Suzaku observations of the low surface brightness cluster A76

TL;DR

Suzaku observations of galaxy cluster A76 reveal a low surface brightness, high central entropy, irregular morphology, and low electron density, indicating it is in an early formation stage with lagging gas compression.

Contribution

This study provides detailed X-ray spectral analysis and radial profiles of A76, highlighting its unique thermodynamic properties and early formation state, which were not previously characterized.

Findings

Global temperature ~3.3 keV and metal abundance ~0.24 solar.

Entropy profile is flat with high central entropy (~400 keV cm^2).

Electron density is extremely low despite high temperature.

Abstract

Context: We present 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. Aims: To understand 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 (~ 0.6 r_200) and 560 kpc (~0.4 r_200) in A76 East and A76 West, respectively. 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 < 0.2r_200) is exceptionally high (~400 keV cm^2). This phenomenon 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 (1e-4 -- 1e-3 cm^-3) for the observed high temperature, suggesting that A76 is in the early phase of cluster formation and the gas compression due to gravitational potential confinement is lagging behind the gas heating.