Was 49b: An Overmassive AGN in a Merging Dwarf Galaxy?

Abstract

We present a combined morphological and X-ray analysis of Was. 49, an isolated, dual-AGN system notable for the presence of a dominant AGN, Was 49b, in the disk of the primary galaxy, Was 49a, at a projected radial distance of 8. kpc from the nucleus. Using X-ray data from Chandra, the Nuclear Spectroscopic Telescope Array, and Swift, we find that this AGN has a bolometric luminosity of L-bol similar to 10(45) erg s(-1), with a black hole mass of M-BH = 1.3(-0.9)(+10)M(circle dot) . Despite the large mass, our analysis of optical data from the Discovery Channel Telescope shows that the supermassive black hole (SMBH) is hosted by a stellar counterpart with a mass of only 5.6(-2.6)(+4.9)M(circle dot), which makes the SMBH potentially larger than expected from SMBH-galaxy scaling relations, and the stellar counterpart exhibits a morphology that is consistent with dwarf elliptical galaxies. Our analysis of the system in the r and K bands indicates that Was. 49 is a minor merger, with the mass ratio of Was 49b to Was 49a between similar to 1:7 and similar to 1:15. This is in contrast with findings that the most luminous merger-triggered AGNs are found in major mergers and that minor mergers predominantly enhance AGN activity in the primary galaxy.