arXiv subjects

P. Popesso

Publications and source records attributed to P. Popesso.

122 records · Page 7Linked to original sources

arXiv2007

The Great Observatories Origins Deep Survey. VLT/FORS2 Spectroscopy in the GOODS-South Field: Part III

Aims. We present the full data set of the spectroscopic campaign of the ESO/GOODS program in the GOODS-South field, obtained with the FORS2 spectrograph at the ESO/VLT. Method. Objects were selected as candidates for VLT/FORS2 observations primarily based on the expectation that the detection and measurement of their spectral features would benefit from the high throughput and spectral resolution of FORS2. The reliability of the redshift estimates is assessed using the redshift-magnitude and color-redshift diagrams, and comparing the results with public data. Results. Including the third part of the spectroscopic campaign (12 masks) to the previous work (26 masks, Vanzella et al. 2005, 2006), 1715 spectra of 1225 individual targets have been analyzed. The actual spectroscopic catalog provides 887 redshift determinations. The typical redshift uncertainty is estimated to be sigma(z) ~ 0.001. Galaxies have been selected adopting different color criteria and using photometric redshifts. The resulting redshift distribution typically spans two domains: from z=0.5 to 2 and z=3.5 to 6.3. The reduced spectra and the derived redshifts are released to the community through the ESO web page http://www.eso.org/science/goods/

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arXiv2007

The reversal of the star formation-density relation in the distant universe

We study the relationship between the local environment of galaxies and their star formation rate (SFR) in the Great Observatories Origins Deep Survey, GOODS, at z~1, from ultradeep imaging at 24 microns with the MIPS camera onboard Spitzer. We show that the star formation-density relation observed locally was reversed at z~1: the average SFR of an individual galaxy increased with local galaxy density when the universe was less than half its present age. Hierarchical galaxy formation models (simulated lightcones from the Millennium model) predicted such a reversal to occur only at only at earlier epochs (z>2) and at a lower level. We present a remarkable structure at z~1.016, containing X-ray traced galaxy concentrations, which will eventually merge into a Virgo-like cluster. This structure illustrates how the individual SFR of galaxies increases with density at the ~1-2 Mpc scale. The SFR of z~1 galaxies is found to correlate with stellar mass suggesting that mass plays a role in the observed star formation-density trend. However the specific SFR (=SFR/M*) decreases with stellar mass while it increases with galaxy density, which implies that the environment does directly affect the star formation activity of galaxies. Major mergers do not appear to be the unique or even major cause for this effect since nearly half (46%) of the luminous infrared galaxies (LIRGs) at z~1 present the HST-ACS morphology of spirals, while only a third present a clear signature of major mergers. The remaining galaxies are divided into compact (9%) and irregular (14%) galaxies. Moreover, the specific SFR of major mergers is only marginally stronger than that of spirals. Reproducing the SFR-density relation at z ~ 1 is a new challenge for models, requiring a correct balance between mergers and in-situ star formation at early epochs.

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arXiv2006

The AGN fraction - velocity dispersion relation in clusters of galaxies

Some previous investigations have found that the fraction (f_AGN) of active galactic nuclei (AGNs) is lower in clusters than in the field. This can result from the suppression of galaxy-galaxy mergers in high-velocity dispersion (sigma_v) clusters, if the formation and/or fueling of AGNs is directly related to the merging process. We investigate the existence of a relation between f_AGN and sigma_v in galaxy clusters in order to shed light on the formation and evolution processes of AGNs and cluster galaxies. Using data from the Sloan Digital Sky Survey we determine f_AGN and sigma_v for the clusters in two samples, extracted from the catalogs of Popesso et al. (2006a) and Miller et al. (2005), and excluding clusters with significant evidence for substructures. We find a significant f_AGN-sigma_v anti-correlation. Clusters with sigma_v lower and, respectively, higher than 500 km/s have AGN fractions of $0.21 \pm 0.01$ and $0.15 \pm 0.01$, on average. The f_AGN-sigma_v relation can be described by a model that assumes f_AGN is proportional to the galaxies merging rate, plus a constant. Since f_AGN increases with decreasing sigma_v, AGNs are likely to have played a significant role in heating the intra-cluster medium and driving galaxy evolution in cluster precursors and groups.

