An Optically Obscured AGN in a Low Mass, Irregular Dwarf Galaxy: A Multi-Wavelength Analysis of J1329+3234

TL;DR

This study identifies a low-mass, irregular dwarf galaxy hosting an actively accreting massive black hole, revealed through multi-wavelength observations, challenging existing notions about black hole presence in such small, bulgeless galaxies.

Contribution

First multi-wavelength confirmation of a massive black hole in a low-mass, bulgeless dwarf galaxy, providing new insights into black hole formation in small galaxies.

Findings

Detection of a hard X-ray source indicating an active black hole

The galaxy's metallicity is not extremely low, ruling out low metallicity as sole cause

This is one of the lowest mass galaxies with evidence of a massive black hole

Abstract

Supermassive black holes (SMBHs) are found ubiquitously in large, bulge-dominated galaxies throughout the local universe, yet little is known about their presence and properties in bulgeless and low-mass galaxies. This is a significant deficiency, since the mass distribution and occupation fraction of nonstellar black holes provide important observational constraints on SMBH seed formation theories and many dwarf galaxies have not undergone major mergers that would erase information on their original black hole population. Using data from the Wide-field Infrared Survey Explorer, we discovered hundreds of bulgeless and dwarf galaxies that display mid-infrared signatures of extremely hot dust highly suggestive of powerful accreting massive black holes, despite having no signatures of black hole activity at optical wavelengths. Here we report, in our first follow-up X-ray investigation of this population, that the irregular dwarf galaxy J132932.41+323417.0 (z = 0.0156) contains a hard, unresolved X-ray source detected by XMM-Newton with luminosity L_2-10 keV = 2.4 x 10^40 erg/s, over two orders of magnitude greater than that expected from star formation, strongly suggestive of the presence of an accreting massive black hole. While enhanced X-ray emission and hot dust can be produced in extremely low metallicity environments, J132932.41+323417.0 is not extremely metal poor (~40% solar). With a stellar mass of 2.0 x 10^8 M_Sol, this galaxy is similar in mass to the Small Magellanic Cloud, and is one of the lowest mass galaxies with evidence for a massive nuclear black hole currently known.