M. Rossetti

Mass profiles and concentration-dark matter relation in X-ray luminous galaxy clusters

Context. Galaxy clusters represent valuable cosmological probes us ing tests that mainly rely on measurements of cluster masses and baryon fractions. X-ray observations represent one of the m ain tools for uncovering these quantities. Aims. We aim to constrain the cosmological parameters Ωm andσ8 using the observed distribution of the both values of the con centrations and dark mass within R200 and of the gas mass fraction within R500. Methods. We applied two different techniques to recover the profiles t he gas and dark mass, described according to the Navarro, Frenk & White (1997, ApJ, 490, 493) functional form, of a samp le of 44 X-ray luminous galaxy clusters observed with XMM-Newton in the redshift range0.1− 0.3. We made use of the spatially resolved spectroscopic data an d of the PSF–deconvolved surface brightness and assumed that hydrostatic equilibrium holds betwee n the intracluster medium and the gravitational potential. We evaluated several systematic uncertainties that affect our reconstr uction of the X-ray masses. Results. We measured the concentration c200, the dark massM200 and the gas mass fraction in all the objects of our sample, pro viding the largest dataset of mass parameters for galaxy clu sters in the redshift range 0.1 − 0.3. We confirm that a tight correlation betweenc200 andM200 is present and in good agreement with the predictions from nu erical simulations and previous observations. When we consider a subsample of relaxed clusters that host a l w entropy core, we measure a flatter c − M relation with a total scatter that is lower by 40 per cent. We conclude, however, th at the slope of thec−M relation cannot be reliably determined from the fitting over a narrow mass range as the one considered in the pr es nt work. From the distribution of the estimates of c200 andM200, with associated statistical (15–25%) and systematic (5–15 %) errors, we used the predicted values from semi-analytic p rescriptions calibrated through N-body numerical runs and obtain σ8 Ω m = 0.45 ± 0.01 (at 2σ level, statistical only) for the subsample of the clusters where the mass reconstruction has been obtai ned more robustly and σ8 Ω m = 0.39 ± 0.02 for the subsample of the 11 more relaxed LEC objects. With the further constrai n from the gas mass fraction distribution in our sample, we b reak 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. We demonstrate that the analysis of the distribution of the c200 −M200 − fgas values represents a mature and competitive technique in the present era of precision cosmology, e ven though it needs more detailed analysis of the output of la rger sets of cosmological numerical simulations to provide definitive a nd robust results.

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.

Is there a hard tail in the Coma Cluster X-ray spectrum?

We report results from a re-analysis of the BeppoSAX observation of Coma and from the analysis of a second, yet unpublished observation of the same object. From our re-analysis of the first observation we find that the statistical evidence for a hard tail is about 2 σ . From the analysis of the second observation which, thanks to the lower background and the longer exposure time, is characterized by a larger signal-to-noise we find no evidence for a hard tail. From the upper limit on the flux of the hard tail, using the standard Inverse Compton formulae, we derive a lower limit for the magnetic of

Cool core remnants in galaxy clusters

{\sim} 0.2{-}0.4\ \mu\rm{Gauss}

Cold fronts in galaxy clusters

consistent with Faraday rotation measurements.

The stripping of a galaxy group diving into the massive cluster A2142

Istituto di Astrofisica Spaziale e Fisica Cosmica, INAF, via Bassini 15, I-20133 Milano, ItalyPreprint online version: October 26, 2009AbstractContext.X ray clusters are conventionally divided into two classes: “cool core” (CC) and “non cool core” (NCC) objects, on the bas isof the observational properties of their central regions. Recent results have shown that the cluster population is bimodal (Cavagnoloet al. 2009).Aims.We want to understand whether the observed distribution of clusters is due to a primordial division into two distinct classesrather than to differences in how these systems evolve across cosmic time.Methods.We systematically search the ICM of NCC clusters in a subsample of the B55 flux limited sample of clusters for regionswhich have some characteristics typical of cool cores, namely low entropy gas and high metal abundanceResults.We find that most NCC clusters in our sample host regions remin iscent of CC, i. e. characterized by relative low entropy gas(albeit not as low as in CC systems) and a metal abundance excess. We have dubbed these structures “cool core remnants”, si nce weinterpret them as what remains of a cool core after a heating event (AGN giant outbursts in a few cases and more commonly mergers).We infer that most NCC clusters have undergone a cool core phase during their life. The fact that most cool core remnants are foundin dynamically active objects provides strong support to scenarios where cluster core properties are not fixed “ab initi o” but evolveacross cosmic time.Key words. Galaxies: clusters: general – X-rays: galaxies: clusters

