arXiv subjects

S. Andreon

Publications and source records attributed to S. Andreon.

At least 307 records · Page 17Linked to original sources

arXiv2010

Galaxy Clusters at z>=1: Gas Constraints from the Sunyaev-Zel'dovich Array

We present gas constraints from Sunyaev-Zel'dovich (SZ) effect measurements in a sample of eleven X-ray and infrared (IR) selected galaxy clusters at z >=1, using data from the Sunyaev-Zel'dovich Array (SZA). The cylindrically integrated Compton-y parameter, Y , is calculated by fitting the data to a two-parameter gas pressure profile. Where possible, we also determine the temperature of the hot intra-cluster plasma from Chandra and XMM-Newton data, and constrain the gas mass within the same aperture (r_2500 ) as Y . The SZ effect is detected in the clusters for which the X-ray data indicate gas masses above ~ 10^13 Msun, including XMMU J2235-2557 at redshift z = 1.39, which to date is one of the most distant clusters detected using the SZ effect. None of the IR-selected targets are detected by the SZA measurements, indicating low gas masses for these objects. For these and the four other undetected clusters, we quote upper limits on Y and Mgas_SZ , with the latter derived from scaling relations calibrated with lower redshift clusters. We compare the constraints on Y and X-ray derived gas mass Mgas_X-ray to self-similar scaling relations between these observables determined from observations of lower redshift clusters, finding consistency given the measurement error.

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arXiv2010

The stellar mass fraction and baryon content of galaxy clusters and groups

[Abridged] The analysis of a sample of 52 clusters with precise and hypothesis-parsimonious measurements of mass shows that low mass clusters and groups are not simple scaled-down versions of their massive cousins in terms of stellar content: lighter clusters have more stars per unit cluster mass. The same analysis also shows that the stellar content of clusters and groups displays an intrinsic spread at a given cluster mass, i.e. clusters are not similar each other in the amount of stars they contain, not even at a fixed cluster mass. The stellar mass fraction depends on halo mass with (logarithmic) slope -0.55+/-0.08 and with 0.15+/-0.02 dex of intrinsic scatter at a fixed cluster mass. The intrinsic scatter at a fixed cluster mass we determine for gas mass fractions is smaller, 0.06+/-0.01 dex. The intrinsic scatter in both the stellar and gas mass fractions is a distinctive signature that the regions from which clusters and groups collected matter, a few tens of Mpc, are yet not representative, in terms of gas and baryon content, of the mean matter content of the Universe. The observed stellar mass fraction values are in marked disagreement with gasdynamics simulations with cooling and star formation of clusters and groups. We found the the baryon (gas+stellar) fraction is fairly constant for clusters and groups with 13.7<lg(mass)<15.0 solar masses and it is offset from the WMAP-derived value by about 6 sigmas. The offset could be related to the possible non universality of the baryon fraction pointed out by our measurements of the intrinsic scatter. Our analysis is the first that does not assume that clusters are identically equal at a given halo mass and it is also more accurate in many aspects. The data and code used for the stochastic computation are distributed with the paper.

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arXiv2010

The scaling relation between richness and mass of galaxy clusters: a Bayesian approach

We use a sample of 53 galaxy clusters at 0.03 < z < 0.1 with available masses derived from the caustic technique and with velocity dispersions computed using 208 galaxies on average per cluster, in order to investigate the scaling between richness, mass and velocity dispersion. A tight scaling between richness and mass is found, with an intrinsic scatter of only 0.19 dex in mass and with a slope one, i.e. clusters which have twice as many galaxies are twice as massive. When richness is measured without any knowledge of the cluster mass or linked parameters (such as r200), it can predict mass with an uncertainty of 0.29+/-0.01 dex. As a mass proxy, richness competes favourably with both direct measurements of mass given by the caustic method, which has typically 0.14 dex errors (vs 0.29) and X-ray luminosity, which offers a similar 0.30 dex uncertainty. The similar performances of X-ray luminosity and richness in predicting cluster masses has been confirmed using cluster masses derived from velocity dispersion fixed by numerical simulations. These results suggest that cluster masses can be reliably estimated from simple galaxy counts, at least at the redshift and masses explored in this work. This has important applications in the estimation of cosmological parameters from optical cluster surveys, because in current surveys clusters detected in the optical range outnumber, by at least one order of magnitude, those detected in X-ray. Our analysis is robust from astrophysical and statistical perspectives. The data and code used for the stochastic computation is distributed with the paper. [Abridged]

