Abstract
To use a molecule as a diagnostic of a single star forming region, one has to observe it in different transitions. That is especially true for galaxies, where the beam filling factor is small and unknown. From studies of Galactic objects, the CS molecule is a good tracer of dense gas. Toward the IR galaxies NGC 253, IC 342 and M 82, Mauersberger and Henkel (1989) observed the
J
=2—1, 3—2 and 5—4 transitions of CS at 3, 2 and 1.3 mm wavelength, repectively. They also measured emission of the isotope C
34
S, which has an abundance ∼ 1/23 that of the main isotope. This study revealed that the H
2
densities in the circumnuclear gas exceed 10
4
, and, in the case of NGC 253, 10
5
cm
−3
. In all three cases, the distribution of the dense gas traced out by CS follows the CO distribution.