Lower mass black holes in the great observatories origins deep survey? Off-nuclear X-ray sources

We have identified a sample of 10 highly reliable off-nuclear (≳2 kpc) X-ray sources at z = 0.03-0.25 in late-type host galaxies within the two Great Observatories Origins Deep Survey (GOODS) fields (i.e., the two Chandra Deep Fields). The combination of the superb spatial resolution and great depth of the Hubble Space Telescope Advanced Camera for Surveys and Chandra ACIS coverage in the GOODS fields is critical to the identification of these sources. We extend the study of this enigmatic population up to higher redshifts and larger look-back times than has been possible before, finding that the fraction of optically luminous galaxies exhibiting luminous off-nuclear sources at z ≈ 0.1 is larger than at the current time (≈36% vs. ≈78%). The X-ray luminosities of the GOODS off-nuclear X-ray sources are comparable to or slightly greater than the integrated point-source X-ray luminosities of nearby galaxies, providing a possible constraint on the X-ray luminosity function at higher star formation rates. X-ray variability demonstrates that several of these sources are likely single accreting black holes; for the majority, however, the number of X-ray counts is too low for X-ray variability studies.