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arXiv2009

JKCS041: a colour-detected galaxy cluster at z_phot=1.9 with deep potential well as confirmed by X-ray data

[Abridged] We report the discovery of JKCS041, a massive near-infrared selected cluster of galaxies z=1.9. The cluster was originally discovered using a modified red-sequence method and was also detected in follow-up Chandra data as extended X-ray source. Optical and near-infrared imaging data alone allow us to show that the detection of JKCS041 is as secure, even in absence of the X-ray data. We investigate the possibility that JKCS041 is not a discrete galaxy cluster at z=1.9, and find other explanations to be unlikely. The X-ray detection and statistical arguments rule out the hypothesis that JKCS041 is actually a blend of groups along the line of sight, and we find that the X-ray emitting gas is too hot and dense to be a filament projected along the line of sight. The cluster has an X-ray core radius of 36.6 arcsec (about 300 kpc), an X-ray temperature of 7.4 keV, a bolometric X-ray luminosity within R500 of 7.6 10^44 erg/s, an estimated mass of M500=2.9 10^14 Msol, the latter derived under the usual (and strong) assumptions. The cluster is composed of 16.4 galaxies within 1.5 arcmin (750 kpc) brighter than K~20.7 mag. The high redshift of JKCS041 is determined from the detection colour, from the detection of the cluster in a galaxy sample formed by z_phot>1.6 galaxies and from a photometric redshift based on 11-band spectral energy distribution fitting. By means of the latter we find the cluster redshift to be 1.84<z<2.12 at 68 % confidence. Therefore, JKCS041 is a cluster of galaxies at z=1.9 with deep potential well, making it the most distant cluster with extended X-ray emission known.

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arXiv2009

A multi-wavelength survey of AGN in the XMM-LSS field: I. Quasar selection via the KX technique

AIMS: We present a sample of candidate quasars selected using the KX-technique. The data cover 0.68 deg^2 of the X-ray Multi-Mirror (XMM) Large-Scale Structure (LSS) survey area where overlapping multi-wavelength imaging data permits an investigation of the physical nature of selected sources. METHODS: The KX method identifies quasars on the basis of their optical (R and z') to near-infrared (Ks) photometry and point-like morphology. We combine these data with optical (u*,g'r',i',z') and mid-infrared (3.6-24 micron) wavebands to reconstruct the spectral energy distributions (SEDs) of candidate quasars. RESULTS: Of 93 sources selected as candidate quasars by the KX method, 25 are classified as quasars by the subsequent SED analysis. Spectroscopic observations are available for 12/25 of these sources and confirm the quasar hypothesis in each case. Even more, 90% of the SED-classified quasars show X-ray emission, a property not shared by any of the false candidates in the KX-selected sample. Applying a photometric redshift analysis to the sources without spectroscopy indicates that the 25 sources classified as quasars occupy the interval 0.7 < z < 2.5. The remaining 68/93 sources are classified as stars and unresolved galaxies.

