HEAVILY OBSCURED AGN IN STAR-FORMING GALAXIES AT z approx = 2

Journal Article · Fri Nov 20 00:00:00 EST 2009 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/706/1/535· OSTI ID:21378180

Treister, E; Kartaltepe, Jeyhan; Le Floc'h, Emeric ^[1]; Cardamone, Carolin N; Schawinski, Kevin; Urry, C Megan; Virani, Shanil ^[2]; Gawiser, Eric ^[3]; Lira, Paulina ^[4]; Damen, Maaike; Taylor, Edward N ^[5]; Justham, Stephen ^[6]

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1. Institute for Astronomy, 2680 Woodlawn Drive, University of Hawaii, Honolulu, HI 96822 (United States)
2. Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520 (United States)
3. Department of Physics and Astronomy, Rutgers University, 136 Frelinghuysen Road, Piscataway, NJ 08854-8019 (United States)
4. Departamento de AstronomIa, Universidad de Chile, Casilla 36-D, Santiago (Chile)
5. Sterrewacht Leiden, Leiden University, NL-2300 RA Leiden (Netherlands)
6. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing (China)

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 {sub 24m}u{sub m}/f{sub 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 approx10{sup 23} cm{sup -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 approx90% 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 approx2 orders of magnitude smaller than for the general AGN population, suggesting column densities of N {sub H} approx> 5 x 10{sup 24} cm{sup -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 approx10{sup 11} M{sub sun} 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 approx 2 of approx10{sup -5} Mpc{sup -3}, finding a strong evolution in the number of L{sub X} >10{sup 44} erg s{sup -1} sources from z = 1.5 to 2.5, possibly consistent with a short-lived heavily obscured phase before an unobscured quasar is visible.

• OSTI ID: 21378180
• Journal Information: Astrophysical Journal, Vol. 706, Issue 1; Other Information: DOI: 10.1088/0004-637X/706/1/535; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
DENSITY
DUSTS
GALACTIC EVOLUTION
HARD X RADIATION
HYDROGEN
MASS
QUASARS
SELF-ABSORPTION
STARS
X-RAY GALAXIES
ABSORPTION
COSMIC RADIO SOURCES
COSMIC RAY SOURCES
COSMIC X-RAY SOURCES
ELECTROMAGNETIC RADIATION
ELEMENTS
EVOLUTION
GALAXIES
IONIZING RADIATIONS
NONMETALS
PHYSICAL PROPERTIES
RADIATIONS
SORPTION
X RADIATION