Author ORCID Identifier

https://orcid.org/0000-0002-0504-565X

Date of Award

2026

Document Type

Thesis (Ph.D.)

Department or Program

Physics and Astronomy

First Advisor

Ryan Hickox

Second Advisor

Elisabeth Newton

Third Advisor

John Thorstensen

Abstract

Within ΛCDM, galaxies form and evolve within dark matter halos, with both central and satellite galaxies contributing to the total stellar mass content. Virtually every massive galaxy harbors a supermassive black hole (SMBH) at its center, with a small fraction of the population growing by actively accreting material; we call this an active galactic nucleus (AGN). The processes giving rise to an AGN phase (i.e., growth of a SMBH) releases enormous amounts of energy in the form of radiation, outflows, and relativistic jets that are able to significantly affect the formation and evolution of galaxies. Blazars are an extreme class of AGNs with relativistic jets oriented at small angles to our line-of-sight and rapid variability.

I leverage observational data and empirical models to investigate AGNs from two distinct perspectives. First, I obtained spectropolarimetric observations of two blazars, PKS 0637–75 and PKS 1510–089, to determine how the head-on orientation of a jet and dominant emission mechanism influence polarimetric variations in the broad lines and continuum. Polarization measurements confirm the conclusions drawn from spectral energy distribution (SED) modeling of the disk–jet contributions to the emission as optical polarization and time variability for PKS 0637–75 are shown to be dominated by the accretion disk while those of PKS 1510–089 are due to both the disk and jet. Second, I investigated how host galaxy properties are related to the incidence of AGN activity by determining the minimum stellar mass necessary for a satellite galaxy to harbor an AGN in the redshift range 0.15 < z < 1.1, detected via different wavelengths. The radio-selected sample comes from LOFAR observations of the Boötes, ELAIS-N1, and Lockman Hole fields and the X-ray-selected sample comes from the Chandra X-ray Observatory of the Boötes field. In our redshift bins I find a mass threshold, log(Mstar/Msun), of 10.3 ± 0.3 , 10.8 ± 0.6, and 10.9 ± 0.3 for the radio-selected sample and of 9.6 ± 0.1, 9.8 ± 0.1, and 9.9 ± 0.1 for the X-ray-selected sample.

In these projects, we saw that a change in the dominant emission process, e.g., thermal or non-thermal, affects continuum polarization measurements (Project 1) and for the first time we place constraints on the minimum stellar mass of satellite galaxies hosting radio-selected (Project 2) and X-ray-selected (Project 3) AGNs.

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