Molecular gas conditions in NGC 4945 and the Circinus galaxy

We present results of a multi-transition study of the dense molecular gas in the central part of the hybrid star-burst/Seyfert galaxies NGC 4945 and the Circinus galaxy. From the results of radiative transfer calculations, we estimate in NGC 4945 $n_{{\rm H}_2}=3 10^3{-}10^4$ cm -3 and $T_{\rm kin}\approx100$ K and in Circinus $n_{{\rm H}_2}=2 10^3{-}10^5$ cm -3 and $T_{\rm kin}\approx50{-}80$ K for the molecular hydrogen density and kinetic temperature, respectively. As well as density/temperature tracing molecules, we have observed C 17 O and C 18 O in each galaxy and the value of C 18 O/C$^{17}{\rm O}\approx6$ for the isotopic column density ratio suggests that both have relatively high populations of massive stars. Finally, although star formation is present, the radiative transfer results combined with the high HCN/CO and (possibly) HCN/FIR, radio/FIR ratios may suggest that, in comparison with Circinus, a higher proportion of the dense gas emission in NGC 4945 may be located in the hypothesised central nuclear disk as opposed to dense star forming cloud cores. Contrary to the literature, which assumes that all of the far-infrared emission arises from star formation, our results suggest that in NGC 4945 some of this emission could arise from an additional source, and so we believe that a revision of the star formation rate estimates may be required for these two galaxies.