X-Ray Study of the Outer Region of Abell 2142 with Suzaku

TL;DR

This study used Suzaku to observe the outer regions of galaxy cluster Abell 2142, revealing temperature and entropy profiles that suggest the intracluster medium is out of thermal equilibrium, with implications for understanding cluster physics.

Contribution

First detailed measurement of temperature and entropy profiles out to the virial radius of Abell 2142 using Suzaku, highlighting deviations from equilibrium models.

Findings

Temperature drops from 9 keV to 3.5 keV at $r_{200}$

Entropy profile flattens and becomes negative near $r_{200}$

Set upper limits on warm-hot intergalactic medium density

Abstract

We observed outer regions of a bright cluster of galaxies A2142 with Suzaku. Temperature and brightness structures were measured out to the virial radius ($r_{200}$) with good sensitivity. We confirmed the temperature drop from 9 keV around the cluster center to about 3.5 keV at $r_{200}$, with the density profile well approximated by the $\beta$ model with $\beta = 0.85$. Within $0.4\r_{200}$, the entropy profile agrees with $r^{1.1}$, as predicted by the accretion shock model. The entropy slope becomes flatter in the outer region and negative around $r_{200}$. These features suggest that the intracluster medium in the outer region is out of thermal equilibrium. Since the relaxation timescale of electron-ion Coulomb collision is expected to be longer than the elapsed time after shock heating at $r_{200}$, one plausible reason of the low entropy is the low electron temperature compared to that of ions. Other possible explanations would be gas clumpiness, turbulence and bulk motions of ICM\@. We also searched for a warm-hot intergalactic medium around $r_{200}$ and set an upper limit on the oxygen line intensity. Assuming a line-of-sight depth of 2 Mpc and oxygen abundance of 0.1 solar, the upper limit of an overdensity is calculated to be 280 or 380, depending on the foreground assumption.