Black hole accretion and host galaxies of obscured quasars in XMM-COSMOS

TL;DR

This study investigates the properties of host galaxies of obscured quasars in the XMM-COSMOS survey, revealing their star formation activity, morphology, and accretion rates, and how these relate to galaxy evolution and black hole growth.

Contribution

It provides new insights into the host galaxy characteristics of obscured quasars, especially their star formation and morphological features, across different redshifts, challenging previous assumptions about galaxy evolution.

Findings

Most obscured quasars reside in massive, bulge-dominated galaxies.

Star formation rates in host galaxies increase with redshift, matching those of normal star-forming galaxies.

Eddington ratios correlate with galaxy morphology, with bulge-dominated hosts having low ratios, and disks or mergers having high ratios.

Abstract

We explore the connection between black hole growth at the center of obscured quasars selected from the XMM-COSMOS survey and the physical properties of their host galaxies. We study a bolometric regime (<Lbol > 8 x 10^45 erg/s) where several theoretical models invoke major galaxy mergers as the main fueling channel for black hole accretion. We confirm that obscured quasars mainly reside in massive galaxies (Mstar>10^10 Msun) and that the fraction of galaxies hosting such powerful quasars monotonically increases with the stellar mass. We stress the limitation of the use of rest-frame color-magnitude diagrams as a diagnostic tool for studying galaxy evolution and inferring the influence that AGN activity can have on such a process. We instead use the correlation between star-formation rate and stellar mass found for star-forming galaxies to discuss the physical properties of the hosts. We find that at z ~1, ~62% of Type-2 QSOs hosts are actively forming stars and that their rates are comparable to those measured for normal star-forming galaxies. The fraction of star-forming hosts increases with redshift: ~71% at z ~2, and 100% at z ~3. We also find that the the evolution from z ~1 to z ~3 of the specific star-formation rate of the Type-2 QSO hosts is in excellent agreement with that measured for star-forming galaxies. From the morphological analysis, we conclude that most of the objects are bulge-dominated galaxies, and that only a few of them exhibit signs of recent mergers or disks. Finally, bulge-dominated galaxies tend to host Type-2 QSOs with low Eddington ratios (lambda<0.1), while disk-dominated or merging galaxies have at their centers BHs accreting at high Eddington ratios (lambda > 0.1).