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arXiv2008

The population of early-type galaxies at 1<z<2 - New clues on their formation and evolution

[Abridged]We present the morphological analysis based on HST-NICMOS observations in the F160W filter of a sample of 32 early-type galaxies (ETGs) at 1<z<2 with spectroscopic confirmation of their redshift and spectral type. We find that ETGs at are composed of two distinct populations, an older population (oETGs) and a younger population (yETGs) whose mean ages differ by about 1.5-2 Gyr. Young ETGs are not denser than local ones since they follow the size-mass relation of local ETGs and luminosity evolution brings them onto the local Kormendy and size-luminosity relations. On the constrary, old ETGs do not follow the size-mass relation of local ETGs and luminosity evolution does not account for the discrepancy they show with respect to the local size-luminosity and Kormendy relations. An increase of their effective radius R_e by a factor 2.5-3 (a density decrease by a factor 15-30) from z~1.5-2 to z~0 is required to bring these galaxies onto the local scaling relations. The different behaviour shown by the two populations implies different formation and evolution scenarios. The older population must have formed at higher-z in a sort of dissipative gas-rich collapse ables to produce remnants which at z~2 are old and compact. The size evolution they must experience from z~2 to z~0 has to leave unchanged their mass to not exceed the local number of high-mass ETGs. Thus, major merging cannot fit this requirement. The younger population of ETGs can be formed later through subsequent episodes of merging which increased progressively their size and assembled their mass down to z~2. At z<2 they evolve purely in luminosity since episodes of major merging would bring them far from the local scaling relations.

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arXiv2008

Morphology of galaxies in the Coma cluster region down to M_B=-14.25. I. A catalog of 473 members

This paper presents morphological type, membership, and U-V color for a sample of galaxies in the Coma cluster direction, complete down to M_B=-15.00 mag and extending down to M_B=-14.25 mag. We have examined 1155 objects from the GMP 1983 catalog on B and V images of the CFH12K camera, and obtained the Hubble type in most cases. Coma cluster membership for 473 galaxies was derived using morphology, apparent size, and surface brightness, and, afterward, redshift. The comparison among morphology- and redshift- memberships and among luminosity functions derived from this morphologically-selected sample, or by using statistical members or spectroscopic members, all show that the morphological membership provided here can be trusted. For the first time, the morphological classification of Coma galaxies reaches magnitudes that are faint enough to observe the whole magnitude range of the giant types, E, S0, and spiral stages. The data presented in this paper makes our sample the richest environment where membership and morphology for complete samples down to faint magnitudes M_B~-15 mag are available, thereby enlarging the baseline of environmental studies.

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arXiv2008

The history of mass assembly of faint red galaxies in 28 galaxy clusters since z=1.3

We measure the relative evolution of the number of bright and faint (as faint as 0.05 L*) red galaxies in a sample of 28 clusters, of which 16 are at 0.50<= z<=1.27, all observed through a pair of filters bracketing the 4000 Angstrom break rest-frame. The abundance of red galaxies, relative to bright ones, is constant over all the studied redshift range, 0<z<1.3, and rules out a differential evolution between bright and faint red galaxies as large as claimed in some past works. Faint red galaxies are largely assembled and in place at z=1.3 and their deficit does not depend on cluster mass, parametrized by velocity dispersion or X-ray luminosity. Our analysis, with respect to previous one, samples a wider redshift range, minimizes systematics and put a more attention to statistical issues, keeping at the same time a large number of clusters.

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arXiv2008

Testing the galaxy cluster mass-observable relations at z = 1 with XMM-Newton and Chandra observations of XLSSJ022403.9-041328

We present an analysis of deep XMM-Newton and Chandra observations of the z=1.05 galaxy cluster XLSSJ022403.9-041328 (hereafter XLSSC 029), detected in the XMM-Newton large scale structure survey. Density and temperature profiles of the X-ray emitting gas were used to perform a hydrostatic mass analysis of the system. This allowed us to measure the total mass and gas fraction in the cluster and define overdensity radii R500 and R2500. The global properties of XLSSC 029 were measured within these radii and compared with those of the local population. The gas mass fraction was found to be consistent with local clusters. The mean metal abundance was 0.18 +0.17 -0.15 Zsol, with the cluster core regions excluded, consistent with the predicted and observed evolution. The properties of XLSSC 029 were then used to investigate the position of the cluster on the M-kT, YX-M, and LX-M scaling relations. In all cases the observed properties of XLSSC 029 agreed well with the simple self-similar evolution of the scaling relations. This is the first test of the evolution of these relations at z > 1 and supports the use of the scaling relations in cosmological studies with distant galaxy clusters.

