2014A&A...565A...3H

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 4mK (15 and 20mJy) in 8.5km/s 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.