The Chandra Deep Field North survey. XV. Optically bright, X-ray-faint sources

A. E. Hornschemeier, F. E. Bauer, D. M. Alexander, W. N. Brandt, W. L.W. Sargent, M. W. Bautz, C. Conselice, G. P. Garmire, D. P. Schneider, G. Wilson

Research output: Contribution to journalReview articlepeer-review

76 Scopus citations


We have analyzed optically bright, X-ray-faint [OBXF; i.e., log (f X/fR) ≲ -2] sources identified in an 178.9 arcmin2 area having high exposure (greater than 1500 ks) within the Chandra Deep Field North 2 Ms survey. We find 43 OBXF sources in this area, making up ≈ 15% of the X-ray sources above a 0.5-2 keV flux of ≈ 2.3 × 10-17 ergs cm-2 s-1. We present spectroscopic identifications for 42 of the OBXF sources and optical spectra for 25, including five previously unpublished redshifts. Deep optical imaging data (either Hubble Space Telescope [HST] or ground-based) are presented for all the OBXF sources; we measure the optical morphologies of the 20 galaxies having HST imaging data. The OBXF population consists mainly of normal and starburst galaxies detected out to cosmologically significant distances (i.e., to a median redshift of z = 0.297 and a full redshift range z = 0.06-0.845). This is notable since these distances equate to look-back times of up to ≈ 8 Gyr; we are thus provided with a window on the X-ray emission from galaxies at redshifts much closer to the cosmic star formation peak than was possible prior to the Chandra X-Ray Observatory. The X-ray luminosity distribution of OBXF sources extends to higher luminosity than does that of " normal " galaxies, indicating that a significant fraction are likely dominated by low-luminosity active galactic nuclei or vigorous star formation. The lowest redshift galaxies (z ≈ 0.06-0.2) have very low X-ray-to-optical flux ratios [i.e., log (fX/fR) ≲ -3], which are consistent with those of normal galaxies in the local universe. By combining the detected X-ray counts, we find the average OBXF X-ray spectrum to be consistent with a Γ ≈ 2.0 power law. The 0.5-2 keV log N-log S for the OBXF galaxies is much steeper (α ≈ -1.7) than for the general X-ray source population. Indeed, the number of OBXF sources has doubled between the 1 and 2 Ms surveys, rising sharply in numbers at faint fluxes. The extragalactic OBXF sources are found to contribute ≈1%-2% of the soft extragalactic X-ray background. We report on the discovery of five candidate off-nuclear ultraluminous X-ray sources (LX ≳ 1039 ergs s-1) with z ≈ 0.1-0.2 within the OBXF population. These sources are "ultraluminous" in that they are typically more X-ray luminous than, e.g., Eddington-limited accretion onto stellar-mass black holes; these sources are found to dominate the X-ray emission of their host galaxies.

Original languageEnglish (US)
Pages (from-to)575-595
Number of pages21
JournalAstronomical Journal
Issue number2 1772
StatePublished - Aug 2003

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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