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.

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.

Citation

Donley, J., Rieke, G., Alexander, D., Egami, E., & Pérez-González, P. (2010). The AGN, star-forming, and morphological properties of luminous IR-bright/optically-faint galaxies. Astrophysical Journal, 719(2), 1393-1407. https://doi.org/10.1088/0004-637x/719/2/1393

• 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
• DOI: https://doi.org/10.1088/0004-637x/719/2/1393
• Keywords: Galaxies, Active, Infrared, X-rays.
• Public URL: https://durham-repository.worktribe.com/output/1479160