Showing papers by "A. Iovino published in 2011"

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

Abstract: 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.

Dissecting photometric redshift for active galactic nucleus using XMM- and Chandra-COSMOS samples

Abstract: In this paper, we release accurate photometric redshifts for 1692 counterparts to Chandra sources in the central square degree of the Cosmic Evolution Survey (COSMOS) field. The availability of a large training set of spectroscopic redshifts that extends to faint magnitudes enabled photometric redshifts comparable to the highest quality results presently available for normal galaxies. We demonstrate that morphologically extended, faint X-ray sources without optical variability are more accurately described by a library of normal galaxies (corrected for emission lines) than by active galactic nucleus (AGN) dominated templates, even if these sources have AGN-like X-ray luminosities. Preselecting the library on the bases of the source properties allowed us to reach an accuracy σ_(Δz/(1+z(spec))~0.015 with a fraction of outliers of 5.8% for the entire Chandra-COSMOS sample. In addition, we release revised photometric redshifts for the 1735 optical counterparts of the XMM-detected sources over the entire 2 deg^2 of COSMOS. For 248 sources, our updated photometric redshift differs from the previous release by Δz > 0.2. These changes are predominantly due to the inclusion of newly available deep H-band photometry (H_(AB) = 24 mag). We illustrate once again the importance of a spectroscopic training sample and how an assumption about the nature of a source together, with the number and the depth of the available bands, influences the accuracy of the photometric redshifts determined for AGN. These considerations should be kept in mind when defining the observational strategies of upcoming large surveys targeting AGNs, such as eROSITA at X-ray energies and the Australian Square Kilometre Array Pathfinder Evolutionary Map of the Universe in the radio band.

Dissecting Photometric redshift for Active Galactic Nuclei using XMM- and Chandra-COSMOS samples

Abstract: With this paper, we release accurate photometric redshifts for 1692 counterparts to Chandra sources in the central square degree of the COSMOS field. The availability of a large training set of spectroscopic redshifts that extends to faint magnitudes enabled photometric redshifts comparable to the highest quality results presently available for normal galaxies. We demonstrate that morphologically extended, faint X-ray sources without optical variability are more accurately described by a library of normal galaxies (corrected for emission lines) than by AGN-dominated templates, even if these sources have AGN-like X-ray luminosities. Preselecting the library on the bases of the source properties allowed us to reach an accuracy sigma_(Delta z/(1+z_spec)) \sim0.015 with a fraction of outliers of 5.8% for the entire Chandra-COSMOS sample. In addition, we release revised photometric redshifts for the 1735 optical counterparts of the XMM-detected sources over the entire 2 sq. deg.of COSMOS. For 248 sources, our updated photometric redshift differs from the previous release by Delta z>0.2. These changes are predominantly due to the inclusion of newly available deep H-band photometry H_AB=24 mag. We illustrate once again the importance of a spectroscopic training sample and how an assumption about the nature of a source together with the number and the depth of the available bands influence the accuracy of the photometric redshifts determined for AGN. These considerations should be kept in mind when defining the observational strategies of upcoming large surveys targeting AGN, such as eROSITA at X-ray energies and ASKAP/EMU in the radio band.

The Impact of Galaxy Interactions on Active Galactic Nucleus Activity in zCosmos

Abstract: 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 × 10^(10) M_☉) in kinematic pairs over the redshift range 0.25 2 × 10^(42) 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.

The impact of galaxy interactions on AGN activity in zCOSMOS

Abstract: 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*>2.5x10^10 M_sun) in kinematic pairs over the redshift range 0.25 2x10^42 erg s^-1) detected by Chandra. We find a higher fraction of 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 to estimate 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_{-7.4}^{+8.4}% 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.

