E. Treister et al. 2009 ApJ 706 535 doi:10.1088/0004-637X/706/1/535
HEAVILY OBSCURED AGN IN STAR-FORMING GALAXIES AT z 
2*
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E. Treister1,10, Carolin N. Cardamone2,3, Kevin Schawinski2,3,10, C. Megan Urry2,3,4, Eric Gawiser5, Shanil Virani2,3, Paulina Lira6, Jeyhan Kartaltepe1, Maaike Damen7, Edward N. Taylor7, Emeric Le Floc'h1, Stephen Justham8 and Anton M. Koekemoer9
Show affiliationsWe 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/fR > 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 ~1023 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 × 1024 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 ~1011 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 LX >1044 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.
- Footnote
- Keywords
galaxies: active; galaxies: Seyfert; X-rays: diffuse background; X-rays: galaxies
- Dates
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Issue 1 (2009 November 20)
Received 2009 June 26, accepted for publication 2009 October 9
Published 2009 November 2
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