Unveiling a population of galaxies harboring low-mass black holes with
  X-rays
  We report the discovery of three low-mass black hole candidates residing in
the centers of low-mass galaxies at z<0.3 in the Chandra Deep Field - South
Survey. These black holes are initially identified as candidate active galactic
nuclei based on their X-ray emission in deep Chandra observations.
Multi-wavelength observations are used to strengthen our claim that such
emission is powered by an accreting supermassive black hole. While the X-ray
luminosities are low at L_X ~ 10^40 erg s^-1 (and variable in one case), we
argue that they are unlikely to be attributed to star formation based on
H\alpha or UV-fluxes. Optical spectroscopy with Keck/DEIMOS and VLT/FORS allows
us to (1) measure accurate redshifts, (2) confirm their low stellar host mass,
(3) investigate the source(s) of photo-ionization, and (4) estimate extinction.
With stellar masses of M* < 3*10^9 M_\sun determined from HST/ACS imaging, the
host galaxies are among the lowest mass systems known to host actively
accreting black holes. We estimate BH masses M_BH ~ 2*10^5 M_\sun based on
scaling relations between BH mass and host properties for more luminous
systems. In one case, a broad component of the H\alpha emission-line profile is
detected thus providing a virial mass estimate. Black holes in such low-mass
galaxies are of considerable interest as the low-redshift analogs to the seeds
of the most massive BHs at high redshift which have remained largely elusive to
date. Our study highlights the power of deep X-ray surveys to uncover such
low-mass systems.
