2006ApJ...651L..21S

The premise that ultraluminous X-ray sources (ULXs) exist beyond the optical extent of nearby galaxies is investigated. A published catalog containing 41 ULX candidates located between 1 and ∼3 times the standard D₂₅isophotal radius of their putative host galaxies is examined. Of these sources, 21 have spectroscopically confirmed distances. All 21 are background objects, giving a 95% probability that at least 37 of the 41 candidates are background sources. In addition, 39 of the 41 sources have X-ray-to-optical flux ratios in the range -1.6<log(F_X/F_O)<+1.3, consistent with those of background active galactic nuclei. [The remaining two are not detected in the optical to a weak limit of m_B∼21.5 mag, corresponding to log(F_X/F_O)≳1.6.] The uniform spatial distribution of the sample is also consistent with a background population. This evidence suggests that ULXs rarely, if at all, exist beyond the distribution of luminous matter in nearby galaxies and, as a consequence, there is no correlation between the population of ULXs and halo objects, such as old globular clusters or Population III remnants.