... galaxy

Based on results collected at the European Southern Observatory, La Silla, Chile.

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... LINERs

Low Ionisation Nuclear Emission-line Regions.

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...1992)

Due to ionised gas (Storchi-Bergmann et al. 1995) ejected from a compact nucleus (Whittle et al. 1988; Pedlar et al. 1989; Christopoulou et al. 1997; Radovich et al. 1998).

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... disks

These pc-scale molecular disks are believed to be associated with the central body responsible for the obscuration of the nuclei in Seyfert galaxies (Antonucci 1993). See Curran (2000) (http://nedwww.ipac.caltech.edu/level5/Curran/
frames.html) for a review.

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... (SEST)

The Swedish-ESO Sub-millimetre Telescope is operated jointly by ESO and the Swedish National Facility for Radio Astronomy, Onsala Space Observatory at Chalmers University of Technology.

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... given

The main beam brightness temperature is defined by $T_{\rm mb}=T_{\rm A}^{*}/\eta_{\rm mb}$, where $T_{\rm A}^*$ is the Rayleigh-Jeans antenna temperature above the atmosphere and $\eta _{\rm mb}$ is the measured main-beam efficiency of the telescope. The velocity integrated intensity is given by $I_{\rm mb}=\int T_{\rm mb} {\rm d}v$.

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... asymmetry

Also seen by Henkel et al. 1990; Henkel et al. (1994).

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...1993)

Papadopoulos & Seaquist (1998) find ratios somewhat lower than this, although Curran et al. (2000) and Curran (2000) suggest that these may be due to pointing errors.

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... only

The CO/CS ratios have not been considered, although these are simultaneously satisfied.

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... temperature

Solutions for the isotopomer intensity ratios only could be found at the lower end of this range i.e. for $n_{{\rm H}_2}\approx10^4-10^5$ cm^-3 (but only for $T_{\rm kin}\approx100$ K). However, unlike $n_{{\rm H}_2}\approx10^5$ cm^-3, applying a molecular hydrogen density of $n_{{\rm H}_2}\approx10^4$ cm^-3 could not satisfy the observed CO/CS ratios. Such a result could be feasible if the CS traced a different component of the gas, but we consider it unlikely that this would have such a similar (low) density and kinetic temperature.

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... analysis

$T_{\rm MEP} \approx T_{\rm kin} \approx T_{\rm ex}$ when $\tau \gg 1$.

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... method

A description of the LTE method is given in e.g. Turner (1991).

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... Circinus

These values are arrived at by taking all isotopomer and inter-molecule ratios into account, i.e. CO, HCN and CS, of which the latter molecule constrains the upper value of $n_{{\rm H}_2}$ (previous section).

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... stars

See Prantzos et al. (1996) for a review.

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... NGC 4945

Based upon the equivalent central dynamical masses of the two galaxies and assuming the same $N_{{\rm H}_{2}}/I_{{\rm CO}}$ conversion factor (Mauersberger et al. 1996; Curran et al. 1998).

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...1985

Assuming distances of 3.7 Mpc (Mauersberger et al. 1996) and 4.0 Mpc (Freeman et al. 1977) to NGC 4945 and Circinus, respectively. Note that this results in a 4% increase in beam area over Circinus which is not sufficient to give a similar HCN/CO ratio as NGC 4945.

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...$\frac{L_{\rm HCN}}{L_{\rm FIR}}~({\rm NGC~4945})\approx2.5\frac{L_{\rm HCN}}{L_{\rm FIR}}~({\rm Circinus})$

NGC 4945 lies closer to the line defined by the Seyfert sample (Fig. 7 of Curran et al. 2000), and while an incomplete sampling of the HCN will affect the ratios in both galaxies, the relative ratios are expected to remain similar. Note also that distance estimates to NGC 4945 vary somewhat, e.g. 6.7 Mpc according to Dahlem et al. (1993); Forbes & Norris (1998), this, however, has no effect on $\frac{L_{\rm HCN}}{L_{\rm FIR}}$.

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... structure

Located well within the molecular ring, which occurs at the inner Lindblad resonance. See Curran (2000) for a review.

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... ratio

For example, from high resolution observations of NGC 1068, Helfer & Blitz (1993); Helfer & Blitz (1995) find that while the CO traces the star-burst ring and the more extended emission, most of the HCN is located in the nucleus with only a relatively small portion located in the ring. Tacconi et al. (1994) also find the HCN to be distributed in warm (>70 K) clouds, in such a fashion.

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...1993)

They consider this rate to be high compared with other star-burst galaxies.

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... line

In the star-burst galaxy NGC 253 the H₂ ortho/para ratio is $\sim$2 (Harrison et al. 1998), corresponding to densities and temperatures of $\sim$10⁵ cm^-3and $\approx$50-100 K, respectively (next paragraph).

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... pc

Where our higher transitions probe in NGC 4945 and Circinus.

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... M 82

Applying our total integrated intensity for HCN $1\rightarrow 0$ (which is identical to the value of Henkel et al. 1990 used by Jackson et al. 1995), gives a $4\rightarrow3/1\rightarrow0$intensity ratio (of $\approx$0.35) for NGC 4945 which is greater than that of M 82 but less than that of NGC 253, perhaps indicating the more prominent AGN.

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... conditions

The reason for this is that they have similar A-coefficients and state energies. The intensity ratio which is insensitive to density and temperature variations, should therefore be mainly dependent on the ratio of the column densities. In fact, due to the 3-fold spin multiplicity of SO, a CS $2\rightarrow 1$ to SO $3_2\rightarrow 2_1$ intensity ratio ratio of 3 implies similar column densities; $N({\rm SO})\approx N({\rm CS})$.

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... respectively

Regarding the previous discussion, although the ratio in NGC 253, is similar to that in M 82 and NGC 4945, perhaps indicating similar conditions, Takano et al. (1995), from the ratios of various other molecular species, postulate additional processes to the formation of molecules in NGC 253, compared to the rest of their sample.

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... typical

Ratios $\le$1 are found in conjunction with outflows, in which SO is enhanced in molecular shocks (e.g. Pineau des Forêts et al. 1993).

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...$N({\rm SO})\approx N({\rm CS})$)

Also values of $\approx$1 for the column density ratio are found in molecular clouds in the Magellanic clouds (Heikkilä 1998).

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... star-burst

The HCN being located in the obscuration (dense component of the molecular ring) surrounding the AGN (Shlosman et al. 1990; Friedli & Martinet 1993; Antonucci 1993; Shaw et al. 1993; Wilson & Tsvetanov 1994; Maiolino & Rieke 1995; Fosbury et al. 1999; Conway 1999; Kohno et al. 1999; Curran et al. 2000). It is possible that some of the higher density (10⁵ cm^-3) gas in Circinus is also located close to the obscuration but this would be confined to within a few pc (Matt et al. 1996; Curran 2000), and so would not contribute significantly to the CS luminosity due to a low beam filling factor.

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