Even though interstellar carbon isotope ratios are locally understood, in extragalactic space beyond the Magellanic Clouds they are almost unexplored. We lack information on objects outside the Local Group of galaxies tracing environments that drastically differ from those in the Milky Way and the Large Magellanic Cloud. We do not know whether our Galaxy is typical for its class of objects or whether its isotopic properties are exceptional. What would the latter imply? Moreover, will we see strong variations in isotopic ratios when observing nearby galaxies with high angular resolution?
An international team led by Xindi Tang from Xinjiang Astronomical Observatory in China and Christian Henkel from Max Planck Institute for Radioastronomy in Germany used The Atacama Large Millimeter/submillimeter Array (ALMA) to reveal carbon isotope ratio in three nearby starburst galaxies. They derived 12C/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 the 3 mm 12CN and 13CN N=1–0 lines in the ALMA Band 3. The 12C/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. They found the highest 12C/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 circumnuclear disk through both the halo and the bar. Low 12C/13C isotopic ratios in the central regions of these starburst galaxies indicate the presence of highly processed material. These results agree with the scenario of 12C/13C ratios slowly decreasing in galaxies with time.
Integrated intensity maps of 12CN and 13CN of NGC 253 (left), NGC 1068 (middle), and NGC 4945 (right).
Original Paper Link:
https://www.aanda.org/articles/aa/abs/2019/09/aa35603-19/aa35603-19.html