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arXiv2008

The evolution of the morphological scale of early-type galaxies since z=2

We present the morphological analysis based on HST-NICMOS observations in the F160W filter of a sample of 30 early-type galaxies spectroscopically confirmed at 1.2<z<2. We derive the effective radius R_e and the mean surface brightness mu_e of galaxies in the rest-frame R-band. We find that early-types at z~1.5 are characterized by a surface brightness (SB) much higher then their local counterparts with comparable R_e. In particular, we find that the mean SB of these early-types should get fainter by ~2.5 mag from z~1.5 to z=0 to match the SB of the local early-types with comparable R_e. This evolution exceeds by a factor two the luminosity evolution expected for early-types in this redshift range and more than a factor three the one derived from the observed luminosity function of galaxies. Consequently, an evolution of the effective radius R_e from the epoch of their formation towards z=0 has to be invoked and the hypothesis of fixed size rejected.

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arXiv2008

Galaxy evolution in the high redshift, colour-selected cluster RzCS 052 at z=1.02

We present deep I and z' imaging of the colour-selected cluster RzCS 052 and study the color-magnitude relation of this cluster, its scatter, the morphological distribution on the red sequence, the luminosity and stellar mass functions of red galaxies and the cluster blue fraction. We find that the stellar populations of early type galaxies in this cluster are uniformly old and that their luminosity function does not show any sign of evolution other than the passive evolution of their stellar populations. We rule out a significant contribution from mergers in the buildup of the red sequence of RzCS 052. The cluster has a large (~30%) blue fraction and and we infer that the evolution of the blue galaxies is faster than an exponentially declining star formation model and that these objects have probably experienced starburst episodes. Mergers are unlikely to be the driver of the observed colour evolution, because of the measured constancy of the mass function, as derived from near-infrared photometry of 32 clusters, including RzCS 052, presented in a related paper. Mechanisms with clustercentric radial dependent efficiencies are disfavored as well, because of the observed constant blue fraction with clustercentric distance.

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arXiv2007

Scaling relations of the colour-detected cluster RzCS 052 at z=1.016 and of some other high redshift clusters

We report on the discovery of the z=1.016 cluster RzCS 052 using a modified red sequence method, followup spectroscopy and X-ray imaging. This cluster has a velocity dispersion of 710+-150 km/s, a virial mass of 4.0e14 Msol (based on 21 spectroscopically confirmed members) and an X-ray luminosity of (0.68+- 0.47)e44 ergs/s in the [1-4] keV band. This optically selected cluster appears to be of richness class 3 and to follow the known L_X-sigma_v relation for high redshift X-ray selected clusters. Using these data, we find that the halo occupation number for this cluster is only marginally consistent with what expected assuming a self-similar evolution of cluster scaling relations, suggesting perhaps a break of them at z~1. We also rule out a strong galaxy merging activity between z=1 and today. Finally, we present a Bayesian approach to measuring cluster velocity dispersions and X-ray luminosities in the presence of a background: we critically reanalyze recent claims for X-ray underluminous clusters using these techniques and find that the clusters can be accommodated within the existing L_X -sigma_v relation.

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arXiv2007

The XMM-LSS survey: the Class 1 cluster sample over the initial 5 square degrees and its cosmological modelling

