Suzaku observations of the low surface brightness cluster A76

¹ Department of PhysicsNara Women’s University, Kitauoyanishimachi, Nara, 630-8501 Nara, Japan
e-mail: naomi@cc.nara-wu.ac.jp
² Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany

Received: 10 April 2013
Accepted: 19 May 2013

Abstract

Aims. We present the results of Suzaku observations of a nearby galaxy cluster, A76 at z = 0.0395. This cluster is characterized by extremely low X-ray surface brightness and is hereafter referred to as the LSB cluster. We investigate the nature and thermodynamic evolution of the LSB cluster by studying the physical properties of the hot intracluster medium in A76.

Methods. We conducted two-pointed Suzaku observations of A76 and examined the global gas properties of the cluster by XIS spectral analysis. We also performed deprojection analysis of annular spectra and derived radial profiles of gas temperature, density, and entropy out to approximately 850 kpc (~0.6r₂₀₀) in A76 East and 560 kpc (~0.4r₂₀₀) in A76 West.

Results. The measured global temperature and metal abundance are approximately 3.3 keV and 0.24 solar, respectively. From the deprojection analysis, the entropy profile is found to be flat with respect to radius. The entropy within the central region (r < 0.2r₂₀₀) is exceptionally high (~400  keV   cm²), a phenomenon that is not readily explained by either gravitational heating or preheating. The X-ray morphology is clumped and irregular, and the electron density is extremely low (10^-4−10^-3  cm^-3) for the observed high temperature, suggesting that A76 is in the early phase of cluster formation and that the gas compression due to gravitational potential confinement is lagging behind the gas heating.