Dissertation Defense: Meredith Powell, Yale University, “The Environments of Accreting Supermassive Black Holes”

Event time: 
Thursday, February 28, 2019 - 9:00am to 10:00am
Watson Center (WTS), A74 See map
60 Sachem Street
New Haven, CT 06511
Event description: 

The details of black hole-galaxy coevolution can be revealed by studying the multi-scale environments of accreting supermassive black holes (SMBH). Using state-of-the-art multiwavelength surveys of complementary depths, volumes, and resolutions, I studied the galaxies and cosmic environments that host Active Galactic Nuclei (AGN) to test current models of supermassive black hole fueling and feedback.

On galactic scales, I investigated the morphology vs. galaxy properties of 5000 galaxies and AGN hosts from the CANDELS/GOODS survey, in order to examine the role of major mergers in their evolution at z~1. The results were consistent with a scenario in which mergers are associated with rapid quenching in galaxies and some of the black hole accretion. However, I found that the majority of the AGN hosts were disk-dominated and therefore not triggered by a major merger event. I also constrained AGN feedback models by searching for hot outflows in the form of extended X-ray emission in two nearby AGN and comparing this emission with their spatially-resolved ionized gas kinematics. The lack of a detection indicated that either the coupling efficiencies between the AGN and ambient medium are small (< 5%), or the density of the medium through which the outflow travels is low ($N_H < 1$ cm$^{-3}$ at a distance of 100 pc).

The significance of major mergers in triggering AGN was further investigated by measuring the halo-scale environments of AGN, which are determined via clustering analyses. By measuring the spatial correlation function of hard X-ray selected AGN from the Swift/BAT AGN Spectroscopic Survey (BASS) and modeling it by populating dark matter halos from the Bolshoi-Planck simulation with empirical halo models, I constrained the halo occupation statistics of local AGN. I found that, on average, the AGN occupy halos consistently with inactive galaxies of the same stellar mass distribution. This suggests that the AGN are dominantly triggered by in-situ processes rather than environmental processes like mergers. However, when disaggregating by column density, I found that obscured AGN reside in denser environments than unobscured AGN, despite no significant differences in their luminosity, redshift, stellar mass, or Eddington ratio distributions. I showed that this could be due to their host halos having statistically different assembly histories. Lastly, I calculated the host halo masses of luminous X-ray AGN around the peak epoch of SMBH growth at z~1.8, which were found to be similar to previous studies of moderate-luminosity X-ray AGN. This suggests that selection biases are the cause for the slight clustering disparity observed between optical and X-ray AGN, rather than luminosity differences expected from distinct triggering mechanisms.

The independent investigations comprising this thesis work are consistent with the scenario where major mergers are not the dominant triggering mechanism for AGN from low-to-moderate redshifts, and instead are mainly triggered by secular processes. Additionally, while AGN feedback may play an important role for some objects, hot quasar winds are not ubiquitous among highly accreting AGN.

Thesis Advisor: Meg Urry