The accretion history of supermassive black holes

Summary

How did the mass of 10⁸−10¹⁰M_⊙ super-massive black holes at the center of massive galaxies in the local Universe build up? Did the bulk of the growth happen in an optically luminous AGN phase? Or did a substantial fraction of SMBH growth occur in a dusty, obscured phase, visible as a luminous infrared galaxy? Has there been substantial SMBH growth in a low luminosity or radiatively inefficient regime after the more luminous AGN phase? These are particularly important questions, given the tight relationship between the mass of galaxy bulges and their SMBHs, suggesting that the formation and evolution of galaxies and SMBHs are intimately linked. We use the multi-wavelength data in the NDWFS Boötes field to address this issue. We have performed an x-ray stacking analysis of ~20,000 red galaxies at z = 0.2−1 to show that the average nuclear accretion rates in these sources are low and decreasing with time. Given the long timescale, significant SMBH mass growth could occur in this regime. We also investigate the nature of an extreme, obscured population of AGN-dominated luminous infrared galaxies which are likely to host SMBHs undergoing a period of rapid and substantial growth.

Introduction

In the present day Universe, most (if not all) massive galaxies contain super-massive black holes (SMBHs). How did the mass of these SMBHs grow as a function of time? The correlation between the mass of SMBHs and the galaxy bulge in which they reside (Magorrian et al. 1998; Gebhardt et al. 2000; Ferrarese & Merritt 2000) suggests that the processes which govern the build up of galaxies and SMBHs are related.