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Hyper-luminous X-ray sources (HLXs; L_X>10^41 erg s^-1) are off-nuclear X-raysources in galaxies and strong candidates for intermediate-mass black holes(IMBHs). We have constructed a sample of 169 HLX candidates by combining X-raydetections from the Chandra Source Catalog (Version 2) with galaxies from theSloan Digital Sky Survey and registering individual images for improvedrelative astrometric accuracy. The spatial resolution of Chandra allows for thesample to extend out to z~0.9. Optical counterparts are detected amongone-fourth of the sample, one-third of which are consistent with dwarf galaxystellar masses. The average intrinsic X-ray spectral slope indicates efficientaccretion, potentially driven by galaxy mergers, and the column densitiessuggest one-third of the sample has significant X-ray absorption. We find that144 of the HLX candidates have X-ray emission that is significantly in excessof the expected contribution from star formation and hot gas, stronglysuggesting that they are produced by accretion onto black holes more massivethan stars. After correcting for an average background or foregroundcontamination rate of 8%, we estimate that at least ~20 of the HLX candidatesare consistent with IMBH masses, and this estimate is potentially several timeshigher assuming more efficient accretion. This catalog currently represents thelargest sample of uniformly-selected, off-nuclear IMBH candidates. Thesesources may represent scenarios in which a low-mass galaxy hosting an IMBH hasmerged with a more massive galaxy and provide an excellent sample for testingmodels of low-mass BH formation and merger-driven growth.

A Catalog of Hyper-luminous X-ray Sources and Intermediate-Mass Black Hole Candidates out to High Redshifts