Keck spectroscopy of ROSAT ultraluminous X-ray source candidates

We present results of optical follow-up observations of ultraluminous X-ray source (ULX) candidates. A published ROSAT catalog of 87 ULX candidates named 19 of these as having optical counterparts in Digitized Sky Survey (DSS) images. Using Keck optical spectroscopy, we classify 16 ULX candidates in this subset as well as one other ULX candidate from the catalog with a counterpart in the DSS. This is one of the largest sets of optical identifications of such objects thus far. Fifteen are background active galactic nuclei (AGNs); two are foreground stars in our Galaxy. These findings are consistent with background and foreground object expectations, as derived from log N - log S relations. The logarithmic X-ray to optical flux ratios of these objects are also typical of background and foreground objects. Finally, we discuss the nature of ULX candidates outside the optical extent of their apparent host galaxies.