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

Donley, J.L.; Rieke, G.H.; Alexander, D.M.; Egami, E.; Pérez-González, P.G.

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


J.L. Donley

G.H. Rieke

E. Egami

P.G. Pérez-González


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.

Journal Article Type Article
Publication Date Aug 1, 2010
Deposit Date Apr 8, 2013
Publicly Available Date Sep 5, 2013
Journal Astrophysical Journal
Print ISSN 0004-637X
Electronic ISSN 1538-4357
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 719
Issue 2
Pages 1393-1407
Keywords Galaxies, Active, Infrared, X-rays.
Public URL


Published Journal Article (884 Kb)

Copyright Statement
© 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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