Electronic Thesis and Dissertation Repository


Doctor of Philosophy




Sarah Gallagher


Although quasar spectra appear remarkably similar, subtle systematic differences are well- known and are linked to the physical conditions in the emitting regions. This thesis char- acterizes the properties of quasar optical and UV spectral features and reveals some of their underlying physical causes by analyzing large spectroscopic datasets from the Sloan Digital Sky Survey.

We first examine the narrow-line region using four optical emission lines with a range of ionization potentials (IPs) using a sample of 16,027 quasars with redshift z ≤ 0.75. We bin the sample using the continuum luminosity and the Hβ width to constrain the accretion rates and black hole masses and create composite spectra from objects in each bin. The luminosities of the high IP lines are constant with redshift consistent with no evolution in the quasar spectral energy distribution (SED) or the host galaxy ISM illuminated by the continuum. In contrast, the [O II] line becomes stronger at higher redshifts pointing to an enhanced star formation contributing to the [O II] at higher redshifts.

In the UV, we explore using unsupervised clustering (specifically the K-means algorithm) in searching for structure in the parameter space of quasar spectral measurements. We use measurements of 4,110 objects from the Paˆris et al. (2014) Catalog and perform K-means on the equivalent widths (EWs), and the red- and blue-half-width at half-maxima of Mg II, C IV, and C III]and use objects in each of the clusters to generate composite spectra. We find that quasars with larger C IV blueshifts and smaller EWs have softer ionizing radiation. This result is particularly interesting when seen using C III].

Next, we use a sample of 2,683 broad absorption line quasars from the Gibson et al. (2009) catalog. We apply K-means using the EWs and minimum and maximum outflow velocities of the C IV absorption trough and generate composite spectra from objects in each of the clusters. We find that objects with deep and broad C IV troughs are more likely to be variable and to show absorption from lower ionization transitions. Conversely, objects with shallower C IV troughs are less likely to be variable and have lower fractions of absorption from lower ionization lines. We examine this behaviour in light of properties such as SED hardness and intrinsic reddening.