Dense molecular gas in Seyfert galaxies

Abstract

We have used the 20 m Onsala and 15 m SEST telescopes to observe the CO J = 1 → 0 (simultaneously with CO J = 2 → 1 at SEST) and HCN J = 1 → 0 (simultaneously with CS J = 3 → 2 at SEST) transitions in 20 Seyfert galaxies. The sample consists of the 18 galaxies detected in CO J = 1 → 0 by Heckman et al. (1989) plus 2 which were not detected in their survey. We have successfully detected all of the galaxies in the CO transition and 13 in HCN of which NGCs 1667, 2273, 5033, 5135, 6814 and Mrk 273 are new detections. For the galaxies in which the beam-width exceeds ≈ 10 kpc we find that LHCN/LCO ≈ 1/6, i.e. a global ratio similar to that of ultraluminous infrared galaxies and over 10 times the ratio for normal spiral galaxies. This implies that LFIR/LCO (LFIR ∼ 1011 L⊙ Seyferts) ≈ 10 LFIR/LCO (normal spirals) and we believe that the far infrared fluxes in our sample arise from star-burst activity, although we cannot rule out the possibility of a contribution from active galactic nuclei.