Cold fronts in galaxy clusters

Cold fronts have been observed in a large number of galaxy clusters. Understanding their nature and origin is of primary importance for the investigation of the internal dynamics of clusters. To gain insight on 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 we correlate their presence with different cluster properties. We have selected a sample of 45 clusters starting from the B55 flux limited sample by Edge et al. (1990) and performed a systematic search of cold fronts. 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 such features in the remaining clusters is related to the presence of 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.Comment: Accepted for publication in Astronomy & Astrophysics. Version with full resolution figures available at: http://www.iasf-milano.inaf.it/~simona/pub/coldfronts/ghizzardi.pd

Metal distribution in sloshing galaxy clusters: the case of A496

We report results from a detailed study of the sloshing gas in the core of A496. We detect the low temperature/entropy spiral feature found in several cores, we also find that conduction between the gas in the spiral and the ambient medium must be suppressed by more than one order of magnitude with respect to Spitzer conductivity. Intriguingly, while the gas in the spiral features a higher metal abundance than the surrounding medium, it follows the entropy vs metal abundance relation defined by gas lying outside the spiral. The most plausible explanation for this behavior is that the low entropy metal rich plasma uplifted through the cluster atmosphere by sloshing, suffers little heating or mixing with the ambient medium. While sloshing appears to be capable of uplifting significant amounts of gas, the limited heat exchange and mixing between gas in and outside the spiral implies that this mechanism is not at all effective in: 1) permanently redistributing metals within the core region and 2) heating up the coolest and densest gas, thereby providing little or no contribution to staving of catastrophic cooling in cool cores.Comment: Accepted for publication on A&

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

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

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&

SMAUG: a new technique for the deprojection of galaxy clusters

This paper presents a new technique for reconstructing the spatial distributions of hydrogen, temperature and metal abundance of a galaxy cluster. These quantities are worked out from the X-ray spectrum, modeled starting from few analytical functions describing their spatial distributions. These functions depend upon some parameters, determined by fitting the model to the observed spectrum. We have implemented this technique as a new model in the XSPEC software analysis package. We describe the details of the method, and apply it to work out the structure of the cluster A1795. We combine the observation of three satellites, exploiting the high spatial resolution of Chandra for the cluster core, the wide collecting area of XMM-Newton for the intermediate regions and the large field of view of Beppo-SAX for the outer regions. We also test the validity and precision of our method by i) comparing its results with those from a geometrical deprojection, ii) examining the spectral residuals at different radii of the cluster and iii) reprojecting the unfolded profiles and comparing them directly to the measured quantities. Our analytical method yields the parameters defining the spatial functions directly from the spectra. Their explicit knowledge allows a straightforward derivation of other indirect physical quantities like the gravitating mass, as well as a fast and easy estimate of the profiles uncertainties.Comment: 24 pages, 11 figures, 3 tables; emulateapj; accepted for publication in the Astrophysical Journa