The Statistical Properties of Galaxies Containing Ultraluminous X‐Ray Objects

We present a statistical analysis of the properties of galaxies containingultraluminous X-ray objects (ULXs). Our primary goal is to establish thefraction of galaxies containing a ULX as a function of ULX luminosity. Oursample is based on ROSAT HRI observations of galaxies. We find that ~ 12% ofgalaxies contain at least one ULX with L_X > 10^39 erg/s and ~ 1% of galaxiescontain at least one ULX with L_X > 10^40 erg/s. These ULX frequencies arelower limits since ROSAT HRI observation would miss absorbed ULXs (i.e., withN_H >~ 10^21 cm^-2) and those within ~ 10" of the nucleus (due to thepositional error circle of the ROSAT HRI). The Hubble type distribution ofgalaxies with a ULX differs significantly from the distribution of types fornearby RC3 galaxies, but does not differ significantly from the galaxy typedistribution of galaxies observed by the HRI in general. We find no increase inthe mean FIR luminosity or FIR / K band luminosity ratio for galaxies with aULX relative to galaxies observed by the HRI in general, however this result isalso most likely biased by the soft bandpass of the HRI and the relatively lownumber of high SFR galaxies observed by the HRI with enough sensitivity todetect a ULX.