Journal Article

Compton-thick active galactic nuclei inside local ultraluminous infrared galaxies

E. Nardini and G. Risaliti

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 415, issue 1, pages 619-628
Published in print July 2011 | ISSN: 0035-8711
Published online July 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.18732.x
Compton-thick active galactic nuclei inside local ultraluminous infrared galaxies

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We present an X-ray analysis of the most luminous obscured active galactic nuclei (AGN) inside local ultraluminous infrared galaxies (ULIRGs). Our sample consists of 10 sources, harbouring AGN components with estimated luminosity in excess of ∼1012 L and yet unidentified at optical wavelengths because of their large obscuration. According to the Chandra and XMM–Newton spectra, only in two cases out of 10 are clear AGN signatures detected at 2–10 keV in the shape of reflected emission. The X-ray flux from the starburst (SB) components, instead, is always broadly consistent with expectations based on their IR emission. The most convincing explanation for the missing AGN detections is therefore the Compton-thickness of the X-ray absorber. In general, the combination of our mid-IR and X-ray spectral analysis suggests that the environment surrounding the AGN component in ULIRGs is much richer in gas and dust than in ordinary active galaxies, and the degree of AGN absorption can be tentatively related to the starburst intensity, indicating a strong interaction between the two processes and supporting the ULIRG/quasar evolutionary scheme.

Keywords: galaxies: active; galaxies: starburst; infrared: galaxies; X-rays: galaxies

Journal Article.  8217 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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