The formation of bulges and black holes: lessons from a census of active galaxies in the SDSS

摘要

We examine the relationship between galaxies, supermassive black holes andAGN using a sample of 22,000 narrow-emission-line AGN drawn from a a sample of122,000 galaxies from the Sloan Digital Sky Survey. We have studied how AGNhost properties compare with those of normal galaxies and how they depend onthe luminosity of the active nucleus. We find that AGN reside in massivegalaxies and have distributions of sizes and concentrations that are similar tothose of the early-type galaxies in our sample. The host galaxies oflow-luminosity AGN have stellar populations similar to normal early-types. Thehosts of high-luminosity AGN have much younger mean stellar ages and asignificant fraction have experienced recent starbursts. High-luminosity AGNare also found in lower density environments. We use the stellar velocitydispersions of the AGN hosts to estimate black hole masses and their[OIII]$\lambda$5007 emission line luminosities to estimate black hole accretionrates. We find that the volume averaged ratio of star formation to black holeaccretion is ~1000, in remarkable agreement with the observed ratio of stellarmass to black hole mass in nearby bulges. Our estimated accretion rates implythat low mass black holes are growing on a timescale that is comparable to theage of the Universe. The growth timescale increases by more than an order ofmagnitude for the most massive black holes in our sample. We conclude that theevolution of the AGN luminosity function is driven by a decrease in thecharacteristic mass scale of actively accreting black holes.