We use cookies to ensure that we give you the best experience on our website. You can change your cookie settings at any time. Otherwise, we'll assume you're OK to continue.

Durham University

Department of Physics

Staff profile

Publication details for Prof David Alexander

Bauer, F.E., Yan, L., Sajina, A. & Alexander, D.M. (2010). X-ray constraints on the active galactic nuclei properties in Spitzer-infrared spectrograph identified z ~ 2 ultraluminous infrared galaxies. The astrophysical journal 710(1): 212-226.

Author(s) from Durham


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 ({ν f_{ν}(24 μm)\over ν f_{ν}(R)})>1, and log ({ν f_{ν }(24 μm)\over ν f_{ν}(8 μm)})>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 ≈1045-1047 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 >~ 1024 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 = 1022 cm-2) of gsim1.7:1 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.