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The AGN, star-forming, and morphological properties of luminous ir-bright/optically-faint galaxies

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2010-08-20
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Donley, J. L.
Rieke, G. H.
Alexander, D. M.
Egami, E.
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American Astronomical Society
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We present the active galactic nucleus (AGN), star-forming, and morphological properties of a sample of 13 MIR-luminous (∫_24 700 μJy) IR-bright/optically-faint galaxies (IRBGs, ∫_24/f_R≲ 1000). While these z ∼ 2 sources were drawn from deep Chandra fields with >200 ks X-ray coverage, only seven are formally detected in the X-ray and four lack X-ray emission at even the 2σ level. Spitzer InfraRed Spectrograph (IRS) spectra, however, confirm that all of the sources are AGN-dominated in the mid-IR, although half have detectable polycyclic aromatic hydrocarbon (PAH) emission responsible for ∼25% of their mid-infrared flux density. When combined with other samples, this indicates that at least 30%–40% of luminous IRBGs have star formation rates in the ultraluminous infrared galaxy (ULIRG) range (∼100–2000 M_⨀ yr^−1). X-ray hardness ratios and MIR to X-ray luminosity ratios indicate that all members of the sample contain heavily X-ray obscured AGNs, 80% of which are candidates to be Compton thick. Furthermore, the mean X-ray luminosity of the sample, log L_2–10 keV(erg s^−1) ∼44.6, indicates that these IRBGs are Type 2 QSOs, at least from the X-ray perspective. While those sources most heavily obscured in the X-ray are also those most likely to display strong silicate absorption in the mid-IR, silicate absorption does not always accompany X-ray obscuration. Finally, ∼70% of the IRBGs are merger candidates, a rate consistent with that of sub-mm galaxies (SMGs), although SMGs appear to be physically larger than IRBGs. These characteristics are consistent with the proposal that these objects represent a later, AGN-dominated, and more relaxed evolutionary stage following soon after the star-formation-dominated one represented by the SMGs.
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© 2010 The American Astronomical Society. J.L.D. thanks STScI for support through the Giacconi Fellowship, and Caltech/JPL for support through contract 1255094 to the University of Arizona, and D. M. A. thanks the Royal Society and Leverhulme Trust for a University Research Fellowship and Philip Leverhulme Prize, respectively. P. G. P.-G. acknowledges support from the Spanish Programa Nacional de Astronomía y Astrofísica under grants AYA 2006-02358 and AYA 2006-15698-C02-02, and from the Ramón y Cajal Program financed by the Spanish Government and the European Union. Finally, we thank Alex Pope, Elise Laird, Yong Shi, Jen Lotz, Laura Hainline, and the anonymous referee for helpful discussions and suggestions that improved the paper.
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