Global cluster morphology and its evolution: X-ray data vs CDM, LCDM and mixed models

The global structure of galaxy clusters and its evolution are tested within a large set of TREESPH simulations, so to allow a fair statistical comparison with available X-ray data. Structure tests are based on the "power ratios", introduced by Buote & Tsai. Cosmological models considered are CDM, LCDM (Omega_L=0.7) and CHDM (1 mass.neu., Omega_h = 0.2). All models are normalized to provide a fair number density of clusters. For each model we run a P3M simulation in a large box, where we select the most massive 40 clusters. Going back to the initial redshift we run a hydro-TREESPH simulation for each of them. In this way we perform a statistical comparison of the global morphology of clusters, for each cosmological model, with ROSAT data, using Student t-test, F-test and K-S test. The last test and its generalization to 2--D distributions are also used to compare the joint distributions of 2 or 3 power ratios. We find that, using DM distribution, instead of gas, as done by some authors, leads to biased results, as baryons are distributed in a less structured way than DM. We also find that the cosmological models considered have different behaviours in these tests: LCDM has the worst performance. CDM and our CHDM have similar scores. The general trend of power ratio distributions is already fit by these models, but a further improvement is expected either from a different DM mix or a non-flat CDM model.Comment: 29 pages (LaTeX,macros included), 9 figure.ps & 3 tables included. To appear on New Astronom

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

X-ray astronomers often divide galaxy clusters into two classes: "cool core" (CC) and "non-cool core" (NCC) objects. The origin of this dichotomy has been the subject of debate in recent years, between "evolutionary" models (where clusters can evolve from CC to NCC, mainly through mergers) and "primordial" models (where the state of the cluster is fixed "ab initio" by early mergers or pre-heating). We found that in a well-defined sample (clusters in the GMRT Radio halo survey with available Chandra or XMM-Newton data), none of the objects hosting a giant radio halo can be classified as a cool core. This result suggests that the main mechanisms which can start a large scale synchrotron emission (most likely mergers) are the same that can destroy CC and therefore strongly supports "evolutionary" models of the CC-NCC dichotomy. Moreover combining the number of objects in the CC and NCC state with the number of objects with and without a radio-halo, we estimated that the time scale over which a NCC cluster relaxes to the CC state, should be larger than the typical life-time of radio-halos and likely shorter than about 3 Gyr. This suggests that NCC transform into CC more rapidly than predicted from the cooling time, which is about 10 Gyr in NCC systems, allowing the possibility of a cyclical evolution between the CC and NCC states.Comment: Accepted for publication in A&

Strong Magnetization Measured in the Cool Cores of Galaxy Clusters

Tangential discontinuities, seen as X-ray edges known as cold fronts (CFs), are ubiquitous in cool-core galaxy clusters. We analyze all 17 deprojected CF thermal profiles found in the literature, including three new CFs we tentatively identify (in clusters A2204 and 2A0335). We discover small but significant thermal pressure drops below all nonmerger CFs, and argue that they arise from strong magnetic fields below and parallel to the discontinuity, carrying 10%-20% of the pressure. Such magnetization can stabilize the CFs, and explain the CF-radio minihalo connection.Comment: PRL accepted, additional control tests adde

Where does the gas fueling star formation in BCGs originate?

We investigate the relationship between X-ray cooling and star formation in brightest cluster galaxies (BCGs). We present an X-ray spectral analysis of the inner regions, 10-40 kpc, of six nearby cool core clusters (z<0.35) observed with Chandra ACIS. This sample is selected on the basis of the high star formation rate (SFR) observed in the BCGs. We restrict our search for cooling gas to regions that are roughly cospatial with the starburst. We fit single- and multi-temperature mkcflow models to constrain the amount of isobarically cooling intracluster medium (ICM). We find that in all clusters, below a threshold temperature ranging between 0.9 and 3 keV, only upper limits can be obtained. In four out of six objects, the upper limits are significantly below the SFR and in two, namely A1835 and A1068, they are less than a tenth of the SFR. Our results suggests that a number of mechanisms conspire to hide the cooling signature in our spectra. In a few systems the lack of a cooling signature may be attributed to a relatively long delay time between the X-ray cooling and the star burst. However, for A1835 and A1068, where the X-ray cooling time is shorter than the timescale of the starburst, a possible explanation is that the region where gas cools out of the X-ray phase extends to very large radii, likely beyond the core of these systems.Comment: to appear in A&

A cluster in a crowded environment: XMM-Newton and Chandra observations of A3558

Combining XMM-Newton and Chandra data, we have performed a detailed study of Abell 3558. Our analysis shows that its dynamical history is more complicated than previously thought. We have found some traits typical of cool core clusters (surface brightness peaked at the center, peaked metal abundance profile) and others that are more common in merging clusters, like deviations from spherical symmetry in the thermodynamic quantities of the ICM. This last result has been achieved with a new technique for deriving temperature maps from images. We have also detected a cold front and, with the combined use of XMM-Newton and Chandra, we have characterized its properties, such as the speed and the metal abundance profile across the edge. This cold front is probably due to the sloshing of the core, induced by the perturbation of the gravitational potential associated with a past merger. The hydrodynamic processes related to this perturbation have presumably produced a tail of lower entropy, higher pressure and metal rich ICM, which extends behind the cold front for about 500 kpc. The unique characteristics of A3558 are probably due to the very peculiar environment in which it is located: the core of the Shapley supercluster