Date of Award
Doctor of Philosophy (PhD)
J. Michael Shull
Paul D. Beale
Julia M. Comerford
James C. Green
The spectra of active galactic nuclei (AGNs) are important probes of astrophysical phenomena on many scales. At the smallest scales, AGN spectra probe the accretion processes that control the growth of super massive black holes (SMBHs) and provide diagnostics of black hole masses. On galactic scales, AGNs play an important role in regulating star formation and controlling the coevolution of galaxies and their central SMBHs. On cosmological scales, AGN spectra, acting as backlights, provide a means for studying via absorption the diffuse gas of the intergalactic medium (IGM), the primary reservoir of baryons in the universe whose ionization state is largely controlled by ionizing flux from AGNs. A comprehensive understanding of the diverse, complex spectra of AGN is necessary to understand any of these phenomena, and because key atomic transitions and absorption edges lie in the ultraviolet (UV), many of these processes are best studied in the UV with space telescopes. In this thesis, I address the nature of several of these physical environments using UV spectra obtained with the Cosmic Origins Spectrograph (COS).
Using a large database of IGM absorption lines, I constrain the metal abundance of the IGM by comparing numerical photoionization models to the observed abundances of ions of carbon, sillicon, and oxygen. This work suggests an enhancement of alpha-process elements relative to solar abundances, and it also tests predictions of the metagalactic ionizing UV background spectrum (UVB).
By combining new and archival spectra of AGNs at redshifts 1.45 ≤ zAGN ≤ 2.14, I construct composite spectra that constrain the typical extreme-UV AGN spectrum, a key input for models of the UVB. I measure a typical EUV slope in frequency space of Fv ∝ v-0.72±0.26 and limit the intrinsic He I 504 Å photoelectric absorption edge opacity to less than 0.047 while also tentatively identifying numerous emission lines.
I also present preliminary results from a large, homogeneous catalog of absorption systems associated with AGNs that presumably trace gas outflows. This work represents the first step toward the largest sample of such absorbers in the low-redshift universe to date, and will enable a more comprehensive understanding of the physical nature of these absorbers and how they relate to their host galaxies.
Tilton, Evan Michael, "The Ultraviolet Spectra of Active Galactic Nuclei: Intrinsic Properties and Intervening Material" (2017). Astrophysical & Planetary Sciences Graduate Theses & Dissertations. 64.