Journal
ASTROPHYSICAL JOURNAL
Volume 769, Issue 2, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/769/2/92
Keywords
stars: formation; X-rays: binaries
Categories
Funding
- National Aeronautics and Space Administration through Chandra Award by the Chandra X-ray Observatory Center [G0-11108A]
- National Aeronautics Space Administration [NAS8-03060]
- NASA [NAS5-98034]
- STFC [ST/K000861/1] Funding Source: UKRI
- Science and Technology Facilities Council [ST/K000861/1] Funding Source: researchfish
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Ultra-luminous X-ray sources (ULX) are X-ray binaries with L-x > 10(39) erg s(-1). 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 (N-ULX(SFR)) in the XMPG sample with N-ULX(SFR) in a comparison sample of galaxies with higher metallicities taken from the Spitzer Infrared Galaxy Sample. 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) similar to 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.
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