Compton-thick active galactic nuclei inside local ultraluminous infrared galaxies

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.