Dartmouth Events

Physics & Astronomy - Virtual PhD Thesis Defense -Christopher Carroll, Dartmouth

Title: "Uncovering Hidden Monsters: Revealing the Full Population of Luminous, Obscured Supermassive Black Holes"

Friday, August 6, 2021
12:00pm – 2:00pm
Zoom: email for link and password
Intended Audience(s): Public
Categories: Lectures & Seminars

Abstract: Supermassive black holes are ubiquitous at the centers of massive galaxies, and grow through periods of accretion in which nuclear material reaches the event horizon of thes exotic objects. During periods of growth, these active galactic nuclei (AGNs) create tremendous amounts of radiation---via conversion of gravitational energy to kinetic power---which dominates across the electromagnetic spectrum. Different energy regimes contribute to our understanding of AGNs in distinctive ways, and all-sky infrared surveys have affirmed that a large fraction of the AGN population is obscured by intervening material, sometimes cabable of concealing even the highest energy X-rays. The exact fraction of the most heavily obscured AGN population is unknown, and no direct method can probe the density of obscuring material. In my dessertation, I provide a reliable technique for selecting heavily obscured AGNs in archival data, and estimates on the total fraction of Compton-think (CT) AGNs. Combining datasets from multiple observations, I created a multiwavelength catalog of 33 million sources from which to search for heavily obscured AGNs. Using a combination of low-resolution spectral templates, I modeled the spectral energy distributions of my catalog sources to identify AGNs. Restricting my sample to X-ray survey fields, I detected a large sample of AGNs lacking X-ray detections, likley due to extreme obscuration. I then conducted a Monte Carlo simulation to forward model the distribution of AGN obscuration, and infer the total fraction of CT AGNs in the universe. My work has confirmed a simple, computationally inexpensive technique for modeling AGNs that is adequate for identifying the most heavily obscured AGNs in extremely large datasets, expanding our ability to select these sources from the vast amount of archival data currently possessed by the AGN community. From my modeling, I determined that at least 50% of total population of AGN are CT, which is in exceptional agreement with the most recent work using completely separate, but comparable, methods. In summary, my work has major implications for our understanding of black hole accretion physics and the role AGNs play in cosmic evolution.

For more information, contact:
Tressena Manning

Events are free and open to the public unless otherwise noted.