Ultraluminous X-ray Source Correlations with Star-Forming Regions
  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 HII
regions in M81 and M101 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 HII regions compared to only 27% (0%) of a control sample
according to the empirical (Fisher) criterion. This is an excess of 3-sigma
above the 32% (27%) expected if the ULXs were randomly distributed within their
galactic hosts. This indicates a ULX preference for young, approximately <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 3e39 erg/s are equally divided among
the group of ULX environments redward of HII 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 HII 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 a <20 solar-mass star.
