Little evidence for entropy and energy excess beyond $r_{500}$ - An end to ICM preheating?

TL;DR

This study investigates the presence of entropy and energy excess in the outer regions of galaxy clusters to evaluate the preheating hypothesis, finding little evidence supporting significant preheating beyond $r_{500}$.

Contribution

The paper provides the first direct observational constraints on ICM preheating beyond $r_{500}$, challenging previous assumptions of high preheating entropy levels.

Findings

Preheating entropy floor of ≥300 keV cm$^2$ is statistically ruled out.

Feedback energy of 1 keV/particle beyond $r_{500}$ is strongly excluded.

Results suggest minimal non-gravitational heating effects in outer cluster regions.

Abstract

Non-gravitational feedback affects the nature of the intra-cluster medium (ICM). X-ray cooling of the ICM and in situ energy feedback from AGN's and SNe as well as {\it preheating} of the gas at epochs preceding the formation of clusters are proposed mechanisms for such feedback. While cooling and AGN feedbacks are dominant in cluster cores, the signatures of a preheated ICM are expected to be present even at large radii. To estimate the degree of preheating, with minimum confusion from AGN feedback/cooling, we study the excess entropy and non-gravitational energy profiles upto $r_{200}$ for a sample of 17 galaxy clusters using joint data sets of {\it Planck} SZ pressure and {\it ROSAT/PSPC} gas density profiles. The canonical value of preheating entropy floor of $\gtrsim 300$ keV cm$^2$, needed in order to match cluster scalings, is ruled out at $\approx 3\sigma$. We also show that the feedback energy of 1 keV/particle is ruled out at 5.2$\sigma$ beyond $r_{500}$. Our analysis takes both non-thermal pressure and clumping into account which can be important in outer regions. Our results based on the direct probe of the ICM in the outermost regions do not support any significant preheating.