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Published February 10, 2010 | Published
Journal Article Open

X-ray Constraints on the Active Galactic Nuclei Properties in Spitzer-Infrared Spectrograph Identified z ~ 2 Ultraluminous Infrared Galaxies

Abstract

We report Chandra X-ray constraints for 20 of the 52 high-redshift ultraluminous infrared galaxies (ULIRGs) identified in the Spitzer Extragalactic First Look Survey with f_ν(24 μm) > 0.9 mJy, log[(νf_ν(24μm)/(νf_ν(R)] > 1, and log[(νf_ν(24μm)/(νf_ν(8μm)] > 0.5. Notably, decomposition of Spitzer mid-infrared IRS spectra for the entire sample indicates that they are comprised predominantly of weak polycyclic aromatic hydrocarbon ULIRGs dominated by hot-dust continua, characteristic of active galactic nuclei (AGNs) activity. Given their redshifts, they have AGN bolometric luminosities of ≈10^(45)-10^(47) erg s^(–1) comparable to powerful quasi-stellar objects (QSOs). This, coupled with their high IR-to-optical ratios and often significant silicate absorption, strongly argues in favor of these mid-IR objects being heavily obscured QSOs. Here we use Chandra observations to further constrain their obscuration. At X-ray energies, we marginally detect two ULIRGs, while the rest have only upper limits. Using the IRS-derived 5.8 μm AGN continuum luminosity as a proxy for the expected X-ray luminosities, we find that all of the observed sources must individually be highly obscured, while X-ray stacking limits on the undetected sources suggest that the majority, if not all, are likely to be at least mildly Compton-thick (N_H ≳ 10^(24) cm^(–2)). With a space density of ≈1.4 × 10^(–7) Mpc^(–3) at z ~ 2, such objects imply an obscured AGN fraction (i.e., the ratio of AGNs above and below N_H = 10^(22) cm^(–2)) of ≳ 1.7:1 even among luminous QSOs. Given that we do not correct for mid-IR extinction effects and that our ULIRG selection is by no means complete for obscured AGNs, we regard our constraints as a lower limit to the true obscured fraction among QSOs at this epoch. Our findings, which are based on extensive multi-wavelength constraints including Spitzer IRS spectra, should aid in the interpretation of similar objects from larger or deeper mid-IR surveys, where considerable uncertainty about the source properties remains and comparable follow-up is not yet feasible.

Additional Information

© 2010 American Astronomical Society. Print publication: Issue 1 (2010 February 10); received 2009 July 8; accepted for publication 2009 December 23; published 2010 January 15. This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory and Caltech under contract with NASA, and the Chandra X-ray Observatory, which is operated by the Smithsonian Astrophysical Observatory under contract with NASA. We thank R. Chary and R. Mushotsky for useful discussions, and F. Fiore for providing data on the COSMOS and GOODS sources. Finally, we thank the anonymous referee for useful comments that improved the content and presentation of the paper. We gratefully acknowledge the financial support of Chandra Postdoctoral Fellowship PF4-50032 (F.E.B.), Chandra Awards GO7-8106X (L.Y.) and GO9-0134C (F.E.B.), the Royal Society (D.M.A.), and the Leverhulme Trust (D.M.A.).

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