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Title:
Heavily Obscured AGN in Star-Forming Galaxies at z sime 2
Authors:
Treister, E.; Cardamone, Carolin N.; Schawinski, Kevin; Urry, C. Megan; Gawiser, Eric; Virani, Shanil; Lira, Paulina; Kartaltepe, Jeyhan; Damen, Maaike; Taylor, Edward N.; Le Floc'h, Emeric; Justham, Stephen; Koekemoer, Anton M.
Affiliation:
AA(Institute for Astronomy, 2680 Woodlawn Drive, University of Hawaii, Honolulu, HI 96822, USA ; Chandra/Einstein Fellow.; ), AB(Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520, USA ; Yale Center for Astronomy and Astrophysics, P.O. Box 208121, New Haven, CT 06520, USA ), AC(Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520, USA ; Yale Center for Astronomy and Astrophysics, P.O. Box 208121, New Haven, CT 06520, USA ; Chandra/Einstein Fellow.), AD(Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520, USA ; Yale Center for Astronomy and Astrophysics, P.O. Box 208121, New Haven, CT 06520, USA ; Department of Physics, Yale University, P.O. Box 208121, New Haven, CT 06520, USA ), AE(Department of Physics and Astronomy, Rutgers University, 136 Frelinghuysen Road, Piscataway, NJ 08854-8019, USA ), AF(Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520, USA ; Yale Center for Astronomy and Astrophysics, P.O. Box 208121, New Haven, CT 06520, USA ), AG(Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile ), AH(Institute for Astronomy, 2680 Woodlawn Drive, University of Hawaii, Honolulu, HI 96822, USA ), AI(Sterrewacht Leiden, Leiden University, NL-2300 RA Leiden, Netherlands ), AJ(Sterrewacht Leiden, Leiden University, NL-2300 RA Leiden, Netherlands ), AK(Institute for Astronomy, 2680 Woodlawn Drive, University of Hawaii, Honolulu, HI 96822, USA ), AL(Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, China ), AM(Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA )
Publication:
The Astrophysical Journal, Volume 706, Issue 1, pp. 535-552 (2009). (ApJ Homepage)
Publication Date:
11/2009
Origin:
IOP
ApJ Keywords:
galaxies: active, galaxies: Seyfert, X-rays: diffuse background, X-rays: galaxies
DOI:
10.1088/0004-637X/706/1/535
Bibliographic Code:
2009ApJ...706..535T

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 ~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 gsim 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 sun and langE(B – V)rang = 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.

Partly based on observations collected at the European Southern Observatory, Chile, under program 080.A-0612.

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