The Formation and Evolution of Supermassive Black Holes: The Extremely Luminous Quasar Survey

Abstract

Quasars allow to probe the formation and evolution of supermassive black holes throughout cosmic time. It is now understood that they are intimately related to properties of their host galaxies and thus are potentially important for galaxy evolution as well. The thesis at hand investigates the luminosity distribution of quasars during the phase of rapid black hole mass growth (z=2.5-4.5) and explores black hole host galaxy co-evolution on the basis of the quasar luminosity function (QLF). We designed the Extremely Luminous Quasar Survey (ELQS) to provide a highly complete sample of very luminous quasars at z=2.5-4.5. In order to achieve a high completeness for quasars at the targeted redshifts, we developed a highly inclusive infrared color cut using the J,K photometric bands of the Two Micron All Sky Survey (2MASS) as well as the W2 photometric band from the Wide Infrared Survey Explorer (WISE) AllWISE survey. With photometric redshifts and further star-quasar classification from Random Forests, a supervised machine-learning technique, the ELQS achieves a completeness of >=75% and a selection efficiency of 80%. We have followed up 184 promising quasar candidates and identified 109 new quasars at z=2.8-4.5 with dereddened SDSS i-band magnitudes of mi<=18.0, which make up ~26% of the full ELQS quasar sample (407 objects). We calculate the bright-end QLF on the basis of the full ELQS quasar sample and find the bright-end slope to be steep. Assuming a single power law with exponential density evolution, we find the data to be well represented by a slope of beta~-4.1 and an exponent of gamma~-0.4 for the density evolution. We constrain the bright-end slope to be steeper than beta<=-3.4 at 99% confidence. We further investigate BH and galaxy co-evolution of the aggregate galaxy population by deriving black hole mass densities (BHMDs) from QLFs and stellar mass densities (SMDs) from a variety of stellar mass functions and the cosmic star formation rate. We compare the BHMDs with the SMDs as a function of redshift and find them to grow in lock-step with time from z~3 to z=0.