The Lack of Halo Ultraluminous X-ray Sources

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 D25 isophotal radius of their putative host galaxies is examined. Twenty-one of these sources have spectroscopically-confirmed distances. All 21 are background objects giving a 95% probability that at least 37 of the 41 candidates are background sources. Thirty-nine of the 41 sources have X-ray-to-optical flux ratios, −1.6 < log(FX/FO) < +1.3, consistent with those of background active galactic nuclei. (The remaining two are not detected in optical to a weak limit of mB ∼ 21.5 mag corresponding to log(FX/FO) ∼> 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 Pop III remnants. Subject headings: galaxies: general — galaxies: halos — X-rays: galaxies — X-rays: general