X-RAY CONSTRAINTS ON THE ACTIVE GALACTIC NUCLEI PROPERTIES IN SPITZER-INFRARED SPECTROGRAPH IDENTIFIED z ∼ 2 ULTRALUMINOUS INFRARED GALAXIES

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(nu)(24 mu m) > 0.9 mJy, log(nu f(nu)(24 mu m)/nu f(nu)(R)) > 1, and log(nu f(nu)(24 mu m)/nu f(nu)(8 mu m)) > 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 approximate to 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 mu m 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 greater than or similar to 10(24) cm(-2)). With a space density of approximate to 1.4 x 10(-7) Mpc(-3) at z similar to 2, such objects imply an obscured AGN fraction (i.e., the ratio of AGNs above and below N-H = 10(22) cm(-2)) of greater than or similar to 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.