An X-ray Selected Sample of Candidate Black Holes in Dwarf Galaxies
  We present a sample of hard X-ray selected candidate black holes (BHs) in 19
dwarf galaxies. BH candidates are identified by cross-matching a parent sample
of ~44,000 local dwarf galaxies (M_stellar < 3 x 10^9 Msun, z<0.055) with the
Chandra Source Catalog, and subsequently analyzing the original X-ray data
products for matched sources. Of the 19 dwarf galaxies in our sample, 8 have
X-ray detections reported here for the first time. We find a total of 43
point-like hard X-ray sources with individual luminosities L(2-10 keV) ~ 10^37
- 10^40 erg/s. Hard X-ray luminosities in this range can be attained by
stellar-mass X-ray binaries (XRBs), and by massive BHs accreting at low
Eddington ratio. We place an upper limit of 53% (10/19) on the fraction of
galaxies in our sample hosting a detectable hard X-ray source consistent with
the optical nucleus, although the galaxy center is poorly defined in many of
our objects. We also find that 42% (8/19) of the galaxies in our sample exhibit
statistically significant enhanced hard X-ray emission relative to the expected
galaxy-wide contribution from low-mass and high-mass XRBs, based on the L(2-10
keV)-M_stellar-SFR relation defined by more massive and luminous systems. For
the majority of these X-ray enhanced dwarf galaxies, the excess emission is
consistent with (but not necessarily due to) a nuclear X-ray source. Follow-up
observations are necessary to distinguish between stellar-mass XRBs and active
galactic nuclei powered by more massive BHs. In any case, our results support
the notion that X-ray emitting BHs in low-mass dwarf galaxies may have had an
appreciable impact on reionization in the early Universe.
