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Issue A&A
Volume 425, Number 1, October I 2004
Page(s) 15 - 32
Section Cosmology (including clusters of galaxies)
DOI http://dx.doi.org/10.1051/0004-6361:20048005



A&A 425, 15-32 (2004)
DOI: 10.1051/0004-6361:20048005

RX J0256.5+0006 : A merging cluster of galaxies at z = 0.36 observed with XMM-NEWTON

S. Majerowicz1, D. M. Neumann1, A. K. Romer2, R. C. Nichol2, D. J. Burke3 and C. A. Collins4

1  CEA/Saclay, Service d'Astrophysique, L'Orme des Merisiers, Bât. 709, 91191 Gif-sur-Yvette Cedex, France
    e-mail: dneumann@cea.fr
2  Physics Department, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA
3  Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
4  Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead L41 1LD, UK

(Received 14 May 2003 / Accepted 9 June 2004)

Abstract
We present here a study based on XMM-NEWTON data of RX J0256.5+0006 , a medium distant ( z=0.36) cluster of galaxies found in the Bright SHARC catalog. The X-ray emitting intracluster medium shows a bimodal structure: one main cluster component and a substructure in the west, which very likely falls onto the main cluster centre. The subcluster shows after subtraction of the main cluster component a cometary shape pointing away from the main cluster centre, suggesting that ram pressure stripping is at work. Despite the indication of interaction between the two components we surprisingly do not find any sign of temperature gradients, which is contradictory to predictions from hydro dynamical simulations of cluster mergers. Due to the non-symmetric form of the main cluster we extract three surface brightness profiles in different sectors around its centre. We see large variations between the profiles, which we quantify by $\beta$ model fitting. The corresponding $r_{\rm c}$'s vary between 0.1-0.5 Mpc and the $\beta$'s between 0.5-1.2. The variations of the profiles and the $\beta$ model parameters indicate that the main cluster is not entirely relaxed. This hypothesis is strengthened further by the fact that the cluster is over luminous with respect to the ( z-evolving) $L_{\rm x}-T$ relation found for nearby clusters.

Galaxy clusters show a high degree of self-similarity. Comparing our profiles to the scaled reference emission measure profile of Arnaud et al. based on nearby clusters, we find that only the profile extracted north-east (NE) of the main cluster centre is similar to this reference profile. This indicates that only the NE profile is representative for the relaxed part of this cluster component.

Based on the $\beta$ model parameters of the NE profile and the spectroscopically fitted temperature of kT=4.9+0.5-0.4 keV we find for the total mass within r500 using the hydrostatic approach $M_{500}\sim 4\times 10^{14}$  $M_\odot$ for the main cluster component. This value is in good agreement with the value ( $M_{500}=3.9\times 10^{14}$  $M_\odot$) obtained using the z-evolving M500-T relation from the HIFLUGCS sample based on nearby clusters. A non- z-evolving M-T relation is only marginally consistent with our result. This is an indication that there exists evolution in the M-T relation, as predicted from simple scaling laws. Calculating the corresponding gas mass fraction we find $f_{\rm g}\sim 18{-}20\%$ which is in good agreement with other work.

We also develop a simple on-axis merger model for the cluster. As input we use the projected distance of the subcluster to the main cluster centre and the velocity difference of main and subcluster based on four galaxy redshifts spectroscopically measured with the Kitt Peak telescope. Together with a simple ram pressure model we find that the most likely physical distance of the subcluster to the main cluster lies between 0.6<d<1.0 Mpc. The coherent results of on-axis merger and ram pressure model suggest that the merger in this cluster is indeed on-axis and not an off-axis merger with a large impact parameter.

We find for the ratio of subcluster to main cluster mass values between 20-30% which indicates that the merger in RX J0256.5+0006 is a major merging event.


Key words: galaxies: clusters: individual: RX J0256.5+0006 -- galaxies: clusters: general -- cosmology: observations

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