Stefano Ettori

The Chandra COSMOS Survey, I: Overview and Point Source Catalog

The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra program that has imaged the central 0.5 deg^2 of the COSMOS field (centered at 10 ^h , +02 ^o ) with an effective exposure of ~160 ks, and an outer 0.4 deg^2 area with an effective exposure of ~80 ks. The limiting source detection depths are 1.9 × 10^(–16) erg cm^(–2) s^(–1) in the soft (0.5-2 keV) band, 7.3 × 10^(–16) erg cm^(–2) s^(–1) in the hard (2-10 keV) band, and 5.7 × 10^(–16) erg cm^(–2) s^(–1) in the full (0.5-10 keV) band. Here we describe the strategy, design, and execution of the C-COSMOS survey, and present the catalog of 1761 point sources detected at a probability of being spurious of <2 × 10^(–5) (1655 in the full, 1340 in the soft, and 1017 in the hard bands). By using a grid of 36 heavily (~50%) overlapping pointing positions with the ACIS-I imager, a remarkably uniform (±12%) exposure across the inner 0.5 deg^2 field was obtained, leading to a sharply defined lower flux limit. The widely different point-spread functions obtained in each exposure at each point in the field required a novel source detection method, because of the overlapping tiling strategy, which is described in a companion paper. This method produced reliable sources down to a 7-12 counts, as verified by the resulting logN-logS curve, with subarcsecond positions, enabling optical and infrared identifications of virtually all sources, as reported in a second companion paper. The full catalog is described here in detail and is available online.

Systematics in the X-ray Cluster Mass Estimators

We examine the systematics affecting the X-ray mass estimators applied to a set of five galaxy clusters resolved at high resolution in hydrodynamic simulations, including cooling, star formation and feedback processes. These simulated objects are processed through the X-ray Map Simulator, X-MAS, to provide Chandra-like long exposures that are analysed to reconstruct the gas temperature, density and mass profiles used as input. These clusters have different dynamic state: we consider a hot cluster with temperature T = 11.4 keV, a perturbed cluster with T = 3.9 keV, a merging object with T = 3.6 keV, and two relaxed systems with T = 3.3 keV and T = 2.7 keV, respectively. These systems are located at z = 0.175 so that their emission fits within the Chandra ACIS-S3 chip between 0.6 and 1.2 R-500. We find that the mass profile obtained via a direct application of the hydrostatic equilibrium ( HE) equation is dependent upon the measured temperature profile. An irregular radial distribution of the temperature values, with associated large errors, induces a significant scatter on the reconstructed mass measurements. At R-2500, the actual mass is recovered within 1 sigma, although we notice this estimator shows high statistical errors due to high level of Chandra background. Instead, the poorness of the beta-model in describing the gas density profile makes the evaluated masses to be underestimated by similar to 40 per cent with respect to the true mass, both with an isothermal and a polytropic temperature profile. We also test ways to recover the mass by adopting an analytic mass model, such as those proposed by Nvarro, Frenk & White and Rasia, Tormen & Moscardini, and fitting the temperature profile expected from the HE equation to the observed one. We conclude that the methods of the HE equation and those of the analytic fits provide a more robust mass estimation than the ones based on the beta-model. In the present work, the main limitation for a precise mass reconstruction is to ascribe to the relatively high level of the background chosen to reproduce the Chandra one. After artificially reducing the total background by a factor of 100, we find that the estimated mass significantly underestimates the true mass profiles. This is manly due (i) to the neglected contribution of the gas bulk motions to the total energy budget and (ii) to the bias towards lower values of the X-ray temperature measurements because of the complex thermal structure of the emitting plasma.

Weighing simulated galaxy clusters using lensing and X-ray

Context. Measuring the mass of galaxy clusters is a key issue in cosmology. Among the methods employed to achieve this goal, the techniques based on lensing and X-ray analyses are perhaps the most widely used. However, the comparison between these mass estimates is often di cult and, in several clusters, the results apparently are inconsistent. Aims. We aim at investigating potential biases in lensing and X-ray methods to measure the cluster mass profiles. Methods. We do so by performing realistic simulations of lensing and X-ray observations that are subsequently analyzed using observational techniques. The resulting mass estimates are compared among them and with the input models. Three clusters obtained from state-of-the-art hydrodynamical simulations, each of which has been projected along three independent lines-of-sight, are used for this analysis. Results. We find that strong lensing models can be trusted over a limited region around the cluster core. Extrapolating the strong lensing mass models to outside the Einstein ring can lead to significant biases in the mass estimates , if the BCG is not modeled properly for example. Weak lensing mass measurements can be largely a ected by substructures, depending on the method implemented to convert the shear into a mass estimate. Using non-parametric methods which combine weak and strong lensing data, the projected masses within R200 can be constrained with a precision of 10%. De-projection of lensing masses increases the scatter around the true masses by more than a factor of two due to cluster triaxiality. X-ray mass measurements have much smaller scatter (about a factor of two smaller than the lensing masses) but they are generally biased low by 5 20%. This bias is entirely ascribable to bulk motions in the gas of our simulated clusters. Using the lensing and the X-ray masses as proxies for the true and the hydrostatic equilibrium masses of the simulated clusters and by averaging over the cluster sample we are able to measure the lack of hydrostatic equilibrium in the systems we have investigated. Conclusions. Although the comparison between lensing and X-ray masses may be di cult in individual systems due to triaxiality and substructures, using a large number of clusters with both lensing and X-ray observations may lead to important information about their gas physics and allow to use lensing masses to calibrate the X-ray scaling relations.

