The Effect of Environment on the Formation of Hα Filaments and Cool Cores in Galaxy Groups and Clusters

We present the results of a combined X-ray and Hα study of 10 galaxy groups and 17 galaxy clusters using the Chandra X-ray Observatory and the Maryland Magellan Tunable Filter. We find no difference in the morphology or detection frequency of Hα filaments in groups versus clusters over the mass range 1013 < M 500 < 1015 M ☉. The detection frequency of Hα emission is shown to be only weakly dependent on the total mass of the system at the 52% confidence level. In contrast, we find that the presence of Hα filaments is strongly correlated with both the global (89% confidence level) and core (84%) intracluster medium (ICM) entropy, as well as the X-ray cooling rate (72%). The Hα filaments are therefore an excellent proxy for the cooling ICM. The Hα filaments are more strongly correlated with the cooling properties of the ICM than with the radio properties of the brightest cluster galaxy; this further supports the scenario where these filaments are directly associated with a thermally unstable, rapidly cooling ICM, rather than radio bubbles. The ICM cooling efficiency, defined as the X-ray cooling rate per unit gas mass, is shown to correlate with the total system mass, indicating that groups are more efficient at cooling than clusters. This result implies that, in systems with cool cores, active galactic nucleus feedback scales with the total mass of the system, in agreement with earlier suggestions.