Abstract
C-12/C-13 line intensity ratios have been derived from several carbon-bearing molecules to confine the range of carbon and oxygen isotope abundance ratios toward the nuclear regions of two infrared bright galaxies. The most stringent limits are obtained from CN mm-wave emission lines. Supplementary measurements toward the Galactic center region indicate that overall I((CN)-C-12)/I((CN)-C-13) line intensity ratios are giving lower limits to the corresponding C-12/C-13 abundance ratio. Toward M 82 and IC 342, we find C-12/C-13 > 40 and > 30. Therefore the smaller C-12/C-13 ratio of our own Galactic center region (25) may not be typical for central regions of galaxies which are more luminous in the infrared. The 12(C)/C-13 limits and data from various isotopic species of CO also infer O-16/O-18 abundance ratios of > 90 and > 125 for M 82 and IC 342, respectively. From the I((HCN)-N-14)/I((HCN)-N-15) line intensity ratio, N-14/N-15 > 100 is derived for M 82.