Heavily obscured agn in star-forming galaxies at z ≃ 2

E. Treister, Carolin N. Cardamone, Kevin Schawinski, C. Megan Urry, Eric Gawiser, Shanil Virani, Paulina Lira, Jeyhan Kartaltepe, Maaike Damen, Edward N. Taylor, Emeric Le Floc'H, Stephen Justham, Anton M. Koekemoer

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65 Scopus citations

Abstract

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/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 ∼1023cm-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 × 1024cm-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.

Original languageEnglish (US)
Pages (from-to)535-552
Number of pages18
JournalAstrophysical Journal
Volume706
Issue number1
DOIs
StatePublished - 2009

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Keywords

  • Galaxies: Active
  • Galaxies: Seyfert
  • X-rays: Diffuse background
  • X-rays: Galaxies

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