Present‐Day Growth of Black Holes and Bulges: The Sloan Digital Sky Survey Perspective

We investigate the accretion-driven growth of supermassive black holes in thelow-redshift Universe using 23,000 ``Type 2'' AGN and the complete sample of123,000 galaxies in the SDSS from which they were drawn. We use the stellarvelocity dispersions of the early type galaxies and AGN hosts to estimate theirblack hole masses and we use the AGN [OIII]5007 emission line luminosities toestimate black hole accretion rates. We find that most present-day accretionoccurs onto black holes with masses < 10E8 Msun that reside in moderatelymassive galaxies (M ~ 10E10 to 10E11.5 Msun) with high stellar densities(~10E8.5 to 10E9.5 Msun/kpc^2) and young stellar populations. Thevolume-averaged accretion rates of low mass black holes (<10E7.5 Msun) implythat this population is growing on a timescale that is comparable to the age ofthe Universe. Around half this growth takes place in AGN that are radiatingwithin a factor of five of the Eddington luminosity. Such systems are rare,making up only 0.2% of the low mass black hole population at the present day.The rest of the growth occurs in lower luminosity AGN. The growth timescale ismore than two orders of magnitude longer for the population of the most massiveblack holes in our sample. The volume averaged ratio of star formation to blackhole accretion in bulge-dominated galaxies is ~1000, in remarkable agreementwith the observed ratio of stellar mass to black hole mass in nearby galaxybulges. We conclude: a) that bulge formation and black hole formation aretightly coupled, even in present-day galaxies; and b) that the evolution of theAGN luminosity function documented in recent optical and x-ray surveys isdriven by a decrease in the characteristic mass scale of accreting black holes.Comment: ApJ, in pres