Lensing and x-ray mass estimates of clusters (simulations)

We present a comparison between weak-lensing and x-ray mass estimates of a sample of numerically simulated clusters. The sample consists of the 20 most massive objects at redshift z = 0.25 and M_vir > 5 × 10^(14) M_☉ h^(−1). They were found in a cosmological simulation of volume 1 h^(−3) Gpc^3, evolved in the framework of a WMAP-7 normalized cosmology. Each cluster has been resimulated at higher resolution and with more complex gas physics. We processed it through Skylens and X-MAS to generate optical and x-ray mock observations along three orthogonal projections. The final sample consists of 60 cluster realizations. The optical simulations include lensing effects on background sources. Standard observational tools and methods of analysis are used to recover the mass profiles of each cluster projection from the mock catalogue. The resulting mass profiles from lensing and x-ray are individually compared to the input mass distributions. Given the size of our sample, we could also investigate the dependence of the results on cluster morphology, environment, temperature inhomogeneity and mass. We confirm previous results showing that lensing masses obtained from the fit of the cluster tangential shear profiles with Navarro–Frenk–White functionals are biased low by ~5–10% with a large scatter (~10–25%). We show that scatter could be reduced by optimally selecting clusters either having regular morphology or living in substructure-poor environment. The x-ray masses are biased low by a large amount (~25–35%), evidencing the presence of both non-thermal sources of pressure in the intra-cluster medium (ICM) and temperature inhomogeneity, but they show a significantly lower scatter than weak-lensing-derived masses. The x-ray mass bias grows from the inner to the outer regions of the clusters. We find that both biases are weakly correlated with the third-order power ratio, while a stronger correlation exists with the centroid shift. Finally, the x-ray bias is strongly connected with temperature inhomogeneities. Comparison with a previous analysis of simulations leads to the conclusion that the values of x-ray mass bias from simulations are still uncertain, showing dependences on the ICM physical treatment and, possibly, on the hydrodynamical scheme adopted.

Searching for Cool Core Clusters at High redshift

Aims. We investigate the detection of Cool Cores (CCs) in the distant galaxy cluster population with the purpose of measuring the CC fraction out to redshift 0.7 ≤ z 0.7, and should also be related to the shorter age of distant clusters, implying less time to develop a cool core.

Tracing the evolution in the iron content of the intra-cluster medium

We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies at z>0.3, which cover a temperature range of 3>kT>15 keV. Our analysis is aimed at measuring the iron abundance in the ICM out to the highest redshift probed to date. We find that the emission-weighted iron abundance measured within (0.15-0.3)R_vir in clusters below 5 keV is, on average, a factor of ~2 higher than in hotter clusters, following Z(T)~0.88T^-(0.47)Z_o, which confirms the trend seen in local samples. We made use of combined spectral analysis performed over five redshift bins at 0.3>z>1.3 to estimate the average emission weighted iron abundance. We find a constant average iron abundance Z_Fe~0.25Z_o as a function of redshift, but only for clusters at z>0.5. The emission-weighted iron abundance is significantly higher (Z_Fe~0.4Z_o) in the redshift range z~0.3-0.5, approaching the value measured locally in the inner 0.15R_vir radii for a mix of cool-core and non cool-core clusters in the redshift range 0.1<z<0.3. The decrease in Z_Fe with redshift can be parametrized by a power law of the form ~(1+z)^(-1.25). The observed evolution implies that the average iron content of the ICM at the present epoch is a factor of ~2 larger than at z=1.2. We confirm that the ICM is already significantly enriched (Z_Fe~0.25Z_o) at a look-back time of 9 Gyr. Our data provide significant constraints on the time scales and physical processes that drive the chemical enrichment of the ICM.

The gas distribution in the outer regions of galaxy clusters

Aims. We present our analysis of a local (z = 0.04-0.2) sample of 31 galaxy clusters with the aim of measuring the density of the X-ray emitting gas in cluster outskirts. We compare our results with numerical simulations to set constraints on the azimuthal symmetry and gas clumping in the outer regions of galaxy clusters.

