Letter to the Editor

Molecular lines as tracers of Compton-thick AGN?^*

¹ INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: ioannis.georgantopoulos@oabo.inaf.it
² Institute of Astronomy and Astrophysics, National Observatory of Athens, I. Metaxa & V. Pavlou str., Palaia Penteli, 15236 Athens, Greece
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany

Received: 28 July 2010
Accepted: 2 September 2010

Abstract

Recent sub-mm CO molecular line observations of nearby ultra-luminous IRAS galaxies, (U)LIRGs with L_IR ≥ 10¹¹ , have shown that exceptionally large gas column densities (N_H ≥ 10²⁵ cm^-2) 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 (0.5–10 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 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 ~ 4 × 10²³ 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 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.