Carbon and oxygen isotope ratios in starburst galaxies: New data from NGC 253 and Mrk 231 and their implications

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: chenkel@mpifr-bonn.mpg.de
² Astronomy Department, Faculty of Science, King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
³ Purple Mountain Observatory, Chinese Academy of Sciences, 21008 Nanjing, PR China
⁴ National Astronomical Observatory of Japan, 2–21–1 Osawa, Mitaka, 181–8588 Tokyo, Japan
⁵ Dept. of Earth and Space Sciences, Chalmers University of Technology, Onsala Observatory, 43994 Onsala, Sweden
⁶ Institute for Computational Cosmology, Dept. of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁷ School of Physics, Cardiff University, Queens Building, The Parade, Cardiff CF24 3AA, UK
⁸ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁹ Observatorio Astronómico Nacional (OAN) – Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
¹⁰ European Southern Observatory, Avda. Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile
¹¹ Institute de Radioastronomie Millimétrique, rue de la Piscine 300, 38406 Saint-Martin d’Hères, France

Received: 31 October 2013
Accepted: 18 February 2014

Abstract

Carbon and oxygen isotope ratios are excellent measures of nuclear processing, but few such data have been taken toward extragalactic targets so far. Therefore, using the IRAM 30-m telescope, CN and CO isotopologues have been measured toward the nearby starburst galaxy NGC 253 and the prototypical ultraluminous infrared galaxy Mrk 231. Toward the center of NGC 253, the CN and ¹³CN N = 1 → 0 lines indicate no significant deviations from expected local thermodynamical equilibrium after accounting for moderate saturation effects (10 and 25%) in the two detected spectral components of the main species. Including calibration uncertainties, which dominate the error budget, the ¹²C/¹³C ratio becomes 40 ± 10. This is larger than the ratio in the central molecular zone of the Galaxy, suggesting a higher infall rate of poorly processed gas toward the central region. Assuming that the ratio also holds for the CO emitting gas, this yields ¹⁶O/¹⁸O = 145 ± 36 and ¹⁶O/¹⁷O = 1290 ± 365 and a ³²S/³⁴S ratio close to the one measured for the local interstellar medium (20–25). No indication of vibrationally excited CN is found in the lower frequency fine structure components of the N = 1 → 0 and 2 → 1 transitions at rms noise levels of 3 and 4 mK (15 and 20 mJy) in 8.5 km s^-1 wide channels. Peak line intensity ratios between NGC 253 and Mrk 231 are ~100 for ¹²C¹⁶O and ¹²C¹⁸O J = 1 → 0, while the ratio for ¹³C¹⁶O J = 1 → 0 is ~250. This and similar ¹³CO and C¹⁸O line intensities in the J = 1 → 0 and 2 → 1 transitions of Mrk 231 suggest ¹²C/¹³C ~ 100 and ¹⁶O/¹⁸O ~ 100, in agreement with values obtained for the less evolved ultraluminous merger Arp 220. Also, when accounting for other (scarcely available) extragalactic data, ¹²C/¹³C ratios appear to vary over a full order of magnitude, from >100 in ultraluminous high redshift galaxies to ~100 in more local such galaxies to ~40 in weaker starbursts that are not undergoing a large scale merger to 25 in the central molecular zone of the Milky Way. With ¹²C being predominantly synthesized in massive stars, while ¹³C is mostly ejected by longer lived lower mass stars at later times, this is qualitatively consistent with our results of decreasing carbon isotope ratios with time and rising metallicity. It is emphasized, however, that both infall of poorly processed material, initiating a nuclear starburst, and the ejecta from newly formed massive stars (in particular in the case of a top-heavy stellar initial mass function) can raise the carbon isotope ratio for a limited amount of time.