Abell 2142 at large scales: An extreme case for sloshing?

Context. Cold fronts have been observed in several galaxy clusters. Understanding their nature and origin is extremely important for investigating the internal dynamics of clusters. Aims. To gain insight into the nature of these features, we carry out a statistical investigation of their occurrence in a sample of galaxy clusters observed with XMM-Newton and correlate this occurrence with different cluster properties. Methods. We selected a sample of 45 clusters starting from the B55 flux limited sample (Edge et al. 1990, MNRAS, 245, 559) and performed a systematic search for cold fronts. Results. We find that a large fraction of clusters host at least one cold front. Cold fronts are easily detected in all systems that are manifestly undergoing a merger event in the plane of the sky, while the presence of these features in the remaining clusters is related to a steep entropy gradient, in agreement with theoretical expectations. Assuming that cold fronts in cool core clusters are triggered by minor merger events, we estimate a minimum of 1/3 merging events per halo per Gyr.

Back and forth from cool core to non-cool core: clues from radio halos

Structure formation in the current Universe operates through the accretion of group-scale systems onto massive clusters. The detection and study of such accreting systems is crucial to understand the build-up of the most massive virialized structures we see today. We report the discovery with XMM-Newton of an irregular X-ray substructure in the outskirts of the massive galaxy cluster Abell 2142. The tip of the X-ray emission coincides with a concentration of galaxies. The bulk of the X-ray emission of this substructure appears to be lagging behind the galaxies and extends over a projected scale of at least 800 kpc. The temperature of the gas in this region is 1.4 keV, which is a factor of ~4 lower than the surrounding medium and is typical of the virialized plasma of a galaxy group with a mass of a few 10^13M_sun. For this reason, we interpret this structure as a galaxy group in the process of being accreted onto the main dark-matter halo. The X-ray structure trailing behind the group is due to gas stripped from its original dark-matter halo as it moves through the intracluster medium (ICM). This is the longest X-ray trail reported to date. For an infall velocity of ~1,200 km s-1 we estimate that the stripped gas has been surviving in the presence of the hot ICM for at least 600 Myr, which exceeds the Spitzer conduction timescale in the medium by a factor of >~400. Such a strong suppression of conductivity is likely related to a tangled magnetic field with small coherence length and to plasma microinstabilities. The long survival time of the low-entropy intragroup medium suggests that the infalling material can eventually settle within the core of the main cluster.

Thermo-dynamic and chemical properties of the intra-cluster medium

We present results obtained with a new XMM-Newton observation of A2142, a famous textbook example of cluster with multiple cold fronts, which has been studied in detail with Chandra but whose large scale properties are presented here for the first time. We report the discovery of a a new cold front, the most distant one ever detected in a galaxy cluster, at about one Mpc from the center to the SE. Residual images, thermodynamics and metal abundance maps are qualitatively in agreement with predictions from numerical simulations of the sloshing phenomenon. However, the scales involved are much larger, similarly to what recently observed in the Perseus cluster. These results show that sloshing is a cluster-wide phenomenon, not confined in the cores, which extends well beyond the cooling region involving a large fraction of the ICM up to almost half of the virial radius. The absence of a cool core and a newly discovered giant radio halo in A2142, in spite of its relaxed X-ray morphology, suggest that large scale sloshing, or the intermediate merger which caused it, may trigger Mpc-scale radio emission and may lead to the disruption of the cluster cool core