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arXiv2006

RASS-SDSS Galaxy Cluster Survey. VI. The dependence of the cluster SFR on the cluster global properties

Using a subsample of 79 nearby clusters from the RASS-SDSS galaxy cluster catalogue of Popesso et al. (2005a), we perform a regression analysis between the cluster integrated star formation rate (Sigma_SFR) the cluster total stellar mass (M_star), the fractions of star forming (f_SF) and blue (f_b) galaxies and other cluster global properties, namely its richness (N_gal, i.e. the total number of cluster members within the cluster virial radius), velocity dispersion (sigma_v), virial mass (M_200), and X-ray luminosity (L_X). All cluster global quantities are corrected for projection effects before the analysis. Galaxy SFRs and stellar masses are taken from the catalog of Brinchmann et al. (2004), which is based on SDSS spectra. We only consider galaxies with M_r <= -20.25 in our analysis, and exclude AGNs. We find that both Sigma_SFR and M_star are correlated with all the cluster global quantities. A partial correlation analysis show that all the correlations are induced by the fundamental one between Sigma_SFR and N_gal, hence there is no evidence that the cluster properties affect the mean SFR or M_star per galaxy. The relations between Sigma_SFR and M_star, on one side, and both N_gal and M_200, on the other side, are linear, i.e. we see no evidence that different clusters have different SFR or different M_star per galaxy and per unit mass. The fraction f_SF does not depend on any cluster property considered, while f_b does depend on L_X. We note that a significant fraction of star-forming cluster galaxies are red (~25% of the whole cluster galaxy population). We conclude that the global cluster properties are unable to affect the SF properties of cluster galaxies, but the presence of the X-ray luminous intra-cluster medium can affect their colors, perhaps through the ram-pressure stripping mechanism.

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arXiv2006

Comparing dynamical and photometric-stellar masses of early-type galaxies at z ~ 1

The purpose of this study is to explore the relationship between galaxy stellar masses, based on multiwavelength photometry spectral template fitting and dynamical masses based on published velocity dispersion measurements, for a sample of 48 early-type galaxies at z ~ 1 with HST/ACS morphological information. We determine photometric-stellar masses and perform a quantitative morphological analysis of cluster and field galaxies at redshift 0.6 < z < 1.2, using ground- and space-based multiwavelegth data available on the GOODS-S field and on the field around the X-ray luminous cluster RDCS1252.9-2927 at z = 1.24. We use multi-band photometry over 0.4-8um from HST/ACS, VLT/ISAAC and Spitzer/IRAC to estimate photometric-stellar masses using Composite Stellar Population (CSP) templates computed with PEGASE.2 models. We compare stellar masses with those obtained using CSPs built with Bruzual & Charlot and Maraston models. We then compare photometric-stellar mass and dynamical mass estimates as a function of morphological parameters obtained from HST/ACS imaging. Based on our sample, which spans the mass range log(Mphot)=[10, 11.5], we find that 1) PEGASE.2, BC03, M05 yield consistent photometric-stellar masses for early-type galaxies at z ~ 1 with a small scatter (0.15 dex rms); 2) adopting a Kroupa IMF, photometric-stellar masses match dynamical mass estimates for early-type galaxies with an average offset of 0.27 dex; 3) assuming a costant IMF, increasing dark matter fraction with the increasing galaxy mass can explain the observed trend.

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arXiv2006

RASS-SDSS Galaxy Cluster Survey. VII. On the Cluster Mass to Light ratio and the Halo Occupation Distribution

