Abstract
We derive molecular-gas-phase12C/13C isotope ratios for the central few hundred parsecs of the three nearby starburst galaxies NGC 253, NGC 1068, and NGC 4945 making use of theλ ∼ 3 mm12CN and13CNN= 1–0 lines in the ALMA Band 3. The12C/13C isotopic ratios derived from the ratios of these lines range from 30 to 67 with an average of 41.6 ± 0.2 in NGC 253, from 24 to 62 with an average of 38.3 ± 0.4 in NGC 1068, and from 6 to 44 with an average of 16.9 ± 0.3 in NGC 4945. The highest12C/13C 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 the 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. Low12C/13C isotopic ratios in the central regions of these starburst galaxies indicate the presence of highly processed material.
Highlights
Even though interstellar carbon isotope ratios are locally understood (e.g., Wilson & Rood 1994; Henkel et al 1994a; Wilson 1999), in extragalactic space beyond the Magellanic Clouds they are almost unexplored
We find that ∼89% and ∼86% of the 12CN and 13CN integrated flux observed by single-dish telescopes is recovered for NGC 253 by our Atacama Large Millimeter/submillimeter Array (ALMA) data, respectively
The line ratios are calculated using velocity-integrated intensities where the 13CN lines are detected with S /N 5σ
Summary
Even though interstellar carbon isotope ratios are locally understood (e.g., Wilson & Rood 1994; Henkel et al 1994a; Wilson 1999), in extragalactic space beyond the Magellanic Clouds they are almost unexplored. A surprising result is that the metal-poor outer Galaxy is not merely providing a “bridge” between the solar neighborhood and the even more metal-poor LMC This is explained by the different age of the bulk of the stellar populations of the outer Galaxy and the LMC and can be exemplified by the 12C/13C and 18O/17O ratios, which are both a measure of “primary” versus “secondary” nuclear processing. While 12C and 18O are produced on rapid timescales primarily via He burning in massive stars, 13C and 17O are predominantly synthesized via CNO processing of 12C and 16O seeds from earlier stellar generations The latter occurs on a slower timescale during the red-giant phase in low- and intermediatemass stars or novae (e.g., Wilson & Rood 1994; Henkel et al.1994a,b)
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