2006A&A...456..505P

The HCN(1-0) rotational line transition traces the dense (n_H2>10⁴cm^–3) fraction of the molecular gas typically located in starforming (SF) regions. In addition, an abnormally high HCN/CO line ratio close to AGNs may indicate then conditions of an X-ray-dominated region. Observed correlations between the CO-, HCN-, and FIR luminosities in nearby non-active, starburst, and low-luminosity active galaxies represent the physical connection between star formation and molecular gas as its fuel. HCN(1-0) has hardly been investigated in nearby high-luminosity AGN within this context. The aim of this study is to compare the HCN luminosity with published CO and IR luminosities to investigate the role of SF in the observed QSO host galaxies. We used the IRAM 30 m for the first time to search for the HCN(1-0) transition in two standard QSO host galaxies at z∼0.1. Our upper limits on L_HCN agree with the known correlations and do not show strong excess abundance or excitation of the HCN due to the luminous active quasar nucleus. The starburst origin of the far-infrared luminosity in the observed QSO hosts cannot be proven unambiguously by the upper limits. We found that the IR/FIR ratio indicates independently of L_IR if a significant amount of AGN heated dust is present.