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

Journal Article · Wed Feb 10 00:00:00 EST 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/710/1/212· OSTI ID:21394529

Bauer, F E ^[1]; Lin, Yan ^[2]; Sajina, A ^[3]; Alexander, D M ^[4]

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1. Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301 (United States)
2. Spitzer Science Center, Caltech, MS 220-6, Pasadena, CA 91125 (United States)
3. Haverford College, Haverford, PA 19041 (United States)
4. Department of Physics, Durham University, Durham DH1 3LE (United Kingdom)

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{sub n}u(24 {mu}m)>0.9 mJy, log((nuf{sub n}u(24mum))/(nuf{sub n}u(R)))>1, and log((((nuf{sub n}u(24mum))/(nuf{sub n}u(8mum)))>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 {approx}10{sup 45}-10{sup 47} erg s{sup -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{sub H} {approx}> 10{sup 24} cm{sup -2}). With a space density of {approx}1.4 x 10{sup -7} Mpc{sup -3} at z {approx} 2, such objects imply an obscured AGN fraction (i.e., the ratio of AGNs above and below N{sub H} = 10{sup 22} cm{sup -2}) of {approx}>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.

• OSTI ID: 21394529
• Journal Information: Astrophysical Journal, Vol. 710, Issue 1; Other Information: DOI: 10.1088/0004-637X/710/1/212; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
ABSORPTION
BOLOMETERS
DUSTS
GALAXY NUCLEI
LUMINOSITY
POLYCYCLIC AROMATIC HYDROCARBONS
RED SHIFT
SPECTRA
X-RAY GALAXIES
AROMATICS
COSMIC RAY SOURCES
COSMIC X-RAY SOURCES
GALAXIES
HYDROCARBONS
MEASURING INSTRUMENTS
OPTICAL PROPERTIES
ORGANIC COMPOUNDS
PHYSICAL PROPERTIES
SORPTION