Disk assembly and the MBH–σe relation of supermassive black holes

Abstract

Recent Hubble Space Telescope observations have revealed that a majority of active galactic nuclei (AGNs) at z ∼ 1–3 are resident in isolated disk galaxies, contrary to the usual expectation that AGNs are triggered by mergers. Here we develop a new test of the cosmic evolution of supermassive black holes (SMBHs) in disk galaxies by considering the local population of SMBHs. We show that substantial SMBH growth in spiral galaxies is required as disks assemble. SMBHs exhibit a tight relation between their mass and the velocity dispersion of the spheroid within which they reside, the M•–σe relation. In disk galaxies the bulge is the spheroid of interest. We explore the evolution of the M•–σe relation when bulges form together with SMBHs on the M•–σe relation and then slowly re-form a disk around them. The formation of the disk compresses the bulge, raising its σe. We present evidence for such compression in the form of larger velocity dispersion of classical bulges compared with elliptical galaxies at the same mass. This compression leads to an offset in the M•–σe relation if it is not accompanied by an increased M•. We quantify the expected offset based on photometric data and show that, on average, SMBHs must grow by ∼50%–65% just to remain on the M•–σe relation. We find no significant offset in the M•–σe relations of classical bulges and of ellipticals, implying that SMBHs have been growing along with disks. Our simulations demonstrate that SMBH growth is necessary for the local population of disk galaxies to have remained on the M•–σe relation.