Azimuthally Resolved X-Ray Spectroscopy to the Edge of the Perseus Cluster

TL;DR

This study provides detailed azimuthally resolved X-ray observations of the Perseus Cluster's outskirts, revealing discrepancies with models and significant thermodynamic variations along different directions.

Contribution

It offers the most detailed large-radius X-ray analysis of a galaxy cluster, highlighting azimuthal variations and deviations from theoretical expectations.

Findings

Density profile flatter than simulations predict

Entropy profile below expected power-law in outskirts

Pressure excess beyond ~0.6r_200 compared to SZ models

Abstract

We present the results from extensive, new observations of the Perseus Cluster of galaxies, obtained as a Suzaku Key Project. The 85 pointings analyzed span eight azimuthal directions out to 2 degrees = 2.6 Mpc, to and beyond the virial radius r_200 ~ 1.8 Mpc, offering the most detailed X-ray observation of the intracluster medium (ICM) at large radii in any cluster to date. The azimuthally averaged density profile for r>0.4r_200 is relatively flat, with a best-fit power-law index of 1.69+/-0.13 significantly smaller than expected from numerical simulations. The entropy profile in the outskirts lies systematically below the power-law behavior expected from large-scale structure formation models which include only the heating associated with gravitational collapse. The pressure profile beyond ~0.6r_200 shows an excess with respect to the best-fit model describing the SZ measurements for a sample of clusters observed with Planck. The inconsistency between the expected and measured density, entropy, and pressure profiles can be explained primarily by an overestimation of the density due to inhomogeneous gas distribution in the outskirts; there is no evidence for a bias in the temperature measurements within the virial radius. We find significant differences in thermodynamic properties of the ICM at large radii along the different arms. Along the cluster minor axis, we find a flattening of the entropy profiles outside ~0.6r_200, while along the major axis, the entropy rises all the way to the outskirts. Correspondingly, the inferred gas clumping factor is typically larger along the minor than along the major axis.