Oral Presentation

The Triggering of Cold Gas Formation and AGN Feedback within Galaxy Clusters

Author(s): Michael Hogan (Waterloo)

Presenter: Michael Hogan (University of Waterloo & Perimeter Institute)

The mechanical actions of jets generated within central dominant galaxies are believed to be responsible for counteracting catastrophic cooling in galaxy clusters. Nevertheless, residual gas cooling from the hot phase does occur. This results in reservoirs of molecular gas within cool-core clusters, whose distribution and dynamics are now able to be mapped with ALMA. These processes are believed to be cyclical, fluctuating between periods of gas cooling and condensation, and periods of increased heating from heightened AGN activity. A crucial piece of this puzzle is understanding what physically triggers gas cooling. Recently, the existence of a cooling threshold when the ratio of the cooling time (tc) to the free-fall time (tff) falls below a certain value (~4-20) has gained popularity. I will discuss our work investigating this possible cooling trigger. We have developed methods to trace the mass profiles, and subsequently acceleration profiles, of a large number of clusters down to the small radii at which this threshold is typically met. We compare the resultant tc/tff values to tracers of both cooling (CO, H-alpha) and AGN feedback (radio), and discuss the relative merits of the tc/tff ratio as a tracer of the onset of cooling, compared to that of the cooling time alone.