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

Abstract

We investigate the accretion-driven growth of supermassive black holes in the low-redshift universe using 23,000 narrow-emission-line ("type 2") active galactic nuclei (AGNs) and the complete sample of 123,000 galaxies in the Sloan Digital Sky Survey from which they were drawn. We use the stellar velocity dispersions of the early-type galaxies and AGN hosts to estimate their black hole masses, and we use the AGN [O III] λ5007 emission line luminosities to estimate black hole accretion rates. We find that most present-day accretion occurs onto black holes with masses less than 10 8 M ☉ that reside in moderately massive galaxies ( M * ≈ 10 10 -10 11.5 M ☉ ) with high stellar surface mass densities (μ * ≈ 10 8.5 -10 9.5 M ☉ kpc -2 ) and young stellar populations. The volume-averaged accretion rates of low-mass black holes (<3 × 10 7 M ☉ ) imply that this population is growing on a timescale that is comparable to the age of the universe. Around half this growth takes place in AGNs that are radiating within a factor of 5 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 AGNs. The growth timescale is more than 2 orders of magnitude longer for the population of the most massive black holes in our sample. The volume-averaged ratio of star formation to black hole accretion in bulge-dominated galaxies is ~1000, in remarkable agreement with the observed ratio of stellar mass to black hole mass in nearby galaxy bulges. We conclude that (1) bulge formation and black hole formation are tightly coupled, even in present-day galaxies, and (2) the evolution of the AGN luminosity function documented in recent optical and X-ray surveys is driven by a decrease in the characteristic mass scale of actively accreting black holes.