"Hidden" Seyfert 2 galaxies and the X-ray background

Edward C. Moran*, Alexei V. Filippenko, Ryan Chornock

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

135 Scopus citations


Obscured active galactic nuclei, which are classified optically as type 2 (narrow line) Seyfert galaxies in the local universe, are by far the most promising candidates for the origin of the hard (2-10 keV) X-ray background radiation. However, optical follow-up observations of faint X-ray sources in deep Chandra images have revealed surprising numbers of apparently normal galaxies at modest redshift. Such objects represent ∼40%-60% of the sources classified in deep Chandra surveys, raising the possibility that the X-ray galaxy population has evolved with cosmic time. Alternatively, most of the faint X-ray galaxies in question are so distant that their angular diameters are comparable to the slit widths used in ground-based spectroscopic observations; thus, their nuclear spectral features may be overwhelmed ("hidden") by host galaxy light. To test this hypothesis, we have obtained integrated spectra of a sample of nearby, well-studied Seyfert 2 galaxies. The data, which accurately simulate observations of distant Chandra sources, demonstrate convincingly that the defining spectral signatures of Seyfert 2s can be hidden by light from their host galaxies. In fact, 60% of the observed objects would not be classified as Seyfert 2s on the basis of their integrated spectra, similar to the fraction of faint X-ray sources identified with "normal" galaxies. Thus, the numbers of narrow-line active galaxies in deep Chandra surveys (and perhaps all ground-based spectroscopic surveys of distant galaxies) are likely to have been underestimated.

Original languageEnglish (US)
Pages (from-to)L71-L74
JournalAstrophysical Journal
Issue number2 II
StatePublished - Nov 10 2002
Externally publishedYes


  • Galaxies: Seyfert
  • X-rays: diffuse background
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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