We present a sample of 29 galaxy clusters from the XMM-LSS survey over an area of some 5deg2 out to a redshift of z=1.05. The sample clusters, which represent about half of the X-ray clusters identified in the region, follow well defined X-ray selection criteria and are all spectroscopically confirmed. For all clusters, we provide X-ray luminosities and temperatures as well as masses. The cluster distribution peaks around z=0.3 and T =1.5 keV, half of the objects being groups with a temperature below 2 keV. Our L-T(z) relation points toward self-similar evolution, but does not exclude other physically plausible models. Assuming that cluster scaling laws follow self-similar evolution, our number density estimates up to z=1 are compatible with the predictions of the concordance cosmology and with the findings of previous ROSAT surveys. Our well monitored selection function allowed us to demonstrate that the inclusion of selection effects is essential for the correct determination of the evolution of the L-T relation, which may explain the contradictory results from previous studies. Extensive simulations show that extending the survey area to 10deg2 has the potential to exclude the non-evolution hypothesis, but that constraints on more refined ICM models will probably be limited by the large intrinsic dispersion of the L-T relation. We further demonstrate that increasing the dispersion in the scaling laws increases the number of detectable clusters, hence generating further degeneracy [in addition to sigma8, Omega_m, L(M,z) and T(M,z)] in the cosmological interpretation of the cluster number counts. We provide useful empirical formulae for the cluster mass-flux and mass-count-rate relations as well as a comparison between the XMM-LSS mass sensitivity and that of forthcoming SZ surveys.

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arXiv2007

The XMM-LSS catalogue: X-ray sources and associated optical data. Version I

Following the presentation of the XMM-LSS X-ray source detection package by Pacaud et al., we provide the source lists for the first 5.5 surveyed square degrees. The catalogues pertain to the [0.5-2] and [2-10] keV bands and contain in total 3385 point-like or extended sources above a detection likelihood of 15 in either band. The agreement with deep logN-logS is excellent. The main parameters considered are position, countrate, source extent with associated likelihood values. A set of additional quantities such as astrometric corrections and fluxes are further calculated while errors on the position and countrate are deduced from simulations. We describe the construction of the band-merged catalogue allowing rapid sub-sample selection and easy cross-correlation with external multi-wavelength catalogues. A small optical CFHTLS multi-band subset of objects is associated wich each source along with an X-ray/optical overlay. We make the full X-ray images available in FITS format. The data are available at CDS and, in a more extended form, at the Milan XMM-LSS database.

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arXiv2007

Obscured and unobscured AGN populations in a hard-X-ray selected sample of the XMDS survey

Our goal is to probe the populations of obscured and unobscured AGN investigating their optical-IR and X-ray properties as a function of X-ray flux, luminosity and redshift within a hard X-ray selected sample of 136 X-ray sources in the XMM Medium Deep Survey (XMDS) with wide multiwavelength coverage. The XMDS area is covered with optical photometry from the VVDS and CFHTLS surveys and infrared Spitzer data. Based on the X-ray luminosity and X-ray to optical ratio, 132 sources are likely AGN, of which 122 have unambiguous optical - IR identification. The observed optical and IR spectral energy distributions of sources are fitted with AGN/galaxy templates in order to classify them and compute photometric redshifts. 70% of the AGN are fitted by a type 2 AGN or a star forming galaxy template and are grouped together in a single class of ``optically obscured'' AGN. They have ``red'' optical colors and generally show significant X-ray absorption from X-ray spectra or hardness ratios (N$_H > 10^{22}$ cm$^{-2}$). Sources with SEDs typical of type 1 AGN have ``blue'' optical colors and exhibit X-ray absorption in about 30% of cases. We performed a stacking analysis for obscured and type 1 AGN. The stacked X-ray spectrum of obscured AGN is flatter than that of type 1 AGN and has an average spectral slope of Gamma = 1.6. The subsample of objects fitted by a galaxy template has an even harder stacked spectrum, with Gamma = 1.2 - 1.3. The obscured fraction is larger at lower fluxes, lower redshifts and lower luminosities. X-ray absorption is less common than ``optical'' obscuration and its incidence is nearly constant with redshift and luminosity. This implies that X-ray absorption is not necessarily related to optical obscuration.

