We derive molecular-gas-phase ^ { 12 } C/ ^ { 13 } C isotope ratios for the central few 100 pc of the three nearby starburst galaxies NGC 253 , NGC 1068 , and NGC 4945 making use of the \lambda \sim 3 mm ^ { 12 } CN and ^ { 13 } CN N = 1–0 lines in the ALMA Band 3 .
The ^ { 12 } C/ ^ { 13 } C isotopic ratios derived from the ratios of these lines range from 30 to 67 with an average of 41.6 \pm 0.2 in NGC 253 , from 24 to 62 with an average of 38.3 \pm 0.4 in NGC 1068 , and from 6 to 44 with an average of 16.9 \pm 0.3 in NGC 4945 .
The highest ^ { 12 } C/ ^ { 13 } C isotopic ratios are determined in some of the outskirts of the nuclear regions of the three starburst galaxies .
The lowest ratios are associated with the northeastern and southwestern molecular peaks of NGC 253 , the northeastern and southwestern edge of the mapped region in NGC 1068 , and the very center of NGC 4945 .
In case of NGC 1068 , the measured ratios suggest inflow from the outer part of NGC 1068 into the circum-nuclear disk through both the halo and the bar .
Low ^ { 12 } C/ ^ { 13 } C isotopic ratios in the central regions of these starburst galaxies indicate the presence of highly processed material .