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Durham University

Department of Physics

Staff profile

Publication details for Prof David Alexander

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. (2011). Revealing a Population of Heavily Obscured Active Galactic Nuclei at z ≈ 0.5-1 in the Chandra Deep Field-South. The astrophysical journal 740(1): 37.

Author(s) from Durham


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.