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

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_* > 2 × 10¹⁰ M_⊙, ultraviolet and infrared-derived SFRs from Spitzer and Galaxy Evolution Explorer, and morphologies from GEMS Hubble Space Telescope/Advanced Camera for Surveys imaging. Using stacking techniques, we find that <25% of all SF occurs in spheroid-dominated galaxies (Sérsic index n > 2.5), while the BHAR that we would expect if the global scalings held is 3 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.