An Entropy Threshold for Strong H-alpha and Radio Emission in the Cores of Galaxy Clusters

TL;DR

This study finds a critical entropy threshold of 30 keV cm^2 in galaxy cluster cores, below which strong H-alpha and radio emissions indicate active star formation and AGN feedback, revealing a key dichotomy in cluster core properties.

Contribution

It identifies a specific entropy threshold that correlates with the presence of multiphase gas, star formation, and AGN activity in galaxy cluster cores, providing insight into feedback mechanisms.

Findings

H-alpha and radio emissions are more common below the 30 keV cm^2 entropy threshold.

Clusters below this threshold often host multiphase gas and star formation.

AGN feedback appears to activate when core gas condenses below the entropy boundary.

Abstract

Our Chandra X-ray Observatory archival study of intracluster entropy in a sample of 222 galaxy clusters shows that H-alpha and radio emission from the brightest cluster galaxy are much more pronounced when the cluster's core gas entropy is < 30 keV cm^2. The prevalence of H-alpha emission below this threshold indicates that it marks a dichotomy between clusters that can harbor multiphase gas and star formation in their cores and those that cannot. The fact that strong central radio emission also appears below this boundary suggests that AGN feedback turns on when the intracluster medium starts to condense, strengthening the case for AGN feedback as the mechanism that limits star formation in the Universe's most luminous galaxies.