The properties and redshift evolution of intermediate-luminosity off-nuclear X-ray sources in the chandra deep fields

B. D. Lehmer, W. N. Brandt, A. E. Hornschemeier, D. M. Alexander, F. E. Bauer, A. M. Koekemoer, D. P. Schneider, A. T. Steffen

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We analyze a population of intermediate-redshift (z ≈ 0.05-0.3) off-nuclear X-ray sources located within optically bright galaxies in the Great Observatories Origins Deep Survey and Galaxy Evolution from Morphology and SEDs fields. A total of 24 off-nuclear source candidates are classified using deep Chandra X-Ray Observatory exposures from the Chandra Deep Field-North, Chandra Deep Field-South, and Extended Chandra Deep Field-South; 15 of these are newly identified. These sources have average X-ray spectral shapes and optical environments similar to those observed for off-nuclear intermediate-luminosity (L x ≥ 10 39 ergs s -1 in the 0.5-2.0 keV band) X-ray objects (IXOs; sometimes referred to as ultraluminous X-ray sources) in the local universe. This sample improves the available source statistics for intermediate-redshift off-nuclear sources with L x ≳ 10 39.5 ergs s -1, and it places significant new constraints on the redshift evolution of the off-nuclear source frequency in field galaxies. The fraction of intermediate-redshift field galaxies containing an off-nuclear source with L x ≳ 10 39 ergs s -1 is suggestively elevated (≈80% confidence level) with respect to that observed for IXOs in the local universe; we calculate this elevation to be a factor of ≈1.9 -1.3-1.4. A rise in this fraction is plausibly expected as a consequence of the observed increase in global star formation density with redshift, and our results are consistent with the expected magnitude of the rise in this fraction.

Original languageEnglish (US)
Pages (from-to)2394-2405
Number of pages12
JournalAstronomical Journal
Issue number5
StatePublished - May 2006

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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