Tracing the X-ray emitting intra-cluster medium of clusters of galaxies beyond r

AIM - Unité Mixte de Recherche CEA - CNRS - Université Paris VII - UMR N° 7158 CEA/Saclay; DSM/DAPNIA/Service d'Astrophysique, CEA/Saclay, L'Orme des Merisiers, Bât. 709, 91191 Gif-sur-Yvette, France e-mail: ddon@cea.fr

Received: 8 March 2005
Accepted: 2 May 2005

Abstract

We present in this paper a study of a sample of 14 nearby clusters of galaxies () observed with the ROSAT/PSPC. We only select clusters with low galactic absorption (  10²⁰ cm^-2) in order to trace the hot X-ray emitting intra-cluster medium (ICM) out to large radii. We convert the X-ray surface brightness profiles of the clusters into emission measure profiles scaled to the classical scaling relations based on the spherical collapse model. We sort the clusters into different temperature bins and stack the scaled emission measure (ScEM) profiles of clusters belonging to the same bin together. This method enhances the statistics of the profiles – especially in outer regions. The stacked profiles allow us to observe a signal out to radii . 
In the center () we find that the ScEM profiles deviate from predicted scaling laws. Hotter clusters have systematically a higher ScEM than cooler clusters. This result is in very good agreement with current studies on the  relation and the entropy-temperature relation () found recently. At radii we find that the ScEM profiles agree well within the error bars, suggesting self-similarity. Fitting beta-models to the overall ScEM profiles, we find for the different subsamples and , which is higher than the canonical value of often found. The beta-model is generally a better representation for hotter than for cooler clusters. We see indications for continuous steepening of the profiles with increasing radius: at radii the profiles are systematically below the beta-model curve with . 
We discuss our results with respect to the observed X-ray luminosity  relation, the gas mass  relation and the total mass  relation. We also address implications on the origin of the observed  relation. Furthermore, we discuss the observed steepness of the X-ray profiles, which falls off more rapidly than predicted from the NFW-profile for cold dark matter halos.