Ultra-Luminous X-Ray Sources in the Most Metal Poor Galaxies
  Ultra-Luminous X-ray sources (ULX) are X-ray binaries with Lx>1E^39 ergs/s.
The most spectacular examples of ULX occur in starburst galaxies and are now
understood to be young, luminous High Mass X-ray Binaries. The conditions under
which ULX form are poorly understood, but recent evidence suggests they may be
more common in low metallicity systems. Here we investigate the hypothesis that
ULX form preferentially in low metallicity galaxies by searching for ULX in a
sample of Extremely Metal Poor Galaxies (XMPG) observed with the Chandra X-ray
Observatory. XMPG are defined as galaxies with log(O/H)+12<7.65, or less than
5% solar. These are the most metal-deficient galaxies known, and a logical
place to find ULX if they favor metal poor systems. We compare the number of
ULX (corrected for background contamination) per unit of star formation (Nulx)
in the XMPG sample with Nulx in a comparison sample of galaxies with higher
metallicities taken from the Spitzer Infrared Galaxy Sample (SINGS). We find
that ULX occur preferentially in the metal poor sample with a formal
statistical significance of 2.3 sigma. We do not see strong evidence for a
trend in the formation of ULX in the high metallicity sample: above 12+log(O/H)
~ 8.0 the efficiency of ULX production appears to be flat. The effect we see is
strongest in the lowest metallicity bin. We discuss briefly the implications of
these results for the formation of black holes in low metallicity gas.
