Molecular lines as tracers of Compton-thick AGN ?

Abstract

Recently, Papadopoulos et al., 2010 using sub-mm CO molecular line observations of nearby ultra-luminous IRAS galaxies, (U)LIRGs, have found that exceptionally large gas column densities (N_H > 10^25 cm-2) can be present across some of the very dense gaseous disks that are typically found in these objects. They also proposed a diagnostic for finding such sources using CO and HCN molecular lines. Given that such high column densities are expected to absorb any X-ray luminous AGN, yielding Compton-thick sources, we set out toexplore 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 (0.5-10 keV) for eleven sources as well as BeppoSAX results (2-100 keV) 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 such systems. X-ray spectroscopy reveals four candidate Compton-thick AGN of which half 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 ~4x10^23 cm-2) 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 as expected cannot 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.