Astron. Astrophys. 329, 443-450 (1998)

Dense gas in nearby galaxies

XI. Interstellar ¹²C/¹³C ratios in the central regions of M 82 and IC 342^*

C. Henkel ¹, Y.-N. Chin ^{2, 3}, R. Mauersberger ⁴ and J.B. Whiteoak <sup>5, 6

1</sup> Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
² Institute of Astronomy and Astrophysics, Academia Sinica, P.O.Box 1-87 Nankang, 115 Taipei, Taiwan
³ Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
⁴ Steward Observatory, The University of Arizona, Tucson, AZ 85721, USA
⁵ Australia Telescope National Facility, Radiophysics Laboratories, P.O. Box 76, Epping, NSW 2121, Australia
⁶ Paul Wild Observatory, Australia Telescope National Facility, CSIRO, Locked Bag 194, Narrabri NSW 2390, Australia

Received 9 September 1996 / Accepted 21 July 1997

Abstract

¹²C/¹³C 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 (¹²CN)/I (¹³CN) line intensity ratios are giving lower limits to the corresponding ¹²C/¹³C abundance ratio. Toward M 82 and IC 342, we find ¹²C/¹³C 40 and 30. Therefore the smaller ¹²C/¹³C 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 ¹²C/¹³C limits and data from various isotopic species of CO also infer ¹⁵O/¹⁸O abundance ratios of 90 and 125 for M 82 and IC 342, respectively. From the I (HC¹⁴N)/I (HC¹⁵N) line intensity ratio, ¹⁴N/¹⁵N 100 is derived for M 82.

Key words: ISM: abundances galaxies: abundances radio lines: ISM radio lines: galaxies

Send offprint requests to:C. Henkel, p220hen@mpifr-bonn.mpg.de

SIMBAD Objects

Contents

• 1. Introduction
• 2. Observations
  • 2.1. 30-m IRAM telescope
  • 2.2. 15-m SEST
• 3. Results
• 4. Discussion
  • 4.1. LTE deviations in ¹²CN and ¹³CN
  • 4.2. Extragalactic isotope ratios
    • 4.2.1. ¹²C/¹³C ratios
    • 4.2.2. Limits to the ¹⁵O/¹⁸O ratio
    • 4.2.3. The ¹⁴N/¹⁵N ratio in M 82
• 5. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997
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