What is a Cool-Core Cluster? A Detailed Analysis of the Cores of the X-ray Flux-Limited HIFLUGCS Cluster Sample

TL;DR

This study analyzes the cores of 64 galaxy clusters using X-ray data, applying multiple diagnostics to distinguish cool-core from non-cool-core clusters and exploring their properties and implications for cooling flow models.

Contribution

It introduces new diagnostics for identifying cool cores and demonstrates the necessity of heating mechanisms to explain observed cooling flows in galaxy clusters.

Findings

72% of clusters have cool cores

44% have strong cool cores

Discrepancy in mass deposition rates indicates heating mechanisms are needed

Abstract

We use the largest complete sample of 64 galaxy clusters (HIghest X-ray FLUx Galaxy Cluster Sample) with available high-quality X-ray data from Chandra, and apply 16 cool-core diagnostics to them, some of them new. We also correlate optical properties of brightest cluster galaxies (BCGs) with X-ray properties. To segregate cool core and non-cool-core clusters, we find that central cooling time, t_cool, is the best parameter for low redshift clusters with high quality data, and that cuspiness is the best parameter for high redshift clusters. 72% of clusters in our sample have a cool core (t_cool < 7.7 h_{71}^{-1/2} Gyr) and 44% have strong cool cores (t_cool <1.0 h_{71}^{-1/2} Gyr). For the first time we show quantitatively that the discrepancy in classical and spectroscopic mass deposition rates can not be explained with a recent formation of the cool cores, demonstrating the need for a heating mechanism to explain the cooling flow problem. [Abridged]