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Ultra-luminous X-ray sources in the most metal poor galaxies

Prestwich, A.H.; Tsantaki, M.; Zezas, A.; Jackson, F.; Roberts, T.P.; Foltz, R.; Linden, T.; Kalogera, V.

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A.H. Prestwich

M. Tsantaki

A. Zezas

F. Jackson

R. Foltz

T. Linden

V. Kalogera


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.


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

Journal Article Type Article
Publication Date Jun 1, 2013
Deposit Date Sep 11, 2013
Publicly Available Date Feb 12, 2014
Journal Astrophysical Journal
Print ISSN 0004-637X
Electronic ISSN 1538-4357
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 769
Issue 2
Article Number 92
Keywords Stars formation, X-rays, Binaries.


Published Journal Article (425 Kb)

Copyright Statement
© 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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