Abstract
Studying the dense molecular gas in the nuclei of nearby spirals with high infrared luminosities, first detections of extragalactic N2H+, SiO, H13CO+, H13CN, and HN13C are reported. The J = 1-0 line of N2H+ was observed in the central parts of NGC 253, Maffei 2, IC 342, M 82, and NGC 6946. A map toward NGC 253 indicates that the spatial distribution of N2H+ follows that of CS. Toward M 82, however, N2H+ is centrally peaked, while strongest emission of other molecules arises from a circumnuclear 'ring'. This difference as well as the peculiar distribution of another ion, HCO+, is explained in terms of gas phase chemistry.
The J = 2-1 and 3-2 lines of the ground vibrational state of SiO (v = 0) were detected toward the nuclear region of NGC 253. H13CO+ was detected in NGC 253, IC 342, and tentatively in M 82. H13CN and HN13C were observed in NGC 253 alone. The integrated line intensity ratio is I(H13CN)/I(HN13C) = 2.2 +/- 0.5. The SiO source toward NGC 253 is extended (>> 1 pc). The intensity of SiO emission relative to that of other high density tracing molecules approaches that in hot, dense Galactic cloud cores like e.g. Orion KL. It is much lower toward M 82, namely by factors greater-than-or-equal-to 2-5. CH3OH is showing a similar behaviour. Since it has been demonstrated that the abundances of SiO and CH3OH increase with temperature, it is suggested that the molecular environment associated with the nucleus of NGC 253 is warmer than the gas of the other nuclei observed. A very effective heating mechanism, e.g. the dissipation of tidal energy, is required to heat up such a large amount of gas.
The main difference between the dense circumnuclear structures in NGC 253 and M 82 is their extent, which explains the complexity of the observed spatial distributions toward M 82 and may also be the cause for the high kinetic temperature in the core of NGC 253. In contrast to known sources in the Galactic disk, galactic center clouds may be warm without being exceptionally dense. As a consequence, typical tracers for 'quiescent' gas as N2H+ and for 'violent' gas as SiO and CH30H may coexist in the nuclear regions of NGC 253 and the Milky Way.