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

TL;DR

This paper reports the discovery of a hidden broad line region in a bulgeless galaxy, enabling black hole mass estimation and insights into SMBH growth without a bulge, using Keck NIR spectroscopy.

Contribution

First detection of a broad line region in a bulgeless Seyfert 2 galaxy, providing rare black hole mass estimates and implications for SMBH formation theories.

Findings

Black hole mass estimated at log(M_BH/M_sun)=6.78 ± 0.50.

Detection of counter-rotating gas within the central kiloparsec.

Possible evidence of recent galaxy interaction triggering SMBH activity.

Abstract

We report the discovery of a buried, active supermassive black hole 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 black hole mass and bulge component. Using virial mass estimators, we obtain a black hole mass of log($M_{\rm{BH}}/M_{\odot}$) = $6.78 \pm 0.50$. This is one of the only Sy2 AGN hosted in a bulgeless galaxy with a virial black hole mass estimate and could provide important constraints on the formation scenarios of the black hole 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 $M_{\rm{BH}}$ - $M_{\rm{stellar}}$ relation to be more reliable than the $M_{\rm{BH}}$ - $M_{\rm{bulge}}$ relation. In addition, we detect extended narrow Pa$\alpha$ emission that allows us to create a rotation curve where we see counter-rotating gas within the central kiloparsec (kpc). Possible causes of this counter-rotation 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.