2009ApJ...703..159S

Maps of low-inclination nearby galaxies in Sloan Digitized Sky Survey u - g, g - r, and r - i colors are used to determine whether ultraluminous X-ray sources (ULXs) are predominantly associated with star-forming regions of their host galaxies. An empirical selection criterion is derived from colors of H II regions in M 81 and M 101 that differentiates between the young, blue stellar component and the older disk and bulge population. This criterion is applied to a sample of 58 galaxies of Hubble type S0 and later and verified through an application of Fisher's linear discriminant analysis. It is found that 60% (49%) of ULXs in optically bright environments are within regions blueward of their host galaxy's H II regions compared to only 27% (0%) of a control sample according to the empirical (Fisher) criterion. This is an excess of 3σ above the 32% (27%) expected if the ULXs were randomly distributed within their galactic hosts. This indicates a ULX preference for young, ≲10 Myr, OB associations. However, none of the ULX environments have the morphology and optical brightness suggestive of a massive young super-star cluster though several are in extended or crowded star-forming (blue) environments that may contain clusters unresolved by Sloan imaging. Ten of the 12 ULX candidates with estimated X-ray luminosities in excess of 3x10³⁹ erg/s are equally divided among the group of ULX environments redward of H II regions and the group of optically faint regions. This likely indicates that the brightest ULXs turn on at a time somewhat later than typical of H II regions; say 10-20 Myr after star formation has ended. This would be consistent with the onset of an accretion phase as the donor star ascends the giant branch if the donor is an ≲20 M_☉star.