A Candidate Massive Black Hole in the Low-metallicity Dwarf Galaxy Pair
  Mrk 709
  The incidence and properties of present-day dwarf galaxies hosting massive
black holes (BHs) can provide important constraints on the origin of
high-redshift BH seeds. Here we present high-resolution X-ray and radio
observations of the low-metallicity, star-forming, dwarf-galaxy system Mrk 709
with the Chandra X-ray Observatory and the Karl G. Jansky Very Large Array.
These data reveal spatially coincident hard X-ray and radio point sources with
luminosities suggesting the presence of an accreting massive BH (M_BH ~
10^5-10^7 Msun). Based on imaging from the Sloan Digital Sky Survey (SDSS), we
find that Mrk 709 consists of a pair of compact dwarf galaxies that appear to
be interacting with one another. The position of the candidate massive BH is
consistent with the optical center of the southern galaxy (Mrk 709 S), while no
evidence for an active BH is seen in the northern galaxy (Mrk 709 N). We derive
stellar masses of M ~ 2.5 x 10^9 Msun and M ~ 1.1 x 10^9 Msun for Mrk 709 S and
Mrk 709 N, respectively, and present an analysis of the SDSS spectrum of the
BH-host Mrk 709 S. At a metallicity of just ~10% solar, Mrk 709 is among the
most metal-poor galaxies with evidence for an active galactic nucleus.
Moreover, this discovery adds to the growing body of evidence that massive BHs
can form in dwarf galaxies and that deep, high-resolution X-ray and radio
observations are ideally suited to reveal accreting massive BHs hidden at
optical wavelengths.
