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