The Discovery of a Hidden Broad-line AGN in a Bulgeless Galaxy: Keck NIR Spectroscopic Observations of SDSS J085153.64+392611.76

Abstract

Abstract We report the discovery of a buried, active supermassive black hole (SMBH) in SDSS J085153.64+392611.76, a bulgeless Seyfert 2 (Sy2) galaxy. Keck near-infrared observations reveal a hidden broad-line region, allowing for the rare case where strong constraints can be placed on both the BH mass and bulge component. Using virial mass estimators, we obtain a BH mass of log(M BH/M ⊙) = 6.78 ± 0.50. This is one of the only Sy2 active galactic nuclei (AGNs) hosted in a bulgeless galaxy with a virial BH mass estimate and could provide important constraints on the formation scenarios of the BH seed population. The lack of a bulge component suggests that the SMBH has grown quiescently, likely caused by secular processes independent of major mergers. In the absence of a detectable bulge component, we find the M BH–M stellar relation to be more reliable than the M BH–M bulge relation. In addition, we detect extended narrow Paα emission that allows us to create a rotation curve where we see counterrotating gas within the central kiloparsec. Possible causes of this counterrotation include a galactic bar or disruption of the inner gas by a recent fly-by of a companion galaxy. This in turn could have triggered accretion onto the central SMBH in the current AGN phase.