Master's Thesis Defense by Isabelle Gauger
Supervisor: Marianne Vestergaard
External examiner: Hans Kjeldsen, Aarhus University
Title: Metal abundances in the broad line region of distant quasars and in their host galaxies
Abstract: The main goal of this thesis is to compare the metal abundance in the broad line region (BLR) of quasars with the metal abundance in their host galaxies. For a sample of high-redshift SDSS quasars, I measure the BLR gas metallicity using different broad emission line flux ratios. Because the broad emission lines (BEL) often heavily blend and some are very weak, a sophisticated fitting procedure is needed to measure individual line fluxes. I have written a program to perform spectral decomposition and line fitting, which was a major part of this thesis project. To convert the measured BEL flux ratios into metallicities, I use theoretical predictions from the literature. The results generally indicate that the metal abundance in the BLR of high-redshift quasars is several times solar, which is in agreement with previous studies.
Due to the large brightness contrast, it is not possible to measure the metallicities in the host galaxies of the sample quasars directly. Instead, I measure the masses of the central black holes (BH) and use empirical relations to estimate the metallicities in the host galaxies based on those measurements. For comparison, I also include a nearby quasar sample in my analysis, for which BH masses and emission line flux measurements are available in the literature. The results indicate that the metal abundance in the BLR of high-redshift quasars is significantly higher than the metallicity in their host galaxies. For the nearby quasars, the metal abundance in the BLR is generally lower than for the high-redshift quasars and the measurements seem to scatter around the host metallicity.
I also investigate the relation between the metal abundance in the BLR and different quasar properties. I find a trend between BLR gas metallicity and BH mass. This trend suggests that the lower BLR gas metallicities of the nearby quasars might be related to their lower BH masses.