We explore the mass-to-light ratio in galaxy clusters and its relation to the cluster mass. We study the relations among the optical luminosity ($L_{op}$), the cluster mass ($M_{200}$) and the number of cluster galaxies within $r_{200}$ ($N_{gal}$) in a sample of 217 galaxy clusters with confirmed 3D overdensity. We correct for projection effects, by determining the galaxy surface number density profile in our cluster sample. This is best fitted by a cored King profile in low and intermediate mass systems. The core radius decreases with cluster mass, and, for the highest mass clusters, the profile is better represented by a generalized King profile or a cuspy Navarro, Frenk & White profile. We find a very tight proportionality between $L_{op}$ and $N_{gal}$, which, in turn, links the cluster mass-to-light ratio to the Halo Occupation Distribution $N_{gal}$ vs. $M_{200}$. After correcting for projection effects, the slope of the $L_{op}-M_{200}$ and $N_{gal}-M_{200}$ relations is found to be $0.92\pm0.03$, close, but still significantly less than unity. We show that the non-linearity of these relations cannot be explained by variations of the galaxy luminosity distributions and of the galaxy M/L with the cluster mass. We suggest that the nonlinear relation between number of galaxies and cluster mass reflects an underlying nonlinear relation between number of subhaloes and halo mass.

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arXiv2006

RASS-SDSS Galaxy Cluster Survey. V. The X-ray-Underluminous Abell Clusters

In this paper we consider a large sample of optically selected clusters, in order to elucidate the physical reasons for the existence of X-ray underluminous clusters. For this purpose we analyze the correlations of the X-ray and optical properties of a sample of 137 spectroscopically confirmed Abell clusters in the SDSS database. We search for the X-ray counterpart of each cluster in the ROSAT All Sky Survey. We find that 40% of our clusters have a marginal X-ray detection or remain undetected in X-rays. These clusters appear too X-ray faint on average for their velocity dispersion determined mass, i.e. they do not follow the scaling relation between X-ray luminosity and virial mass traced by the other clusters. On the other hand, they do follow the general scaling relation between optical luminosity and virial mass. We refer to these clusters as the X-ray-Underluminous Abell clusters (AXU clusters, for short) and designate as 'normal' the X-ray detected Abell systems. We examine the distributions and properties of the galaxy populations of the normal and the AXU clusters, separately. The AXU clusters are characterized by leptokurtic (more centrally concentrated than a Gaussian) velocity distribution of their member galaxies in the outskirts ($1.5 < r/r_{200} \leq 3.5$), as expected for the systems in accretion. In addition, the AXU clusters have a higher fraction of blue galaxies in the external region and show a marginally significant paucity of galaxies at the center. Our results seem to support the interpretation that the AXU clusters are systems in formation undergoing a phase of mass accretion. Their low X-ray luminosity should be due to the still accreting Intracluster gas or to an ongoing merging process.

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arXiv2006

The Great Observatories Origins Deep Survey VLT/FORS2 Spectroscopy in the GOODS-South Field: Part II

We present the second campaign of the ESO/GOODS program of spectroscopy of faint galaxies in the GOODS-South field. Objects were selected as candidates for VLT/FORS2 observations primarily based on the expectation that the detection and measurement of their spectral features would benefit from the high throughput and spectral resolution of FORS2. The reliability of the redshift estimates is assessed using the redshift-magnitude and color-redshift diagrams and comparing the results with public data. 807 spectra of 652 individua targets have been obtained in service mode with the FORS2 spectrograph at the ESO/VLT, providing 501 redshift determinations. The typical redshift uncertainty is estimated to be sigma_z ~ 0.001. Galaxies have been selected adopting three different color criteria and using the photometric redshifts.The resulting redshift distribution typically spans two redshift domains: from z=0.5 to 2 and z=3.5 to 6.2. In particular, 94 B435-,V606-,i775 "dropout" Lyman break galaxies have been observed, yielding redshifts for 65 objects in the interval 3.4<z<6.2. Three sources have been serendipitously discovered in the redshift interval 4.8<z<5.5. Together with the previous release, 930 sources have now been observed and 724 redshift determinations have been carried out. The reduced spectra and the derived redshifts are released to the community through the ESO web page http://www.eso.org/science/goods/. Large scale structures are clearly detected at z=0.666, 0.734, 1.096, 1.221, 1.300, and 1.614. A sample of 34 sources with tilted [OII]3727 emission has been identified, 32 of them in the redshift range 0.9<z<1.5.

