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 ({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.