Heavily Obscured Quasar Host Galaxies at z~2 are Disks, Not Major Mergers

TL;DR

This study shows that heavily obscured quasar host galaxies at z~2 are predominantly disk galaxies, not major mergers, indicating secular processes mainly drive black hole growth rather than mergers.

Contribution

It provides evidence that most heavily obscured quasar hosts at z~2 are disk-dominated, challenging the idea that major mergers are the primary mechanism for black hole growth.

Findings

Only 4-25% of host galaxies are major mergers.

90% of hosts are disk-dominated or have significant disks.

Major mergers are not the dominant process in black hole growth at this epoch.

Abstract

We explore the nature of heavily obscured quasar host galaxies at z~2 using deep Hubble Space Telescope WFC3/IR imaging of 28 Dust Obscured Galaxies (DOGs) to investigate the role of major mergers in driving black hole growth. The high levels of obscuration of the quasars selected for this study act as a natural coronagraph, blocking the quasar light and allowing a clear view of the underlying host galaxy. The sample of heavily obscured quasars represents a significant fraction of the cosmic mass accretion on supermassive black holes as the quasars have inferred bolometric luminosities around the break of the quasar luminosity function. We find that only a small fraction (4%, at most 11-25%) of the quasar host galaxies are major mergers. Fits to their surface brightness profiles indicate that 90% of the host galaxies are either disk dominated, or have a significant disk. This disk-like host morphology, and the corresponding weakness of bulges, is evidence against major mergers and suggests that secular processes are the predominant driver of massive black hole growth. Finally, we suggest that the co-incidence of mergers and AGN activity is luminosity dependent, with only the most luminous quasars being triggered mostly by major mergers.