Chandra stacking constraints on the contribution of 24 μm Spitzer sources to the unresolved cosmic X-ray background

A. T. Steffen, W. N. Brandt, D. M. Alexander, S. C. Gallagher, B. D. Lehmer

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We employ X-ray stacking techniques to examine the contribution from X-ray-undetected, mid-infrared-selected sources to the unresolved, hard (6-8 keV) cosmic X-ray background (CXB). We use the publicly available, 24 μm Spitzer Space Telescope MIPS catalogs from the Great Observatories Origins Deep Survey (GOODS)-North and South fields, which are centered on the 2 Ms Chandra Deep Field-North and 1 Ms Chandra Deep Field-South, to identify bright (S 24 μm > 80 μJy) mid-infrared sources that may be powered by heavily obscured AGNs. We measure a significant stacked X-ray signal in all of the X-ray bands examined, including, for the first time, a significant (3.2 σ) 6-8 keV stacked X-ray signal from an individually undetected X-ray source population. We find that the X-ray-undetected MIPS sources make up ≈2% (or less) of the total CXB below 6 keV, but ≈6% in the 6-8 keV band. The 0.5-8 keV stacked spectrum is consistent with a hard power law (Γ = 1.44 ± 0.07), with the spectrum hardening at higher X-ray energies. Our findings show that these bright MIPS sources do contain obscured AGNs, but are not the primary source of the unresolved ∼40% of 6-8 keV CXB. Our study rules out obscured, luminous quasars as a significant source of the remaining unresolved CXB and suggests that it most likely arises from a large population of obscured, high-redshift (z ≳ 1), Seyfert-luminosity AGNs.

Original languageEnglish (US)
Pages (from-to)L25-L28
JournalAstrophysical Journal
Issue number1
StatePublished - Sep 20 2007

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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