A textbook example of ram-pressure stripping in the Hydra A/A780 cluster

In the current epoch, one of the main mechanisms driving the growth of galaxy clusters is the continuous accretion of group-scale halos. In this process, the ram pressure applied by the hot intracluster medium on the gas content of the infalling group is responsible for stripping the gas from its dark-matter halo, which gradually leads to the virialization of the infalling gas in the potential well of the main cluster. Using deep wide-field observations of the poor cluster Hydra A/A780 with XMM-Newton and Suzaku, we report the discovery of an infalling galaxy group 1.1 Mpc south of the cluster core. The presence of a substructure is confirmed by a dynamical study of the galaxies in this region. A wake of stripped gas is trailing behind the group over a projected scale of 760 kpc. The temperature of the gas along the wake is constant at kT ~ 1.3 keV, which is about a factor of two less than the temperature of the surrounding plasma. We observe a cold front pointing westwards compared to the peak of the group, which indicates that the group is currently not moving in the direction of the main cluster, but is moving along an almost circular orbit. The overall morphology of the group bears remarkable similarities with high-resolution numerical simulations of such structures, which greatly strengthens our understanding of the ram-pressure stripping process

X-ray characterisation of the massive galaxy clusterClG-J104803.7+313843 at z=0.76 with XMM-Newton

We present the characterisation of the massive cluster ClG-J$104803.7+313843$ at $z=0.76$ performed using a serendipitous XMM-Newton observation. High redshift and massive objects represent an ideal laboratory to benchmark our understanding of how cluster form and assembly formation driven mainly by gravity.Leveraging the high throughput of XMM-Newton we were firstly able to determine the redshift of the object, shedding light on ambiguous photometric redshift associations. We investigated the morphology of this cluster which shows signs of merging activities in the outskirts and a flat core. We also measured the radial density profile up to $R_{500}$. With these quantities in hand, we were able to determine the mass, $M_{500}=5.64^{+0.79}_{-0.62} \times 10^{14}M_{\odot}$, using the YX proxy. This quantity improves previous measurement of the mass of this object by a factor of $\sim 3.5$. The characterisation of one cluster at such mass and redshift regime is fundamental as these objects are intrinsically rare, the number of objects discovered so far being less than $\sim 25$. Our study highlights the importance of using X-ray observations in combination with ancillary multi-wavelength data to improve our understanding of high-z and massive clustersComment: Submitted to A&

Gas sloshing, cold fronts, Kelvin-Helmholtz instabilities and the merger history of the cluster of galaxies Abell 496

We investigate the origin and nature of the multiple sloshing cold fronts in the core of Abell 496 by direct comparison between observations and dedicated hydrodynamical simulations. Our simulations model a minor merger with a 4{\times}10^13M{\circ} subcluster crossing A496 from the south-west to the north-north-east, passing the cluster core in the south-east at a pericentre distance 100 to a few 100 kpc about 0.6 to 0.8 Gyr ago. The gas sloshing triggered by the merger can reproduce almost all observed features, e.g. the characteristic spiral-like brightness residual distribution in the cluster centre and its asymmetry out to 500 kpc, also the positions of and contrasts across the cold fronts. If the subcluster passes close (100 kpc) to the cluster core, the resulting shear flows are strong enough to trigger Kelvin-Helmholtz instabilities that in projection resemble the peculiar kinks in the cold fronts of Abell 496. Finally, we show that sloshing does not lead to a significant modification of the global ICM profiles but a mild oscillation around the initial profiles.Comment: MNRAS, accepted, 19 page

Low heat conduction in white dwarf boundary layers?

