M. Roncarelli

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.

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.

The Sunyaev–Zel'dovich effects from a cosmological hydrodynamical simulation: large-scale properties and correlation with the soft X-ray signal

Using the results of a cosmological hydrodynamical simulation of the concordance Lambda cold dark matter model, we study the global properties of the Sunyaev‐Zel’dovich (SZ) effects, both considering the thermal (tSZ) and the kinetic (kSZ) component. The simulation follows gravitation and gas dynamics and includes also several physical processes that affect the baryonic component, like a simple reionization scenario, radiative cooling, star formation and supernova feedback. Starting from the outputs of the simulation we create mock maps of the SZ signals due to the large structures of the Universe integrated in the range 0 z 6. We predict that the Compton y-parameter has an average value of (1.19 ± 0.32) × 10 −6 and is lognormally distributed in the sky; half of the whole signal comes from z 2. The Doppler b-parameter shows approximately a normal distribution with vanishing mean value and an s.d. of 1.6 × 10 −6 , with a significant contribution from high-redshift (z > 3) gas. We find that the tSZ effect is expected to dominate the primary cosmic microwave background anisotropies for � 3000 in the Rayleigh‐Jeans limit, while interestingly the kSZ effect dominates at all frequencies at very high multipoles (� 7 × 10 4 ). We also analyse the cross-correlation between the two SZ effects and the soft (0.5‐2 keV) X-ray emission from the intergalactic medium and we obtain a strong correlation between the three signals, especially between X-ray emission and tSZ effect (r� � 0.8‐0.9) at all angular scales.

Large-scale inhomogeneities of the intracluster medium: improving mass estimates using the observed azimuthal scatter

Achieving a robust determination of the gas density profile i n cluster outskirts is a crucial point in order to measure their baryonic content and to use them as cosmological probes. The difficulty in obtaining this measurement lies not only in the low surface brightness of the ICM, but also in the inhomogeneities of the gas associated to clumps, asymmetries and accretion patterns. Using a set of hydrodynamical simulations of 62 galaxy clusters and groups we study this kind of inhomogeneities, focusing on the ones on the large scale that, unlike clumps, are the most difficult to identify. To this purpose we introduce the concept of residual clumpiness, CR, that quantifies the large-scale inhomogeneity of the ICM. A fter showing that this quantity can be robustly defined for relaxed systems, we characterize how it varies with radius, mass and dynamical state of the halo. Most importantly, we observe that it introduces an overestimate in the determination of the density profile f rom the X-ray emission, which translates into a systematic overestimate of 6 (12) per cent in the measurement of Mgas at R200 for our relaxed (perturbed) cluster sample. At the same time, the increase of CR with radius introduces also a �2 per cent systematic underestimate in the measurement of the hydrostatic-equilibrium mass (Mhe), which adds to the previous one generating a systematic �8.5 per cent overestimate in fgas in our relaxed sample. Since the residual clumpiness of the ICM is not directly observable, we study its correlation with the azimuthal scatter in the X-ray surface brightness of the halo, a quantity that is well -constrained by current measurements, and in the y-parameter profiles that is at reach of the forthcoming SZ exp eriments. We find that their correlation is highly significant ( rS = 0.6 0.7), allowing to define the azimuthal scatter measured in the X-ray surface brightness profile and in the y-parameter as robust proxies of CR. After providing a function that connects the two quantitie s, we obtain that correcting the observed gas density profiles using the a zimuthal scatter eliminates the bias in the measurement of Mgas for relaxed objects, which becomes 0 ± 2 per cent up to 2R200, and reduces it by a factor of 3 for perturbed ones. This method allows also to eliminate the systematics on the measurements of Mhe and fgas, although a significant halo to halo scatter remains.

Properties of the diffuse X‐ray background in a high‐resolution hydrodynamical simulation

We study the properties of the diffuse X-ray background by using the results of a cosmological hydrodynamical simulation of the concordance Lambda cold dark matter (Lambda CDM) model. The simulation follows gravitational and gas dynamics and includes a treatment of physical processes, such as radiative cooling, star formation and supernova feedback. From the simulation outputs, we produce a set of two-dimensional maps of the intergalactic medium X-ray emission integrated over redshift. We find that the signal in the soft (0.5-2 keV) band is lognormally distributed with a mean intensity of about 4 x 10(-12) erg s(-1) cm(-2) deg(-2); approximately 40 per cent of the emission originates from warm-hot gas ( defined as baryons with 10(5) < T < 10(7) K), and 90 per cent comes from structures at z < 0.9. Since the spectrum is soft, being mostly provided by the intergalactic medium at low temperature, the total mean intensity in the hard (2-10 keV) X-ray band is smaller by a factor of about 4. In order to constrain the physical processes included in our simulation, we compare our results with the observed upper limit (1.2 +/- 0.3) x 10(-12) erg s(-1) cm(-2) deg(-2) of the soft X-ray emission due to diffuse gas. To this purpose, we remove the contributions of observable extended objects ( groups and clusters of galaxies) from the simulated maps by adopting different detectability criteria which are calibrated on the properties of systems at intermediate redshifts observed by Chandra. We show that the simulated diffuse soft X-ray emission is consistent with the present observed upper limit. However, if future measurements will decrease the level of the unresolved X-ray background by a factor of 2, a more efficient feedback mechanism should be required to suppress the soft emission of the gas residing within filaments and group-size haloes.

