Ultraluminous X-ray Source Correlations with Star-Forming Regions

TL;DR

This study investigates the spatial correlation between ultraluminous X-ray sources (ULXs) and star-forming regions in nearby galaxies, revealing ULXs are preferentially located near young stellar populations, with implications for their formation and evolution.

Contribution

The paper introduces an empirical color-based criterion to identify ULX environments and demonstrates their association with young star-forming regions using a large galaxy sample.

Findings

60% of ULXs are near blue HII regions, compared to 27% in control samples.

ULXs are often found in environments indicative of <10 Myr star formation.

Brightest ULXs may activate 10-20 Myr after star formation, suggesting a delay in accretion onset.

Abstract

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.