2010A&A...520L...4G

Recent sub-mm CO molecular line observations of nearby ultra-luminous IRAS galaxies, (U)LIRGs with L_IR≥10¹¹L_☉, have shown that exceptionally large gas column densities (N_H≥10²⁵/cm²) can be present across some of the very dense gaseous disks that are typically found in these objects. In particular, Papadopoulos et al. proposed a diagnostic for finding such obscured sources using CO and HCN molecular lines. Given that these high column densities are expected to absorb any X-ray luminous active galactic nuclei (AGN), yielding Compton-thick sources, we set out to explore whether this can be discerned using X-ray observations. More specifically we examine X-ray spectral observations of 14 sources in their sample, using public Chandra observations (0.5-10keV) for eleven sources as well as BeppoSAX results (2-100keV) from the literature for another three sources. Our goal is to find candidate Compton-thick AGN and to check whether the molecular line selection criterion is successful in selecting these systems. X-ray spectroscopy reveals four candidate Compton-thick AGN, of which two fall within the high obscuration region in the molecular line ratio diagnostics. Of the remaining five sources falling into the ``high dust obscuration'' box, one (Mrk273) is highly obscured (N_H∼4x10²³/cm²), while in the other four the X-ray emission is most probably associated with star-forming processes rather than an AGN on the basis of their X-ray and mid-infrared properties. Overall, we argue that although this method cannot of course recover all Compton-thick AGN, there are no examples of X-ray luminous AGN inside that region that have low obscuration, suggesting that this method is efficient in finding heavily obscured AGN in dust-enshrouded star-forming galaxies. The above results bear important implications for future joint ALMA and X-ray observations for the detection of Compton-thick AGN.