The scatter in the radial profiles of X-ray luminous galaxy clusters as diagnostic of the thermodynamical state of the ICM

We study the azimuthal scatter in the radial profiles of X-ray luminous galaxy clusters, with two sets of high-resolution cosmological re-simulations o btained with the codes ENZO and GADGET2. The average gas profiles are computed for different angular sectors of the cluster projected volume, and compared with the mean cluster profile s at ach radius from the center. We report that in general the level of azimuthal scatter is fo und to be∼ 10 per cent for gas density, temperature and entropy inside R200, and∼ 25 per cent for X-ray luminosity for the same volume. These values generally doubles going to 2R200 from the cluster center, and are generally found to be higher (by ∼ 20 − 40 per cent) in the case of perturbed systems. A comparison with results from recent Suzakuobservations is discussed, showing the possibility to simply interpret the large azimuthal scatter of observab les in light of our simulated results.

STUDYING THE WARM-HOT INTERGALACTIC MEDIUM IN EMISSION

We assess the possibility of detecting the warm-hot intergalactic medium in emission and characterizing its physical conditions and spatial distribution through spatially resolved X-ray spectroscopy, in the framework of the recently proposed DIOS, EDGE, Xenia, and ORIGIN missions, all of which are equipped with microcalorimeter-based detectors. For this purpose, we analyze a large set of mock emission spectra, extracted from a cosmological hydrodynamical simulation. These mock X-ray spectra are searched for emission features showing both the O VII Kα triplet and O VIII Lyα line, which constitute a typical signature of the warm-hot gas. Our analysis shows that 1 Ms long exposures and energy resolution of 2.5 eV will allow us to detect about 400 such features per deg2 with a significance ≥5σ and reveals that these emission systems are typically associated with density ~100 above the mean. The temperature can be estimated from the line ratio with a precision of ~20%. The combined effect of contamination from other lines, variation in the level of the continuum, and degradation of the energy resolution reduces these estimates. Yet, with an energy resolution of 7 eV and all these effects taken into account, one still expects about 160 detections per deg2. These line systems are sufficient for tracing the spatial distribution of the line-emitting gas, which constitute an additional information, independent from line statistics, to constrain the poorly known cosmic chemical enrichment history and the stellar feedback processes.

Missing baryons traced by the galaxy luminosity density in large-scale WHIM filaments

We propose a new approach to the problem of the missing baryons. Building on the common assumption that the missing baryons are in the form of the warm hot intergalactic medium (WHIM), we also assume here that the galaxy luminosity density can be used as a tracer of the WHIM. This last assumption is supported by our discovery of a significant correlation between the WHIM density and the galaxy luminosity density in recent hydrodynamical simulations. We also found that the percentage of the gas mass in the WHIM phase is substantially higher (by a factor of similar to 1.6) within large-scale galactic filaments, i.e. similar to 70%, compared to the average in the full simulation volume of similar to 0.1 Gpc(3). The relation between the WHIM overdensity and the galaxy luminosity overdensity within the galactic filaments is consistent with a linear one: delta(whim) = 0.7 +/- 0.1 x delta(0.9 +/- 0.2)(LD) We then applied our procedure to the line of sight towards the blazar H2356-309 and found evidence of WHIM that corresponds to the Sculptor Wall (SW) (z similar to 0.03 and log N-H = 19.9(-0.3)(+0.1)) and Pisces-Cetus (PC) superclusters (z similar to 0.06 and log N-H = 19.7(-0.3)(+0.2)), in agreement with the redshifts and column densities of the X-ray absorbers identified recently. This agreement indicates that the galaxy luminosity density and galactic filaments are reliable signposts for the WHIM and that our method is robust for estimating WHIM density. The signal that we detected cannot originate in the halos of nearby galaxies because they cannot account for the high WHIM column densities that our method and X-ray analysis consistently find in the SW and PC superclusters.

Searching for galaxy clusters in the Kilo-Degree Survey

In this paper, we present the tools used to search for galaxy clusters in the Kilo Degree Survey (KiDS), and our first results. The cluster detection is based on an implementation of the optimal filtering technique that enables us to identify clusters as over-densities in the distribution of galaxies using their positions on the sky, magnitudes, and photometric redshifts. The contamination and completeness of the cluster catalog are derived using mock catalogs based on the data themselves. The optimal signal to noise threshold for the cluster detection is obtained by randomizing the galaxy positions and selecting the value that produces a contamination of less than 20%. Starting from a subset of clusters detected with high significance at low redshifts, we shift them to higher redshifts to estimate the completeness as a function of redshift: the average completeness is ~ 85%. An estimate of the mass of the clusters is derived using the richness as a proxy. We obtained 1858 candidate clusters with redshift 0 <z_c <0.7 and mass 13.5 <log(M500/Msun) <15 in an area of 114 sq. degrees (KiDS ESO-DR2). A comparison with publicly available Sloan Digital Sky Survey (SDSS)-based cluster catalogs shows that we match more than 50% of the clusters (77% in the case of the redMaPPer catalog). We also cross-matched our cluster catalog with the Abell clusters, and clusters found by XMM and in the Planck-SZ survey; however, only a small number of them lie inside the KiDS area currently available.

Constraints on Ωm and σ8 from the potential-based cluster temperature function

The abundance of galaxy clusters is in principle a powerful tool to constrain cosmological parameters, especially