ALMA view of the ¹²C/¹³C isotopic ratio in starburst galaxies^⋆

¹ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 830011 Urumqi, PR China
e-mail: tangxindi@xao.ac.cn
² Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 830011 Urumqi, PR China
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: chenkel@mpifr-bonn.mpg.de
⁴ Astronomy Department, King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
⁵ European Southern Observatory, Alonso de Córdova 3107, Vitacura Casilla, 763 0355 Santiago, Chile
⁶ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura Casilla, 763 0355 Santiago, Chile
⁷ Argelander Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁸ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Observatory, 43992 Onsala, Sweden
⁹ Observatorio de Madrid, OAN-IGN, Alfonso XII, 3, 28014 Madrid, Spain
¹⁰ Ioffe Physical-Technical Institute, Polytekhnicheskaya Str. 26, 194021 St. Petersburg, Russia
¹¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
¹² Department of Physics and Astronomy, UCL, Gower St., London WC1E 6BT, UK
¹³ Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, PR China
¹⁴ Center for Astrophysics, Guangzhou University, 510006 Guangzhou, PR China
¹⁵ School of Astronomy and Space Science, Nanjing University, 210093 Nanjing, PR China

Received: 3 April 2019
Accepted: 15 June 2019

Abstract

We derive molecular-gas-phase 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 λ ∼ 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. 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 circum-nuclear 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.