Black hole accretion and host galaxies of obscured quasars in XMM-COSMOS

Abstract: Aims. We explore the connection between black hole growth at the center of obscured quasars selected from the XMM-COSMOS survey and the physical properties of their host galaxies. We study a bolometric regime ( ⟨ L_(bol) ⟩ = 8 × 10^(45) erg s^(-1)) where several theoretical models invoke major galaxy mergers as the main fueling channel for black hole accretion. Methods. To derive robust estimates of the host galaxy properties, we use an SED fitting technique to distinguish the AGN and host galaxy emission. We evaluate the effect on galaxy properties estimates of being unable to remove the nuclear emission from the SED. The superb multi-wavelength coverage of the COSMOS field allows us to obtain reliable estimates of the total stellar masses and star formation rates (SFRs) of the hosts. We supplement this information with a morphological analysis of the ACS/HST images, optical spectroscopy, and an X-ray spectral analysis. Results. We confirm that obscured quasars mainly reside in massive galaxies (M_⋆ > 10^(10)M_⊙) and that the fraction of galaxies hosting such powerful quasars monotonically increases with the stellar mass. We stress the limitation of the use of rest-frame color − magnitude diagrams as a diagnostic tool for studying galaxy evolution and inferring the influence that AGN activity can have on such a process. We instead use the correlation between SFR and stellar mass found for star-forming galaxies to discuss the physical properties of the hosts. We find that at z ~ 1, ≈62% of Type-2 QSOs hosts are actively forming stars and that their rates are comparable to those measured for normal star-forming galaxies. The fraction of star-forming hosts increases with redshift: ≈ 71% at z ~ 2, and 100% at z ~ 3. We also find that the evolution from z ~ 1 to z ~ 3 of the specific SFR of the Type-2 QSO hosts is in excellent agreement with that measured for star-forming galaxies. From the morphological analysis, we conclude that most of the objects are bulge-dominated galaxies, and that only a few of them exhibit signs of recent mergers or disks. Finally, bulge-dominated galaxies tend to host Type-2 QSOs with low Eddington ratios (λ 0.1).

The evolution of quiescent galaxies at high redshifts (z≥ 1.4)

Abstract: 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 deg^2 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 (ma_( 3.6 μm) 2.5 (log_ρ[M_⊙ Mpc^(−3)]~ 6), meaning that efficient star formation had to take place before that time.

The evolution of quiescent galaxies at high redshift (z > 1.4)

Abstract: We have studied the evolution of high redshift quiescent galaxies over an effective area of ~1.7 deg^2 in the COSMOS field. Galaxies have been divided according to their star-formation activity and the evolution of the different populations has been investigated in detail. We have studied an IRAC (mag_3.6 1.4 with multi-wavelength coverage. We have derived accurate photometric redshifts (sigma=0.06) and other important physical parameters through a SED-fitting procedure. We have divided our sample into actively star-forming, intermediate and quiescent galaxies depending on their specific star formation rate. We have computed the galaxy stellar mass function 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 (log(M/M_sun)~10.65), weakly star forming stellar populations with low dust extinction (E(B-V) 11, while the quiescent population increases from 10% to 50% 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 (2007) model is the one that better reproduces the data. Finally, we calculate the stellar mass density of the star-forming and quiescent populations finding that there is already a significant number of quiescent galaxies at z > 2.5 (rho~6.0 MsunMpc^-3).

The zCOSMOS-Bright survey: the clustering of early and late galaxy morphological types since z≃ 1

Abstract: We measure the spatial clustering of galaxies as a function of their morphological type at z similar or equal to 0.8, for the first time in a deep redshift survey with full morphological information. This is obtained by combining high-resolution Hubble Space Telescope imaging and Very Large Telescope spectroscopy for about 8500 galaxies to with accurate spectroscopic redshifts from the zCOSMOS-Bright redshift survey. At this epoch, early-type galaxies already show a significantly stronger clustering than late-type galaxies on all probed scales. A comparison to the Sloan Digital Sky Survey Data at z similar or equal to 0.1 shows that the relative clustering strength between early and late morphological classes tends to increase with cosmic time at small separations, while on large scales it shows no significant evolution since z similar or equal to 0.8. This suggests that most early-type galaxies had already formed in intermediate and dense environments at this epoch. Our results are consistent with a picture in which the relative clustering of different morphological types between z similar or equal to 1 and 0 reflects the evolving role of environment in the morphological transformation of galaxies, on top of a global evolution driven by mass.

Comparison of the VIMOS-VLT deep survey with the Munich semi-analytical model - I. Magnitude counts, redshift distribution, colour bimodality, and galaxy clustering

Abstract: Aims. This paper presents a detailed comparison between high-redshift observations from the VIMOS-VLT Deep Survey (VVDS) and predictions from the Munich semi-analytical model of galaxy formation. In particular, we focus this analysis on the magnitude, redshift, and colour distributions of galaxies, as well as their clustering properties. Methods. We constructed 100 quasi-independent mock catalogues, using the output of the semi-analytical model presented in De Lucia & Blaizot (2007, MNRAS, 375, 2). We then applied the same observational selection function of the VVDS-Deep survey, so as to carry out a fair comparison between models and observations. Results. We find that the semi-analytical model reproduces well the magnitude counts in the optical bands. It tends, however, to overpredict the

