Feedback Regulated Star Formation in Cool Core Clusters of Galaxies

The classical "cooling flow" model historically associated with "cool core" clusters of galaxies fails in the absence of an external, non-gravitational heating mechanism needed to offset catastrophic radiative losses of the X-ray bright intracluster medium (ICM). Numerous proposed solutions exist, including feedback from active galactic nuclei (AGN), which may elegantly calibrate fundamental relationships such as the coupled co-evolution of black holes and the stellar component of their host galaxies. AGN feedback cannot completely offset cooling at all times, however, as the brightest cluster galaxies (BCGs) in cool core clusters harbor extensive warm (∼10⁴ K) and cold (10 < T < 10⁴ K) gas reservoirs whose physical properties are regulated by ongoing star formation and an unknown, non-stellar heating mechanism.

We present a doctoral thesis broadly related to these issues, particularly as they pertain to cooling flows, the triggering of AGN activity, and the associated energetic feedback that may play a critical role in heating the ambient environment on tens to hundreds of kiloparsec scales. We begin with a summary of the relevant background material, and in Chapter 2 we present a multiwavelength study of effervescent AGN heating in the cool core cluster Abell 2597. Previously unpublished Chandra X-ray data show the central regions of the hot intracluster medium (ICM) to be highly anisotropic on the scale of the BCG, permeated by a network of kpc-scale X-ray cavities, the largest of which is cospatial in projection with extended 330 MHz radio emission. We present spectral maps of projected, modeled gas properties fit to the X-ray data. The X-ray temperature map reveals two discrete, "hard-edged'' structures, including a ∼15 kpc "cold filament'' and an arc of hot gas which in projection borders the inner edge of the large X-ray cavity. We interpret the latter in the context of the effervescent AGN heating model, in which cavity enthalpy is thermalized as the ambient keV gas rushes to refill the wake of the buoyant bubble. The hot arc revealed in the temperature map may be one of the first instances in which ICM/ISM heating by AGN feedback is directly observed. The ∼15 kpc soft excess filament, part of which is cospatial with extended 1.3 GHz radio emission, may be associated with dredge-up of low entropy gas by the propagating radio source. Results from our study of the hot X-ray gas are framed in the context of inferred young stellar component ages associated with the central emission line nebula in the BCG. We find that inferred ages of the young stellar component are both younger and older than the inferred ages of the X-ray cavities, suggesting that low levels of star formation have managed to persist amid the AGN feedback-driven excavation of the X-ray cavity network.

In Chapter 3 we present Hubble Space Telescope far-ultraviolet imaging of seven BCGs in cool core clusters selected on the basis of elevated star formation rates. We find that even at low levels, star formation provides a dominant contribution to the ionizing photon reservoir required to power the observed luminosities of the emission line nebula. Weak, compact radio sources are observed in each of these seven BCGs. The combination of higher SFR and lower radio power is consistent with a scenario wherein a low state of AGN feedback allows for increased residual condensation from the ambient X-ray atmosphere, accounting for the elevated star formation rates.

In Chapter 4 we present a comparison study of episodic star formation and AGN activity in the giant radio galaxy 3C 236, which is not associated with a cluster. We find that an episodic AGN/starburst connection can be fostered by a non-steady transport of gas to the nucleus. These results are then compared with Abell 2597, enabling a better understanding of the roles that may be played by cooling flows vs. mergers and hot vs. cold accretion modes in depositing the gaseous reservoirs that fuel both star formation and AGN activity.

In Chapter 5 we broaden the context of the thesis with a search for high redshift Fanaroff-Riley class I radio galaxies, which may act as observable "beacons" for assembling protoclusters. Probing the epoch of cluster assembly will be critical to a better understanding of the evolution of the cool core phenomenon and the history of cluster entropy regulation in general. The relative inability of X-ray cluster selection techniques to extend to these redshifts necessitates alternative detection methods, one of which we describe in this thesis.

Finally, in Chapter 6 we discuss the main conclusions of this thesis, which can be summarized as follows: (1) AGN feedback is real, and likely plays a dominant role in regulating the pathway of entropy loss from hot ambient medium to cold gas to star formation; (2) AGN feedback does not establish an impassable "entropy floor" below which gas cannot cool; and (3) star formation plays an important role in determining the temperature and ionization of the warm (∼10⁴ K) and cold (10 < T < 10⁴ K) gas phases in brightest cluster galaxies.