A Catalog of Hyper-luminous X-ray Sources and Intermediate-Mass Black
  Hole Candidates out to High Redshifts
  Hyper-luminous X-ray sources (HLXs; L_X>10^41 erg s^-1) are off-nuclear X-ray
sources in galaxies and strong candidates for intermediate-mass black holes
(IMBHs). We have constructed a sample of 169 HLX candidates by combining X-ray
detections from the Chandra Source Catalog (Version 2) with galaxies from the
Sloan Digital Sky Survey and registering individual images for improved
relative astrometric accuracy. The spatial resolution of Chandra allows for the
sample to extend out to z~0.9. Optical counterparts are detected among
one-fourth of the sample, one-third of which are consistent with dwarf galaxy
stellar masses. The average intrinsic X-ray spectral slope indicates efficient
accretion, potentially driven by galaxy mergers, and the column densities
suggest one-third of the sample has significant X-ray absorption. We find that
144 of the HLX candidates have X-ray emission that is significantly in excess
of the expected contribution from star formation and hot gas, strongly
suggesting that they are produced by accretion onto black holes more massive
than stars. After correcting for an average background or foreground
contamination rate of 8%, we estimate that at least ~20 of the HLX candidates
are consistent with IMBH masses, and this estimate is potentially several times
higher assuming more efficient accretion. This catalog currently represents the
largest sample of uniformly-selected, off-nuclear IMBH candidates. These
sources may represent scenarios in which a low-mass galaxy hosting an IMBH has
merged with a more massive galaxy and provide an excellent sample for testing
models of low-mass BH formation and merger-driven growth.
