A (CO)-C-12, (CO)-C-13, AND CS STUDY OF NGC-2146 AND IC-342

We present observations of the (CO)-C-12 and (CO)-C-13 J = 2-1 and 1-0, as well as CS J = 2-1 lines in the nuclear regions of NGC 2146 and IC 342, made with the 14 m FCRAO telescope. Through comparisons with radiative transfer calculations, we find that the (12)C0/(CO)-C-13 J = 1-0 line ratio, in conjunction with the (CO)-C-13 J = 2-1/J = 1-0 line ratio, provides significant constraints on the physical conditions of the bulk of molecular gas. The densities and temperatures in the nuclear regions of the two actively star-forming galaxies are found to be higher than the typical values of the disk GMCs in the Milky Way but similar to those in the molecular clouds of the Galactic center. The derived physical conditions of the gas depend on the abundance ratio of (CO)-C-12/H-2, used in the model calculations. With (CO)-C-12/H-2 = 10(-4) and (CO)-C-12/(CO)-C-13 = 60, the average temperature in the nuclear region of NGC 2146 is estimated to be around 55 K, and the average density is about 2 x 10(4) cm(-3). The average temperature and density ascertained in the nuclear region of IC 342 are lower than those in NGC 2146, with T similar to 25 K and n(H-2) similar to 4 x 10(3) cm(-3). In spite of the difference in the physical conditions of these two galaxies, the values of the N(H-2)/I-CO conversion factor, which depends on the root n(H-2)/T-CO ratio, seem to be within a factor of 2 of the Galactic value at the centers of the two galaxies, provided that they have the same CO abundance. CS J = 2-1 emission is detected toward the centers of NGC 2146 and IC 342, confirming the conclusion reached in previous works that a large amount of dense gas (with n[H-2] greater than or similar to 10(4) cm(-3)) exists in the centers of many galaxies.