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arXiv2007

Spectral Energy Distributions of Hard X-ray selected AGNs in the XMDS Survey

We present the spectral energy distributions (SEDs) of a hard X-ray selected sample. The sample contains 136 sources with F(2-10 keV)>10^-14 erg/cm^2/s and 132 are AGNs. The sources are detected in a 1 square degree area of the XMM-Newton-Medium Deep Survey where optical data from the VVDS, CFHTLS surveys, and infrared data from the SWIRE survey are available. Based on a SED fitting technique we derive photometric redshifts with sigma(1+z)=0.11 and 6% of outliers and identify AGN signatures in 83% of the objects. This fraction is higher than derived when a spectroscopic classification is available. The remaining 17+9-6% of AGNs shows star-forming galaxy SEDs (SF class). The sources with AGN signatures are divided in two classes, AGN1 (33+6-1%) and AGN2 (50+6-11). The AGN1 and AGN2 classes include sources whose SEDs are fitted by type 1 and type 2 AGN templates, respectively. On average, AGN1s show soft X-ray spectra, consistent with being unabsorbed, while AGN2s and SFs show hard X-ray spectra, consistent with being absorbed. The analysis of the average SEDs as a function of X-ray luminosity shows a reddening of the IR SEDs, consistent with a decreasing contribution from the host galaxy at higher luminosities. The AGNs in the SF classes are likely obscured in the mid-infrared, as suggested by their low L(3-20micron)/Lcorr(0.5-10 keV) ratios. We confirm the previously found correlation for AGNs between the radio luminosity and the X-ray and the mid-infrared luminosities. The X-ray-radio correlation can be used to identify heavily absorbed AGNs. However, the estimated radio fluxes for the missing AGN population responsible for the bulk of the background at E>10 keV are too faint to be detected even in the deepest current radio surveys.

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arXiv2006

The buildup of stellar mass of galaxies in clusters during the last two thirds of the universe age and associated methodological issues

We have measured the 3.6 micron luminosity and mass evolution of about 1000 galaxies in 32 clusters at 0.2<z<1.25 with a special attention to methodological issues, as emphasized in this proceeding contribution. We find that the luminosity of our galaxies evolves as an old and passively evolving population formed at high redshift without any need for additional redshift-dependent evolution. Models with a prolonged stellar mass growth are rejected by the data with high confidence. The data also reject models in which the age of the stars is the same at all redshifts. Similarly, the characteristic stellar mass evolves, in the last two thirds of the universe age, as expected for a stellar population formed at high redshift. Together with the old age of stellar populations derived from fundamental plane studies, our data seems to suggest that massive early-type cluster galaxies have been completely assembled at high redshift, and not only that their stars are old. The quality of the data allows us to derive the LF and mass evolution homogeneously over the whole redshift range, using a single estimator. The Schechter function describes the galaxy luminosity function well. The characteristic luminosity at z=0.5 is found to be 16.30 mag, with an uncertainty of 10 per cent.

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arXiv2006

XMM-LSS discovery of a z=1.22 galaxy cluster

We present details of the discovery of XLSSJ022303.0-043622, a z=1.2 cluster of galaxies. This cluster was identified from its X-ray properties and selected as a z>1 candidate from its optical/near-IR characteristics in the XMM Large-Scale Structure Survey (XMM-LSS). It is the most distant system discovered in the survey to date. We present ground-based optical and near IR observations of the system carried out as part of the XMM-LSS survey. The cluster has a bolometric X-ray luminosity of 1.1 +/- 0.7 x 10^44 erg/s, fainter than most other known z>1 X-ray selected clusters. In the optical it has a remarkably compact core, with at least a dozen galaxies inside a 125 kpc radius circle centred on the X-ray position. Most of the galaxies within the core, and those spectroscopically confirmed to be cluster members, have stellar masses similar to those of massive cluster galaxies at low redshift. They have colours comparable to those of galaxies in other z>1 clusters, consistent with showing little sign of strong ongoing star formation. The bulk of the star formation within the galaxies appears to have ceased at least 1.5 Gyr before the observed epoch. Our results are consistent with massive cluster galaxies forming at z>1 and passively evolving thereafter. We also show that the system is straightforwardly identified in Spitzer/IRAC 3.6 and 4.5 micron data obtained by the SWIRE survey emphasising the power and utility of joint XMM and Spitzer searches for the most distant clusters.

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