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

Donley, J.L. and Rieke, G.H. and Alexander, D.M. and Egami, E. and Pérez-González, P.G. (2010) 'The AGN, star-forming, and morphological properties of luminous IR-bright/optically-faint galaxies.', Astrophysical journal., 719 (2). pp. 1393-1407.

Abstract

We present the active galactic nucleus (AGN), star-forming, and morphological properties of a sample of 13 MIR-luminous (f 24 >~ 700 μJy) IR-bright/optically-faint galaxies (IRBGs, f 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 sun 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.

Item Type:Article
Keywords:Galaxies, Active, Infrared, X-rays.
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Status:Peer-reviewed
Publisher Web site:http://dx.doi.org/10.1088/0004-637X/719/2/1393
Publisher statement:© 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
Date accepted:No date available
Date deposited:05 September 2013
Date of first online publication:August 2010
Date first made open access:No date available

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