5 Summary

We have detected CO  $1\rightarrow 0$ in all and HCN  $1\rightarrow 0$ in 13 of the 20 Seyfert galaxies observed. The detections include six new detections in HCN for NGCs 1667, 2273, 5033, Mrk 273, NGC 5135 and NGC 6814. Similar to ULIRGs, for the distant sources we find a global HCN to CO luminosity ratio of $\approx1/6$ which is an order of magnitude greater than in normal spiral galaxies and, again as in the case of ULIRGs (Scoville et al. 1991), this implies that the CO appears to be confined to the nuclear region (otherwise an even lower luminosity ratio is obtained). The $\frac{{\rm CO} ~1\rightarrow0}{{\rm HCN} ~1\rightarrow0}$ intensity ratios obtained are significantly lower than those in moderate infrared luminosity ( $L_{\rm FIR}<10^{11}~L_{\odot}$) galaxies. Also, perhaps because of the centralisation, our sample does not appear to suffer from the same selection effect at play, due to the difficulty in detecting HCN in such distant galaxies, cf. $\frac{{\rm CO} ~1\rightarrow0}{{\rm HCN} ~1\rightarrow0}>20$ at velocities  km s^-1 for moderate infrared luminosity galaxies (Bryant 1997), although a selection effect may be responsible for the high CO/HCN ratios here.

We also find that the HCN to FIR luminosity ratio is similar to that for normal spiral galaxies through to ULIRGs. This result implies that there is no excess in the far infrared continuum (which could be due to an AGN) in our sample, although if the denser gas tracers form part of the obscuration rather than star-forming clouds, the FIR to HCN ratio would imply an additional FIR source. In any case, as the results stand there is no overwhelming evidence to invoke a contribution from an AGN.

Acknowledgements

We are grateful to the anonymous referee for his helpful comments and we would like to thank Per Bergman, Lars E.B. Johansson and the SEST and Onsala telescope operators. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.