THE NONLINEAR BIASING OF THE zCOSMOS GALAXIES UP TO z ∼ 1 FROM THE 10k SAMPLE

Abstract: We use the overdensity field reconstructed in the volume of the COSMOS area to study the nonlinear biasing of the zCOSMOS galaxies. The galaxy overdensity field is reconstructed using the current sample of ~8500 accurate zCOSMOS redshifts at I(AB) = b(delta,z,R) delta is of the following characteristic shape. The function vanishes in the most underdense regions and then sharply rises in a nonlinear way towards the mean densities. is almost a linear tracer of the matter in the overdense regions, up to the most overdense regions in which it is nonlinear again and the local effective slope of vs. delta is smaller than unity. The function is evolving only slightly over the redshift interval 0.4

X-ray Groups of Galaxies at 0.5<z<1 in zCOSMOS: Increased AGN Activities in High Redshift Groups

Abstract: We present a photometric and spectroscopic study of galaxies at 0.5

The zCOSMOS redshift survey : Influence of luminosity, mass and environment on the galaxy merger rate

Abstract: The contribution of major mergers to galaxy mass assembly along cosmic time is an important ingredient to the galaxy evolution scenario We aim to measure the evolution of the merger rate for both luminosity/mass selected galaxy samples and investigate its dependence with the local environment We use a sample of 10644 spectroscopically observed galaxies from the zCOSMOS redshift survey to identify pairs of galaxies destined to merge, using only pairs for which the velocity difference and projected separation of both components with a confirmed spectroscopic redshift indicate a high probability of merging We have identified 263 spectroscopically confirmed pairs with r_p^{max} = 100 h^{-1} kpc We find that the density of mergers depends on luminosity/mass, being higher for fainter/less massive galaxies, while the number of mergers a galaxy will experience does not depends significantly on its intrinsic luminosity but rather on its stellar mass We find that the pair fraction and merger rate increase with local galaxy density, a property observed up to redshift z=1 We find that the dependence of the merger rate on the luminosity or mass of galaxies is already present up to redshifts z=1, and that the evolution of the volumetric merger rate of bright (massive) galaxies is relatively flat with redshift with a mean value of 3*10^{-4} (8*10^{-5} respectively) mergers h^3 Mpc^{-3} Gyr^{-1} The dependence of the merger rate with environment indicates that dense environments favors major merger events as can be expected from the hierarchical scenario The environment therefore has a direct impact in shapping-up the mass function and its evolution therefore plays an important role on the mass growth of galaxies along cosmic time

The Role of Galaxy Interaction in Environmental Dependence of the Star Formation Activity at z~1.2

Abstract: In order to understand environmental effects on star formation in high-redshift galaxies, we investigate the physical relationships between the star formation activity, stellar mass, and environment for z ~1.2 galaxies in the 2 deg^2 COSMOS field. We estimate star formation using the [OII] emission line and environment from the local galaxy density. Our analysis shows that for massive galaxies M_*>10^10 M_sun, the fraction of [OII] emitters in high-density environments is 1.7 times higher than in low-density environments, while the [OII] emitter fraction does not depend on environment for low-mass M_* 10^10 M_sun. In addition, massive galaxies are more likely to have companions in high-density environments. However, although the "number" of star forming galaxies increases for massive galaxies with close companions and in dense environments, the "average" star formation rate of star forming galaxies at a given mass is independent of environment and the presence/absence of a close companion. These results suggest that interactions and/or mergers in high-density environment could induce star formation in massive galaxies at z~1.2, increasing the fraction of star-forming galaxies with M_* > 10^10 M_sun.

The bimodality of the 10k zCOSMOS-bright galaxies up to z ~ 1: a new statistical and portable classification based on optical galaxy properties

Abstract: Aims. Our goal is to develop a new and reliable statistical method to classify galaxies from large surveys. We probe the reliability of the method by comparing it with a three-dimensional classification cube, using the same set of spectral, photometric and morphological parameters. Methods. We applied two different methods of classification to a sample of galaxies extracted from the zCOSMOS redshift survey, in the redshift range 0.5 ≲ z ≲ 1.3. The first method is a combination of three independent classification schemes – a spectroscopic one based on the strength of the continuum break at 4000 A and the rest-frame equivalent width of the [O ii] emission line, a photometric one based on the observed B − z colours, and a morphological one. The second method exploits an entirely new approach based on statistical analyses like principal component analysis (PCA) and unsupervised fuzzy partition (UFP) clustering method. The PCA+UFP method has also been applied to a lower redshift sample (z ≲ 0.5), exploiting the same set of data but replacing the spectroscopic indicators with the equivalent width of Hα. Results. The comparison between the two methods shows fairly good agreement on the definition on the two main populations, the early-type and the late-type galaxies. Our PCA+UFP method of classification is robust, flexible and capable of identifying the two main populations of galaxies as well as an intermediate population. The intermediate galaxy population shows many of the properties of “green valley” galaxies, and constitutes a more coherent and homogeneous population. The large redshift range of the studied sample allows us to characterize downsizing: galaxies with masses of the order of 3 × 10^(10) M_⊙ are predominantly found in the transition from the late-type to the early-type group at z > 0.5, while galaxies with lower masses, of the order of 10^(10) M_⊙, are in transition at later epochs. Galaxies with M 5 × 10^(10 M_⊙) have mostly completed their transition to the early-type regime before z ~ 1.

