Hubble Space Telescope Imaging of the Active Dwarf Galaxy RGG 118

TL;DR

This study uses Hubble imaging to analyze the structure of the dwarf galaxy RGG 118, revealing a pseudobulge and an under-massive black hole, supporting secular growth models for black holes in disk galaxies.

Contribution

First detailed structural analysis of RGG 118 with Hubble imaging, confirming the galaxy's pseudobulge and the black hole's under-massiveness relative to bulge relations.

Findings

RGG 118 has a pseudobulge and spiral disk morphology.

The central black hole is under-massive compared to bulge relations.

Black hole growth likely occurs through secular processes.

Abstract

RGG 118 (SDSS 1523+1145) is a nearby ($z=0.0243$), dwarf disk galaxy ($M_{\ast}\approx2\times10^{9} M_{\odot}$) found to host an active $\sim50,000$ solar mass black hole at its core (Baldassare et al. 2015). RGG 118 is one of a growing collective sample of dwarf galaxies known to contain active galactic nuclei -- a group which, until recently, contained only a handful of objects. Here, we report on new \textit{Hubble Space Telescope} Wide Field Camera 3 UVIS and IR imaging of RGG 118, with the main goal of analyzing its structure. Using 2-D parametric modeling, we find that the morphology of RGG 118 is best described by an outer spiral disk, inner component consistent with a pseudobulge, and central PSF. The luminosity of the PSF is consistent with the central point source being dominated by the AGN. We measure the luminosity and mass of the "pseudobulge" and confirm that the central black hole in RGG 118 is under-massive with respect to the $M_{BH}-M_{\rm bulge}$ and $M_{BH}-L_{\rm bulge}$ relations. This result is consistent with a picture in which black holes in disk-dominated galaxies grow primarily through secular processes.