Ultraluminous X-ray Source Correlations with Star-Forming Regions - Astrophysics > Astrophysics of GalaxiesReport as inadecuate

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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 sourcesULXs are predominantly associated with star-forming regions of their hostgalaxies. An empirical selection criterion is derived from colors of HIIregions in M81 and M101 that differentiates between the young, blue stellarcomponent and the older disk and bulge population. This criterion is applied toa sample of 58 galaxies of Hubble type S0 and later and verified through anapplication of Fisher-s linear discriminant analysis. It is found that 60%49% of ULXs in optically-bright environments are within regions blueward oftheir host galaxy-s HII regions compared to only 27% 0% of a control sampleaccording to the empirical Fisher criterion. This is an excess of 3-sigmaabove the 32% 27% expected if the ULXs were randomly distributed within theirgalactic hosts. This indicates a ULX preference for young, approximately <10Myr, OB associations. However, none of the ULX environments have the morphologyand optical brightness suggestive of a massive young super star cluster thoughseveral are in extended or crowded star-forming blue environments that maycontain clusters unresolved by Sloan imaging. Ten of the 12 ULX candidates withestimated X-ray luminosities in excess of 3e39 erg-s are equally divided amongthe group of ULX environments redward of HII regions and the group of opticallyfaint regions. This likely indicates that the brightest ULXs turn on at a timesomewhat later than typical of HII regions; say 10-20 Myr after star formationhas ended. This would be consistent with the onset of an accretion phase as thedonor star ascends the giant branch if the donor is a <20 solar-mass star.

Author: Douglas A. Swartz, Allyn F. Tennant, Roberto Soria

Source: https://arxiv.org/


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