4 Summary

We have observed a number of molecules, in several transitions in the central positions of NGC 4945 and the Circinus galaxy. From a mean-escape probability analysis of the data we conclude the following:

1.

In NGC 4945 the CO traces molecular hydrogen densities of $n_{{\rm H}_2}\approx3~10^3$ cm^-3 and HCN, CS and HCO⁺ trace densities of $\sim$10⁴ cm^-3;

2.

In Circinus these values are $\approx$2 10³ cm^-3 and $\sim$10⁵ cm^-3.

This result implies that the molecular gas in Circinus may be found in denser cores than in NGC 4945. In conjunction with the (relatively) high HCN/CO (and possibly HCN/FIR and radio/FIR) ratio(s) in NGC 4945, this result suggests that much of the HCN and FIR radiation may arise from the AGN rather than the star-burst (up to 50% of the bolometric luminosity in NGC 4945 may be due to the AGN, Spoon et al. 2000). If this is to be believed, then previous calculations of the star formation rate in this galaxy may be significant overestimates.

We also find that the kinetic temperature of the gas to be $T_{\rm kin}=100$ K and $T_{\rm kin}=50{-}80$ K in NGC 4945 and Circinus, respectively (from H₂ lines, Spoon et al. 2000 also find warm gas, $T_{\rm ex}=160$ K, in NGC 4945). Could the higher temperature in NGC 4945 be related to the asymmetric shapes of (the temperature tracing) molecular profiles? A temperature gradient across the observed region could cause the possible self-absorption: Molecular rings (which have been insinuated in both these galaxies) are expected to have higher temperatures in their inner radius due to heating from the AGN and star formation, and so we may expect such asymmetric profiles in both cases. The fact that this is not seen in the Circinus spectra may simply imply a smaller gradient in accordance with the lower temperature derived. We hope that future interferometric (ALMA) studies of these galaxies will answer this through detailed mapping of temperature tracing molecules.

We find column densities (per unit line-width) of $\approx$2 10¹⁷ cm $^{-2}~({\rm km~s}^{-1})^{-1}$ for ¹²CO in both galaxies. Combined with the ¹³CO results, these values suggest a ¹²C/¹³C ratio of $\approx$50 for NGC 4945 and $\approx$60-80 for Circinus. These values are typical of Galactic clouds ($\approx$40-90, Goldsmith 1987), but higher than in the Galactic centre.

We also report the first detection of SO in these galaxies, thus increasing the number of extragalactic SO detections to six (Petuchowski & Bennett 1992; Johansson et al. 1994; Takano et al. 1995; Chin et al. 1998; Heikkilä et al. 1999).

Finally, worth noting is the fact that both the MEP and LTE methods suggest that the gas in NGC 4945 is warmer than in Circinus (by roughly the same factor), and so perhaps LTE methods could provide a useful simple means of determining the relative gas temperatures in a larger sample of Seyfert galaxies.

Acknowledgements

We wish to thank the referee Christian Henkel for his helpful comments, Francisco Azagra at SEST for locating some of the older data for us and the operators in general for their help with the observations. Thanks also to Michael Olberg, John Black and Silvana Nikolic at Onsala for their help with RADEX. Also at Onsala, Antonis Polatidis who made modifications (not possible from SEST) to Figs. 3 and 4. AH acknowledges financial support from the Academy of Finland through grant No. 1011055.