The Effect of Environment on the Formation of Halpha Filaments and Cool
  Cores in Galaxy Groups and Clusters
  We present the results of a combined X-ray and Halpha 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 Halpha filaments in groups versus clusters, over the
mass range 10^13 < M_500 < 10^15 Msun. The detection frequency of Halpha
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 Halpha
filaments is strongly correlated with both the global (89% confidence level)
and core (84%) ICM entropy, as well as the X-ray cooling rate (72%). The Halpha
filaments are therefore an excellent proxy for the cooling ICM. The Halpha
filaments are more strongly correlated with the cooling properties of the ICM
than with the radio properties of the BCG; 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, AGN
feedback scales with the total mass of the system, in agreement with earlier
suggestions.