Mass profiles and c − MDM relation in X-ray luminous galaxy clusters

Context. Galaxy clusters represent valuable cosmological probes using tests that mainly rely on measurements of cluster masses and baryon fractions. X-ray observations represent one of the main tools for uncovering these quantities. Aims. We aim to constrain the cosmological parameters Ωm and σ8 using the observed distribution of the both values of the concentrations and dark mass within R200 and of the gas mass fraction within R500. Methods. We applied two different techniques to recover the profiles the gas and dark mass, described according to the Navarro, Frenk & White (1997, ApJ, 490, 493) functional form, of a sample of 44 X-ray luminous galaxy clusters observed with XMM-Newton in the redshift range 0.1−0.3. We made use of the spatially resolved spectroscopic data and of the PSF–deconvolved surface brightness and assumed that hydrostatic equilibrium holds between the intracluster medium and the gravitational potential. We evaluated several systematic uncertainties that affect our reconstruction of the X-ray masses. Results. We measured the concentration c200, the dark mass M200 and the gas mass fraction in all the objects of our sample, providing the largest dataset of mass parameters for galaxy clusters in the redshift range 0.1−0.3. We confirm that a tight correlation between c200 and M200 is present and in good agreement with the predictions from numerical simulations and previous observations. When we consider a subsample of relaxed clusters that host a low entropy core, we measure a flatter c − M relation with a total scatter that is lower by 40 per cent. We conclude, however, that the slope of the c − M relation cannot be reliably determined from the fitting over a narrow mass range as the one considered in the present work. From the distribution of the estimates of c200 and M200, with associated statistical (15–25%) and systematic (5–15%) errors, we used the predicted values from semi-analytic prescriptions calibrated through N-body numerical runs and obtain σ8 Ω 0.60±0.03 m = 0.45 ± 0.01 (at 2σ level, statistical only) for the subsample of the clusters where the mass reconstruction has been obtained more robustly and σ8 Ω 0.56±0.04 m = 0.39 ± 0.02 for the subsample of the 11 more relaxed LEC objects. With the further constraint from the gas mass fraction distribution in our sample, we break the degeneracy in the σ8 − Ωm plane and obtain the best-fit values σ8 ≈ 1.0 ± 0. 2( 0.83 ± 0.1 when the subsample of the more relaxed objects is considered) and Ωm = 0.26 ± 0.02. Conclusions. Analysis of the distribution of the c200 − M200 − fgas values represents a mature and competitive technique in the present era of precision cosmology, even though it needs more detailed analysis of the output of larger sets of cosmological numerical simulations to provide definitive and robust results.

The baryon fraction in hydrodynamical simulations of galaxy clusters

We study the baryon mass fraction in a set of hydrodynamical simulations of galaxy clusters performed using the Tree+SPH code GADGET-2. We investigate the dependence of the baryon fraction upon radiative cooling, star formation, feedback through galactic winds, conduction and redshift. Both the cold stellar component and the hot X-ray-emitting gas have narrow distributions that, at large cluster-centric distances r R 500, are nearly independent of the physics included in the simulations. Only the non-radiative runs reproduce the gas fraction inferred from observations of the inner regions (r ≈ R 2500 )o fmassive clusters. When cooling is turned on, the excess star formation is mitigated by the action of galactic winds, yet not by the amount required by observational data. The baryon fraction within a fixed overdensity increases slightly with redshift, independent of the physical processes involved in the accumulation of baryons in the cluster potential well. In runs with cooling and feedback, the increase in baryons is associated with a larger stellar mass fraction that arises at high redshift as a consequence of more efficient gas cooling. For the same reason, the gas fraction appears less concentrated at higher redshift. We discuss the possible cosmological implications of our results, and find that two assumptions generally adopted, i.e. (1) mean value of Y b = f b/(� b/� m) not evolving with redshift, and (2) a fixed ratio between f star and f gas independent of radius and redshift, might not be valid. In the estimate of the cosmic matter density parameter, this implies some systematic effects of the order of �� m/� m +0.15 for non-radiative runs and �� m/� m ≈ +0.05 and −0.05 for radiative simulations. Ke yw ords: methods: numerical ‐ galaxies: clusters: general ‐ cosmology: miscellaneous ‐ X-rays: galaxies.

Simulated X-ray galaxy clusters at the virial radius: slopes of the gas density, temperature and surface brightness profiles

Using a set of hydrodynamical simulations of nine galaxy clusters with masses in the range 1.5 x 10 14 < M vir < 3.4 x 10 15 M ⊙ , we have studied the density, temperature and X-ray surface brightness profiles of the intracluster medium in the regions around the virial radius. We have analysed the profiles in the radial range well above the cluster core, the physics of which are still unclear and matter of tension between simulated and observed properties, and up to the virial radius and beyond, where present observations are unable to provide any constraints. We have modelled the radial profiles between 0.3R 200 and 3R 200 with power laws with one index, two indexes and a rolling index. The simulated temperature and [0.5-2] keV surface brightness profiles well reproduce the observed behaviours outside the core. The shape of all these profiles in the radial range considered depends mainly on the activity of the gravitational collapse, with no significant difference among models including extraphysics. The profiles steepen in the outskirts, with the slope of the power-law fit that changes from -2.5 to -3.4 in the gas density, from -0.5 to -1.8 in the gas temperature and from -3.5 to -5.0 in the X-ray soft surface brightness. We predict that the gas density, temperature and [0.5-2] keV surface brightness values at R 200 are, on average, 0.05, 0.60, 0.008 times the measured values at 0.3R 200 . At 2R 200 , these values decrease by an order of magnitude in the gas density and surface brightness, by a factor of 2 in the temperature, putting stringent limits on the detectable properties of the intracluster-medium (ICM) in the virial regions.