Observational constraints on the co-evolution of supermassive black holes and galaxies

TL;DR

This study investigates the relationship between star formation and black hole activity in galaxies, revealing that they mostly occur in different events, challenging the idea of their simple co-evolution, and highlighting the role of galaxy structure.

Contribution

It provides observational evidence that star formation and black hole accretion are offset in time and occur in different galaxy events, refining models of galaxy and black hole co-evolution.

Findings

Less than 25% of star formation occurs in spheroid-dominated galaxies.

Expected black hole accretion rates are three times higher than observed in these galaxies.

Major star formation and black hole activity happen in different events, not simultaneously.

Abstract

The star formation rate (SFR) and black hole accretion rate (BHAR) functions are measured to be proportional to each other at z < ~3. This close correspondence between SF and BHA would naturally yield a BH mass-galaxy mass correlation, whereas a BH mass-bulge mass correlation is observed. To explore this apparent contradiction we study the SF in spheroid-dominated galaxies between z=1 and the present day. We use 903 galaxies from the COMBO-17 survey with M* >2x10^10M_sun, ultraviolet and infrared-derived SFRs from Spitzer and GALEX, and morphologies from GEMS HST/ACS imaging. Using stacking techniques, we find that <25% of all SF occurs in spheroid-dominated galaxies (Sersic index n>2.5), while the BHAR that we would expect if the global scalings held is three times higher. This rules out the simplest picture of co-evolution, in which SF and BHA trace each other at all times. These results could be explained if SF and BHA occur in the same events, but offset in time, for example at different stages of a merger event. However, one would then expect to see the corresponding star formation activity in early-stage mergers, in conflict with observations. We conclude that the major episodes of SF and BHA occur in different events, with the bulk of SF happening in isolated disks and most BHA occurring in major mergers. The apparent global co-evolution results from the regulation of the BH growth by the potential well of the galactic spheroid, which includes a major contribution from disrupted disk stars.