The redshift dependence of the MBH–Mhost relation in quasars.

This Thesis is focused on the study of the evolution of the MBH–Lhost, Mhost relations as a function of redshift. The aim is to probe whether (and how) the black hole – host galaxy relations evolve during Cosmic ages. This would contribute in constraining the possible formation histories of galaxies and in unveiling the mechanisms regulating the co-existence of black holes and galaxies. The present study addresses the joint evolution of black holes and galaxies from an observational point of view. Our sample consists of 96 quasars with 0 < z < 3, half of which are radio loud (chapter 2). The stellar luminosities of the host galaxies are taken from imaging studies available in the literature, and from new observations led by our group (chapter 3). The stellar mass is derived from the host galaxy luminosity, under certain assumptions on the star formation history of the galaxy (chapter 4). Spectroscopic determinations of the black hole masses are derived from the analysis of new data taken on purpose (chapters 3 and 5). The whole dataset is available in the appendix sections A and B. From the study of the MBH/Mhost ratio at low-z, where evolutionary effects are negligible, we infer some constraints on the geometry of the broad line region. When moving to high redshift (z = 2−3), we find that the MBH–Lhost relation is practically unchanged. On the other hand, because of the evolution of the stellar mass-to-light ratio, the MBH–Mhost relation shows a clear evolution, in the sense that, for a given host galaxy mass, black holes at high redshift are ∼ 8 times more massive than their low-z counter-parts. This trend cannot be explained in terms of selection effects, and is confirmed after relaxing some operative assumptions. Our results are compared to those of similar works available in the literature. Sketches of possible interpretative scenarios are also discussed (chapter 6). Conclusions and future perspectives are presented in chapter 7. The present work principally deals with the analyses and studies published in Decarli et al. (2008a, on the broad line region geometry), Kotilainen et al. (2009, on the luminosity of quasar host galaxies at z > 2), Decarli et al. (2009b, on the spectroscopic dataset) and Decarli et al. (2009c, on the evolution of the BH–host galaxy relations). A complete list of my publications during the PhD is provided in appendix C.