X-ray spectra of dwarf novae in quiescence observed by Chandra and XMM-Newton provide new information on the boundary layers of their accreting white dwarfs. Comparison of observations and models allows us to extract estimates for the thermal conductivity in the accretion layer and reach conclusions on the relevant physical processes. We calculate the structure of the dense thermal boundary layer that forms under gravity and cooling at the white dwarf surface on accretion of gas from a hot tenuous ADAF-type coronal inflow. The distribution of density and temperature obtained allows us to calculate the strength and spectrum of the emitted X-ray radiation. They depend strongly on the values of thermal conductivity and mass accretion rate. We apply our model to the dwarf nova system VW Hyi and compare the spectra predicted for different values of the thermal conductivity with the observed spectrum. We find a significant deviation for all values of thermal conductivity that are a sizable fraction of the Spitzer conductivity. A good fit arises however for a conductivity of about 1% of the Spitzer value. This also seems to hold for other dwarf nova systems in quiescence. We compare this result with thermal conduction in other astrophysical situations. The highly reduced thermal conductivity in the boundary layer requires magnetic fields perpendicular to the temperature gradient. Locating their origin in the accretion of magnetic fields from the hot ADAF-type coronal flow we find that dynamical effects of these fields will lead to a spatially intermittent, localized accretion geometry at the white dwarf surface.Comment: 8 pages, 5 figs, to appear in Astronomy & Astrophysic

Mixed Models with n>1 and Large Scale Structure constraints

Recent data on CBR anisotropies show a Doppler peak higher than expected in CDM cosmological models, if the spectral index $n=1$. However, CDM and LCDM models with n>1 can hardly be consistent with LSS data. Mixed models, instead, whose transfer function is naturally steeper because of free--streaming in the hot component, may become consistent with data if n>1, when Omega_h is large. This is confirmed by our detailed analysis, extended both to models with a hot component whose momentum space distribution had a thermal origin (like massive neutrinos), and to models with a non--cold component arising from heavier particle decay. In this work we systematically search models which fulfill all constraints which can be implemented at the linear level. We find that a stringent linear constraint arises from fitting the extra-power parameter Gamma. Other significant constraints arise comparing the expected abundances of galaxy clusters and high-z systems with observational data. Keeping to models with Gamma \geq 0.13, a suitable part of the space parameter still allows up to \sim 30% of hot component (it is worth outlining that our stringent criteria allow only models with 0.10 \mincir Omega_h \mincir 0.16, if n \leq 1). We also outline that models with such large non--cold component would ease the solution of the so--called baryon catastrophe in galaxy clusters.Comment: 28 pages + 9 figures, uses elsart.sty, to be published in New Astronom

Sloshing cold fronts in galaxy groups and their perturbing disk galaxies: an X-ray, Optical and Radio Case Study

We present a combined X-ray, optical, and radio analysis of the galaxy group IC 1860 using the currently available Chandra and XMM data, literature multi-object spectroscopy data and GMRT data. The Chandra and XMM imaging and spectroscopy reveal two surface brightness discontinuities at 45 and 76 kpc shown to be consistent with a pair of cold fronts. These features are interpreted as due to sloshing of the central gas induced by an off-axis minor merger with a perturber. This scenario is further supported by the presence of a peculiar velocity of the central galaxy IC 1860 and the identification of a possible perturber in the optically disturbed spiral galaxy IC 1859. The identification of the perturber is consistent with the comparison with numerical simulations of sloshing. The GMRT observation at 325 MHz shows faint, extended radio emission contained within the inner cold front, as seen in some galaxy clusters hosting diffuse radio mini-halos. However, unlike mini-halos, no particle reacceleration is needed to explain the extended radio emission, which is consistent with aged radio plasma redistributed by the sloshing. There is strong analogy of the X-ray and optical phenomenology of the IC 1860 group with two other groups, NGC 5044 and NGC 5846, showing cold fronts. The evidence presented in this paper is among the strongest supporting the currently favored model of cold-front formation in relaxed objects and establishes the group scale as a chief environment to study this phenomenon.Comment: 22 pages, 21 figures, accepted for publication in the Astrophysical Journa

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

We present results obtained with a new XMM-Newton observation of A2142, a textbook example of a 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 new cold front, the most distant one ever detected in a galaxy cluster, at about 1 Mpc from the center to the SE. Residual images, thermodynamics, and metal abundance maps qualitatively agree with predictions from numerical simulations of the sloshing phenomenon. However, the scales involved are much larger, similar to what has been recently observed in the Perseus cluster. These results show that sloshing is a cluster-wide phenomenon and is not confined in the cores. Sloshing extends well beyond the cooling region, involving a high 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 that caused it, may trigger Mpc-scale radio emission and may lead to the disruption of the cluster cool core