Revealing a population of heavily obscured active galactic nuclei at z ≈ 0.5-1 in the Chandra deep field-south.

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.