HEAVILY OBSCURED AGN IN STAR-FORMING GALAXIES ATz≃ 2

We study the properties of a sample of 211 heavily obscured active galactic nucleus (AGN) candidates in the extended Chandra Deep Field-South selecting objects with f 24 μm / f R > 1000 and R − K >4.5. Of these, 18 were detected in X-rays and found to be obscured AGNs with neutral hydrogen column densities of ∼10 23 cm −2 . In the X-ray-undetected sample, the following evidence suggests a large fraction of heavily obscured (Compton-thick) AGN: (1) The stacked X-ray signal of the sample is strong, with an observed ratio of soft to hard X-ray counts consistent with a population of ∼90% heavily obscured AGNs combined with 10% star-forming galaxies. (2) The X-ray-to-mid-IR ratios for these sources are significantly larger than that of star-forming galaxies and ∼2 orders of magnitude smaller than for the general AGN population, suggesting column densities of N H ≳ 5 × 10 24 cm −2 . (3) The Spitzer near- and mid-IR colors of these sources are consistent with those of the X-ray-detected samples if the effects of dust self-absorption are considered. Spectral fitting to the rest-frame UV/optical light (dominated by the host galaxy) returns stellar masses of ∼10 11 M ☉ and 〈 E ( B − V )〉 = 0.5, and reveals evidence for a significant young stellar population, indicating that these sources are experiencing considerable star formation. This sample of heavily obscured AGN candidates implies a space density at z ∼ 2 of ∼10 −5 Mpc −3 , finding a strong evolution in the number of L X >10 44 erg s −1 sources from z = 1.5 to 2.5, possibly consistent with a short-lived heavily obscured phase before an unobscured quasar is visible.