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arXiv2005

RASS-SDSS Galaxy Cluster Survey. IV. A ubiquitous dwarf galaxy population in clusters

We analyze the Luminosity Functions (LFs) of a subsample of 69 clusters from the RASS-SDSS galaxy cluster catalog. When calculated within the cluster physical sizes, given by r200 or r500, all the cluster LFs appear to have the same shape, well fitted by a composite of two Schechter functions with a marked upturn and a steepening at the faint-end. Previously reported cluster-to-cluster variations of the LF faint-end slope are due to the use of a metric cluster aperture for computing the LF of clusters of different masses. We determine the composite LF for early- and late-type galaxies, where the typing is based on the galaxy u-r colors. The late-type LF is well fitted by a single Schechter function with a steep slope (alpha=-2.0 in the r band, within r200}). The early-type LF instead cannot be fitted by a single Schechter function, and a composite of two Schechter functions is needed. The faint-end upturn of the global cluster LF is due to the early-type cluster galaxies. The shape of the bright-end tail of the early-type LF does not seem to depend upon the local galaxy density or the distance from the cluster center. The late-type LF shows a significant variation only very near the cluster center. On the other hand, the faint-end tail of the early-type LF shows a significant and continuous variation with the environment. We provide evidence that the process responsible for creating the excess population of dwarf early type galaxies in clusters is a threshold process that occurs when the density exceeds ~500 times the critical density of the Universe. We interpret our results in the context of the 'harassment' scenario, where faint early-type cluster galaxies are predicted to be the descendants of tidally-stripped late-type galaxies.

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arXiv2004

The RASS-SDSS Galaxy Cluster Survey. III. Scaling relations of galaxy clusters

We use the RASS-SDSS galaxy cluster sample to compare the quality of optical and X-ray luminosities as predictors of other cluster properties such as their masses, temperatures, and velocity dispersions. We use the SDSS spectroscopic data to estimate the velocity dispersions and the virial masses of a subsample of 69 clusters within r_{500} and r_{200}. The ASCA temperature of the intra-cluster medium, T_X, is retrieved from the literature for a subsample of 49 clusters. For this subsample we estimate the cluster masses also by using the mass-temperature relation. We show that the optical luminosity, L_{op}, correlates with the cluster mass much better than the X-ray luminosity, L_X. L_{op} can be used to estimate the cluster mass with an accuracy of 40% while L_X can predict the mass only with a 55% accuracy. We show that correcting $L_X$ for the effect of a cool core at the center of a cluster, lowers the scatter of the $L_X-M$ relation only by 3%. We find that the scatter observed in the L_{op}-L_X relation is determined by the scatter of the L_X-M relation. The mass-to-light ratio in the SDSS i band clearly increases with the cluster mass with a slope 0.2\pm0.08. The optical and X-ray luminosities correlate in excellent way with both T_X and σ_V with an orthogonal scatter of 20% in both relations. Moreover, L_{op} and L_X can predict with the same accuracy both variables. We conclude that the cluster optical luminosity is a key cluster parameter since it can give important information about fundamental cluster properties such as the mass, the velocity dispersion, and the temperature of the intra-cluster medium.

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arXiv2004

ROSAT-SDSS Galaxy Clusters Survey. Correlating X-ray and optical properties

For a detailed comparison of the appearance of cluster of galaxies in X-rays and in the optical, we have compiled a comprehensive database of X-ray and optical properties of a sample of clusters based on the largest available X-ray and optical surveys: the ROSAT All Sky Survey (RASS) and the Sloan Digital Sky Survey (SDSS). The RASS-SDSS sample comprises 114 X-ray selected clusters. For each system we have uniformly determined the X-ray and optical properties . For a subsample of 53 clusters we have also compiled the temperature and the iron abundance from the literature. The total optical luminosity can be determined with a typical uncertainty of 20% with a result independent of the choice of local or global background subtraction. We searched for parameters which provide the best correlation between the X-ray luminosity and the optical properties and found that the z band luminosity determined within a cluster aperture of 0.5 Mpc h_{70}^{-1} provides the best correlation with a scatter of about 60%. The resulting correlation of L_X and L_{op} in the z and i bands shows a logarithmic slope of 0.38, a value not consistent with the assumption of a constant M/L. Consistency is found, however, for an increasing M/L with luminosity. This is confirmed by our analysis of the cluster mass to light ratio for a subsample of 53 clusters. We found a significant dependence of M/L on the cluster mass with a logarithmic slope ranging from 0.27 in the i and r bands to 0.22 in the z band.

