4 Summary

We have used the observational results of Curran et al. (2000) in order to investigate the possibility that there may generally be more molecular gas in type 2 than in type 1 Seyfert galaxies. Whereas our results may be consistent with those of Maiolino et al. (1997) in that we may find no difference in the mean CO/FIR luminosity ratio between the two classes, for sources in which $L_{\rm FIR}\sim10^{11}~L_{\odot}$, for less luminous Seyferts we find that the ratio in type 2 may be at least three times that in type 1 sources. This factor disappears when only the CO within the beam for the near-by ( $L_{\rm FIR}\sim10^{10}~L_{\odot}$) sources is considered. This possibly implies that the additional CO in the near-by type 2 sample may not be associated with the bulk of the FIR (or HCN, Curran et al. 2000) emission. Could this additional CO trace the reservoir of molecular gas required to power the star-burst activity, which is lacking in type 1 Seyferts (Heckman 1987; Pogge 1989; Moorwood 1996; Hunt et al. 1997; Oliva et al. 1999)? Referring to Fig. 8 of Curran et al. (2000), this could well be the case; the CO/FIR luminosity ratio may generally decrease with increasing FIR luminosity, and since in type 1 Seyferts a higher fraction of the luminosity must arise from the un-obscured AGN (e.g. Lawrence & Elvis 1982), we suggest that a higher fraction of the FIR flux in type 2 (cf. type 1) Seyferts arises from star-burst activity. Although this contribution may decrease with increasing $L_{\rm FIR}$ it will always constitute a greater portion of the FIR flux than in the type 1 Seyferts.

From the CO detections we have also modelled the molecular gas distribution according to the molecular ring in the Circinus galaxy. From our results we conclude that:

1.

For the sample the molecular ring does appear to be share a similar inclination as the galactic disc, although this alignment is tighter for type 2 Seyferts;

2.

Type 2 nuclei show a slight preference for edge-on galaxies and type 1s for face-on systems, and in most cases, as with the ring and the main disc, the torus and ring tend to be aligned within $\approx30^{\circ}$. This result could, however, be misleading if the 100 pc-scale ring contributes to the obscuration (Maiolino & Rieke 1995).

Could the alignment between the host and ring in type 2 Seyferts be related to a greater molecular gas abundance? Curran et al. (1999) have shown that the orientation of the molecular ring should have little effect on the observed luminosity of the CO for our sample, but if there is indeed a gas deficit in type 1 Seyferts, perhaps it is due to a reduced gas flow causing instabilities and allowing the molecular ring to be further departed from the galactic plane. Since there appear to be larger gas concentrations in barred galaxies (Sakamoto et al. 1998) and bars may be more common in type 2 Seyferts (Pogge 1989), we suggest that the reservoir of gas which appears to be present in these galaxies may provide the stability necessary to hold the molecular ring coplanar to the galactic disc. Further evidence for alignment is given by the results of Maiolino et al. (1997); Papadopoulos & Seaquist (1998) who attribute the excess of star-burst activity in type 2 Seyferts to the material in the obscuring torus still being funnelled down from the global scale via the 100 pc-scale ring, thus suggesting that the torus may be but a dense component of the molecular ring (Shlosman et al. 1990; Friedli & Martinet 1993; Shaw et al. 1993; Wilson & Tsvetanov 1994; Fosbury et al. 2000; Conway 1999; Dopita et al. 1998; van Langevelde et al. 2000). This scenario would suggest that, in general, type 1 nuclei are more evolved than their "edge-on'' counterparts; a notion supported by Blandford (1990); Maiolino et al. (1997); Rigopoulou et al. (1997); Dopita (1998); Papadopoulos & Seaquist (1998).

The ideas presented here are of course highly speculative, although it is hoped that the results from the model prove useful in guiding future interferometric observations.

Acknowledgements

I would like to thank S. Aalto whose idea was to study the relative gas abundances and also L.E.B. Johansson and M. Olberg for their help. I would also like to thank the referee R. Antonucci for his very helpful comments. 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.