Revealing a Population of Heavily Obscured Active Galactic Nuclei at z ≈ 0.5-1 in the Chandra Deep Field-South

Luo, B.; Brandt, W.N.; Xue, Y.Q.; Alexander, D.M.; Brusa, M.; Bauer, F.E.; Comastri, A.; Fabian, A.C.; Gilli, R.; Lehmer, B.D.; Rafferty, D.A.; Schneider, D.P.; Vignali, C.

Abstract

Heavily obscured (N H >~ 3 × 1023 cm-2) active galactic nuclei (AGNs) not detected even in the deepest X-ray surveys are often considered to be comparably numerous to the unobscured and moderately obscured AGNs. Such sources are required to fit the cosmic X-ray background (XRB) emission in the 10-30 keV band. We identify a numerically significant population of heavily obscured AGNs at z ≈ 0.5-1 in the Chandra Deep Field-South (CDF-S) and Extended Chandra Deep Field-South by selecting 242 X-ray undetected objects with infrared-based star-formation rates (SFRs) substantially higher (a factor of 3.2 or more) than their SFRs determined from the UV after correcting for dust extinction. An X-ray stacking analysis of 23 candidates in the central CDF-S region using the 4 Ms Chandra data reveals a hard X-ray signal with an effective power-law photon index of Γ = 0.6+0.3 -0.4, indicating a significant contribution from obscured AGNs. Based on Monte Carlo simulations, we conclude that 74% ± 25% of the selected galaxies host obscured AGNs, within which ≈95% are heavily obscured and ≈80% are Compton-thick (CT; N H > 1.5 × 1024 cm-2). The heavily obscured objects in our sample are of moderate intrinsic X-ray luminosity (≈(0.9-4) × 1042 erg s-1 in the 2-10 keV band). The space density of the CT AGNs is (1.6 ± 0.5) × 10-4 Mpc-3. The z ≈ 0.5-1 CT objects studied here are expected to contribute ≈1% of the total XRB flux in the 10-30 keV band, and they account for ≈5%-15% of the emission in this energy band expected from all CT AGNs according to population-synthesis models. In the 6-8 keV band, the stacked signal of the 23 heavily obscured candidates accounts for <5% of the unresolved XRB flux, while the unresolved ≈25% of the XRB in this band can probably be explained by a stacking analysis of the X-ray undetected optical galaxies in the CDF-S (a 2.5σ stacked signal). We discuss prospects to identify such heavily obscured objects using future hard X-ray observatories.

Citation

Luo, B., Brandt, W., Xue, Y., Alexander, D., Brusa, M., Bauer, F., …Vignali, C. (2011). Revealing a Population of Heavily Obscured Active Galactic Nuclei at z ≈ 0.5-1 in the Chandra Deep Field-South. Astrophysical Journal, 740(1), Article 37. https://doi.org/10.1088/0004-637x/740/1/37

• Journal Article Type: Article
• Publication Date: Oct 1, 2011
• Deposit Date: Apr 8, 2013
• Publicly Available Date: Jan 3, 2014
• Journal: Astrophysical Journal
• Print ISSN: 0004-637X
• Electronic ISSN: 1538-4357
• Publisher: American Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 740
• Issue: 1
• Article Number: 37
• DOI: https://doi.org/10.1088/0004-637x/740/1/37
• Keywords: Cosmic background radiation, Galaxies, Photometry, Starburst, Infrared, X-rays.