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arXiv2004

The RASS-SDSS galaxy cluster survey

Solid observational evidences indicate a strong dependence of the galaxy formation and evolution on the environment. In order to study in particular the interaction between the intracluster medium and the evolution of cluster galaxies, we have created a large database of clusters of galaxies based on the largest available X-ray and optical surveys: the ROSAT All Sky Survey (RASS), and the Sloan Digital Sky Survey (SDSS). We analyzed the correlation between the total optical and the X-ray cluster luminosity. The resulting correlation of L_X and L_{op} shows a logarithmic slope of 0.6, a value close to the self-similar correlation. We analysed also the cluster mass to light ratio, by finding a significant dependence of M/L on the cluster mass with a logarithmic slope ranging from 0.27 in the i and r bands to 0.22 in the z band.

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arXiv2004

ROSAT-SDSS Galaxy Clusters Survey. I. The Catalog and the correlation of X-ray and optical properties

For a detailed comparison of the appearance of cluster of galaxies in X-rays and in the optical, we have compiled a comprehensive database of X-ray and optical properties of a sample of clusters based on the largest available X-ray and optical surveys: the ROSAT All Sky Survey (RASS) and the Sloan Digital Sky Survey (SDSS). The X-ray galaxy clusters of this RASS-SDSS catalog cover a wide range of masses, from groups of 10^{12.5} solar masses to massive clusters of 10^{15} solar masses in the redshift range from 0.002 to 0.45. The RASS-SDSS sample comprises all the X-ray selected objects already observed by the Sloan Digital Sky Survey (114 clusters). For each system we have uniformly determined the X-ray and optical properties. The total optical luminosity can be determined with a typical uncertainty of 20% with a result independent of the choice of local or global background subtraction. We found that the z band cluster luminosity determined within an aperture of 0.5 Mpc h_{70}^(-1) provides the best correlation with the X-ray luminosity with a scatter of about 60-70%. The scatter decreases to less than 40% if the correlation is limited to the bright X-ray clusters. The resulting correlation of L_X and L_{op} in the z and i bands shows a logarithmic slope of 0.38, a value not consistent with the assumption of a constant M/L. Consistency is found, however, for an increasing M/L with luminosity as suggested by other observations. We also investigated the correlation between L_{op} and the X-ray temperature obtaining the same result.

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arXiv2003

Photometric redshifts with the Multilayer Perceptron Neural Network: application to the HDF-S and SDSS

We present a technique for the estimation of photometric redshifts based on feed-forward neural networks. The Multilayer Perceptron (MLP) Artificial Neural Network is used to predict photometric redshifts in the HDF-S from an ultra deep multicolor catalog. Various possible approaches for the training of the neural network are explored, including the deepest and most complete spectroscopic redshift catalog currently available (the Hubble Deep Field North dataset) and models of the spectral energy distribution of galaxies available in the literature. The MLP can be trained on observed data, theoretical data and mixed samples. The prediction of the method is tested on the spectroscopic sample in the HDF-S (44 galaxies). Over the entire redshift range, $0.1<z<3.5$, the agreement between the photometric and spectroscopic redshifts in the HDF-S is good: the training on mixed data produces sigma_z(test) ~ 0.11, showing that model libraries together with observed data provide a sufficiently complete description of the galaxy population. The neural system capability is also tested in a low redshift regime, 0<z<0.4, using the Sloan Digital Sky Survey Data Release One (DR1) spectroscopic sample. The resulting accuracy on 88108 galaxies is sigma_z(test) ~ 0.022. Inputs other than galaxy colors - such as morphology, angular size and surface brightness - may be easily incorporated in the neural network technique. An important feature, in view of the application of the technique to large databases, is the computational speed: in the evaluation phase, redshifts of 10^5 galaxies are estimated in few seconds.

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