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Durham University

Department of Physics

Staff profile

Publication details for Prof Tim Roberts

Prestwich, A.H., Tsantaki, M., Zezas, A., Jackson, F., Roberts, T.P., Foltz, R., Linden, T. & Kalogera, V. (2013). Ultra-luminous X-ray sources in the most metal poor galaxies. Astrophysical Journal 769(2): 92.

Author(s) from Durham


Ultra-luminous X-ray sources (ULX) are X-ray binaries with Lx > 1039 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 (NULX(SFR)) in
the XMPG sample with NULX(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 2.3σ. 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.