The radial and azimuthal profiles of Mg II absorption around 0.5 < z < 0.9 zCOSMOS galaxies of different colors, masses and environments

Abstract: 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 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 equivalent width at close galactocentric radii as compared to the red galaxies. Amongst the blue galaxies, there is a correlation between Mg II equivalent width 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: A Homogeneous Galaxy Group Catalogue Upto z ~ 1

Abstract: We present a homogeneous and complete catalogue of optical galaxy groups identified in the VIMOS-VLT spectroscopic Deep Survey (VVDS). We use mock catalogues extracted from the Millennium simulation to study the potential systematics that might affect the overall distribution of the identified systems. We train on these mock catalogues the adopted group-finding technique (the Voronoi-Delaunay Method, VDM), to recover in an unbiased way the redshift and velocity dispersion distributions of groups and maximise the level of completeness (C) and purity (P) of the group catalogue. We identify 318(/144) VVDS groups with at least 2(/3) members within 0. 2 ≤ z ≤ 1. 0, globally with C = 60% and P = 50%. We use the group sample to study the redshift evolution of the fraction f b of blue galaxies within 0. 2 ≤ z ≤ 1 in both groups and in the total VVDS galaxy sample.

The Role of Galaxy Stellar Mass in the Colour–Density Relation up to z ~ 1

Abstract: It is well known that galaxy properties correlate with the local environment in which galaxies reside. In contrast, it is still matter of debate why and when these environmental dependences originate, and whether only one “main” property depends on environment, thus driving all the others environmental dependences via the correlations among properties themselves. We use the first ∼ 10, 000 spectra of the zCOSMOS sample (I ≤ 22. 5) to study the role of galaxy stellar mass in the colour–density relation up to z = 1. We confirm that within a luminosity-limited sample (M B ≤ − 20. 5 − z) red galaxies reside mainly in high densities (δ) at least up to z = 1. This trend becomes weaker for increasing redshifts, and it is mirrored by the D n 4, 000 \r{A} break–density relation. We also find that up to z ∼ 1 the fraction of galaxies with log(EW[OII]) ≥ 1. 15 is higher for lower δ. Given the triple dependence among galaxy colours, stellar mass and δ, the colour–δ relation that we find can be due to the broad range of stellar masses embedded in the sample. We find that once mass is fixed the colour-δ relation is globally flat up to z ∼ 1 for galaxies with log(M ∕ M sun ) ≥ 10. 7. 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 sun ) ≤ 10. 7 depends on δ. We suggest a scenario in which the colour depends primarily on stellar mass, but for an intermediate mass regime (10. 2 ≤ log(M ∕ M sun ) ≤ 10. 7) the local density modulates this dependence. These lower mass galaxies formed more recently, in an epoch when evolved structures were already in place, and their longer SFH allowed environment-driven physical processes to operate during longer periods of time.

The DPOSS II distant compact group survey: the EMMI-NTT spectroscopic sample

Abstract: This paper presents the results of the redshift survey of 138 candidate compact groups from the DPOSS II catalog (Iovino et al., 2003; de Carvalho et al. 2005), which extends the available redshift range of spectroscopically confirmed compact groups of galaxies to z~0.2. The aims of the survey are to confirm group membership via spectroscopic redshift information, to measure the characteristic parameters of the confirmed groups, namely mass, radius, luminosity, velocity dispersion and crossing time, and to compare them with those of nearby compact groups. Using available information from the literature, we also studied the surrounding group environment and searched for additional, previously unknown, group members, or larger scale structures to whom the group might be associated. Of the 138 observed groups, 96 had three or more concordant galaxies, i.e. a 70% success rate. Of these 96, 62 are isolated on the sky, while the other 34 are close on the sky to a larger scale structure. The remaining objects turned out to be couple of pairs or chance projection of galaxies on the sky. Group environment and the characteristics parameters (mass, crossing time, velocity dispersion) are evaluated and discussed.