that massive ETGs have undergone 0.89 mergers (0.43 major and 0.46 minor) since z ~ 1, leading to a mass growth of ~30%. We find that μ ≥ 1/10 mergers can explain ~55% of the observed size evolution of these galaxies since z ~ 1. Another ~20% is due to the progenitor bias (younger galaxies are more extended) and we estimate that very minor mergers (μ < 1/10) could contribute with an extra ~20%. The remaining ~5% should come from other processes (e.g., adiabatic expansion or observational effects). This picture also reproduces the mass growth and the velocity dispersion evolution of these galaxies. We conclude from these results, and after exploring all the possible uncertainties in our picture, that merging is the main contributor to the size evolution of massive ETGs at z ≲ 1, accounting for ~50−75% of that evolution in the last 8 Gyr. Nearly half of the evolution due to mergers is related to minor (μ < 1/4) events.
Accurate photometric redshifts are among the key requirements for precision weak lensing measurements. Both the large size of the Sloan Digital Sky Survey (SDSS) and the existence of large ...spectroscopic redshift samples that are flux-limited beyond its depth have made it the optimal data source for developing methods to properly calibrate photometric redshifts for lensing. Here, we focus on galaxy–galaxy lensing in a survey with spectroscopic lens redshifts, as in the SDSS. We develop statistics that quantify the effect of source redshift errors on the lensing calibration and on the weighting scheme, and show how they can be used in the presence of redshift failure and sampling variance. We then demonstrate their use with 2838 source galaxies with spectroscopy from DEEP2 and zCOSMOS, evaluating several public photometric redshift algorithms, in two cases including a full p(z) for each object, and find lensing calibration biases as low as <1 per cent (due to fortuitous cancellation of two types of bias) or as high as 20 per cent for methods in active use (despite the small mean photoz bias of these algorithms). Our work demonstrates that lensing-specific statistics must be used to reliably calibrate the lensing signal, due to asymmetric effects of (frequently non-Gaussian) photoz errors. We also demonstrate that large-scale structure (LSS) can strongly impact the photoz calibration and its error estimation, due to a correlation between the LSS and the photoz errors, and argue that at least two independent degree-scale spectroscopic samples are needed to suppress its effects. Given the size of our spectroscopic sample, we can reduce the galaxy–galaxy lensing calibration error well below current SDSS statistical errors.
We study the impact of the environment on the evolution of galaxies in the zCOSMOS 10 k 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 near-infrared (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, not reproducible by a single Schechter function. As a result of this analysis, 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, ranging from isolated galaxies to rich group members. The evolution toward z = 0 of the transition mass ℳcross, i.e., the mass at which the early- and late-type GSMFs match each other, is more rapid in high density environments, because of a difference in the evolution of the normalisation of GSMFs compared to the total one in the considered environment. The same result is found by studying the relative contributions of different galaxy types, implying that there is a more rapid evolution in overdense regions, in particular for intermediate stellar masses. The rate of evolution is different for sets of galaxy types divided on the basis of their SEDs or their morphologies, tentatively suggesting that the migration from the blue cloud to the red sequence occurs on a shorter timescale than the transformation from disc-like morphologies to ellipticals. Our analysis suggests that environmental mechanisms of galaxy transformation start to be more effective at z < 1. The comparison of the observed GSMFs to the same quantities derived from a set of mock catalogues based on semi-analytical models shows disagreement, in both low and high density environments: in particular, 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.
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