Amanda Bredahl Larssen MSc defense

Title: Dust reddening of quasars: A study of redshift and luminosity dependence of extinction, and of dust as main explanation of quasar UV SED diversity.

Supervisor: Marianne Vestergaard, DARK
Censor: Hans Kjeldsen, Aarhus University

Abstract: Establishing an accurate account of the properties of dust in quasar nuclei and host galaxies has historically been hampered by the fact that the intrinsically emitted spectrum is not unique and universal. For a large sample of quasar spectra obtained from the Sloan Digital Sky Survey I investigate the extinction of quasars to test if and how it changes as a function of redshift and source luminosity - an aspect that has not previously been investigated. I also test if the diversity of the observed quasars can be explained by dust extinction alone.

I select a subsample of nearly 2500 UV spectra in the redshift range 1.90 < z < 2.35 with luminosities in the range 46.3 < log (L_bol /erg s⁻¹) < 48.2, for which I measure the amount of extinction through continuum fits for two models of extinction, namely dust from the Small Magellanic Cloud (SMC) and the Large Magellanic Cloud (LMC), respectively.

Dust in quasars is usually assumed to be SMC like, but I find that the extinction in many cases can be described equally well with the LMC dust model. This has interesting implications for our understanding of quasars. I see no dependence on source luminosity or redshift for the average amount of extinction of quasars for either the SMC or LMC dust models. I do however find indications of an evolution in the type of dust affecting quasars, with LMC like dust more common at higher redshifts. This is an intriguing result that deserves a closer investigation in the future.

Link to thesis –>>here