A2163: Merger events in the hottest Abell galaxy cluster

Abstract

Located at z = 0.203, A2163 is a rich galaxy cluster with an intra-cluster medium (ICM) that exhibits extraordinary properties, including an exceptionally high X-ray luminosity, average temperature, and a powerful and extended radio halo. The irregular and complex morphology of its gas and galaxy structure suggests that this cluster has recently undergone major merger events that involve two or more cluster components. In this paper, we study the gas structure and dynamics by means of spectral-imaging analysis of X-ray data obtained from XMM-Newton and Chandra observations. From the evidence of a cold front, we infer the westward motion of a cool core across the E-W elongated atmosphere of the main cluster A2163-A. Located close to a galaxy over-density, this gas "bullet" appears to have been spatially separated from its galaxy (and presumably dark matter component) as a result of high-velocity accretion. From gas brightness and temperature profile analysis performed in two opposite regions of the main cluster, we show that the ICM has been adiabatically compressed behind the crossing "bullet" possibly because of shock heating, leading to a strong departure of the ICM from hydrostatic equilibrium in this region. Assuming that the mass estimated from the Y X proxy best indicates the overall mass of the system and that the western cluster sector is in approximate hydrostatic equilibrium before subcluster accretion, we infer a merger scenario between two subunits of mass ratio 1:4, leading to a present total system mass of M 500 1.9 × 10 15 M . Additional analysis of the spatially-separated northern subcluster A2163-B does not show any evidence of strong interaction with the main cluster A2163-A, leading us to infer that the physical distance separating the northern subcluster and the main component is longer than the projected separation of these components. The exceptional properties of A2163 present various similarities with those of 1E0657-56, the so-called "bullet-cluster". These similarities are likely to be related to a comparable merger scenario.

Dark matter distribution in the merging cluster Abell 2163

“…This scenario remains fully compatible with the present observation of the dark matter distribution traced by weak lensing and is verified by numerical simulations of cluster mergers including both dark matter and intracluster gas physics (Roettiger et al 1997). Moreover, it has been recently confirmed by the analysis of Bourdin et al (2011), who show evidence of a cool gas "bullet" close to the cluster center that is clearly separated from its galaxy component and identified here as the second mass peak of the cluster.…”

“…Interestingly, we are able to measure a mass ratio ∼3:1 between the 2 clumps, although this result must be taken with caution. This is not too different from the mass ratio 4:1 estimated by Bourdin et al (2011) from interpretating of the merging scenario. Our ratio 3:1 also seems closer to the apparent ratio in the galaxy distribution and could be more representative of the total mass repartition.…”

“…Even the much less significant peaks in the mass distribution are coincident with other subclusters, namely the northern subcluster A2163-B, and other peripheral clumps C and D. The correlation between the dark matter distribution and the cluster galaxies density distribution clearly demonstrates that they both trace a noncollisional component in merging clusters. These maps are very different from the X-ray map (Govoni et al 2004;Bourdin et al 2011), which traces the gas distribution with the following characteristics: a single peaked distribution, with an E-W elongation and a peak center significantly off-center compared to BCG1. Maurogordato et al (2008) propose in their paper a scenario in which we observe A2163 in a post-merger phase, less than 1 Gyr after the main collision.…”

Planckintermediate results

“…The XMM-Newton data have been prepared and analysed using the procedure described in detail in Bourdin et al (2011), andMazzotta (2008). We estimated the Y X = M gas × T parameter of Coma iteratively using the Y X − M 500 scaling relation calibrated from hydrostatic mass estimates in a nearby cluster sample observed with XMM-Newton (Arnaud et al 2010); we find R 500 ≈ (47 ± 1)arcmin ≈ (1.31 ± 0.03) Mpc and we use this value throughout the paper.…”

“…We estimated the Y X = M gas × T parameter of Coma iteratively using the Y X − M 500 scaling relation calibrated from hydrostatic mass estimates in a nearby cluster sample observed with XMM-Newton (Arnaud et al 2010); we find R 500 ≈ (47 ± 1)arcmin ≈ (1.31 ± 0.03) Mpc and we use this value throughout the paper. To study the surface brightness and temperature radial profiles we use the forward approach described in Bourdin et al (2011) taking care to project the temperature profile using the formula appropriate for spectroscopy; i.e., we use the spectroscopic-like temperature introduced by Mazzotta et al (2004).…”