A large ¹²C/¹³C isotopic ratio in M 82 and NGC 253

¹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
e-mail: smartin@eso.org
² Instituto de Radioastronomía Milimétrica (IRAM), Avda. Divina Pastora 7, Local 20, 18012 Granada, Spain
³ Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁴ Joint ALMA Observatory, Av. El Golf 40, Piso 18, Las Condes, Santiago, Chile

Received: 10 May 2010
Accepted: 28 June 2010

Abstract

Aims. Our aim is to derive carbon isotopic ratios from optically thin tracers in the central regions of the starburst galaxies M 82 and NGC 253.

Methods. We present high-sensitivity observations of CCH and two of its ¹³C isotopologues, C¹³CH and ¹³CCH, as well as the optically thin emission from C¹⁸O and ¹³C¹⁸O. We assume the column density ratio between isotopologues is representative of the ¹²C/¹³C isotopic ratio.

Results. From CCH, lower limits to the ¹²C/¹³C isotopic ratio of 138 in M 82, and 81 in NGC 253, are derived. Lower limits to the ¹²C/¹³C ratios from CO isotopologues support these.¹³C¹⁸O is tentatively detected in NGC 253, which is the first reported detection in the extragalactic ISM. Based on these limits, we infer ratios of ¹⁶O/¹⁸O  >  350 and  > 300 in M 82 and NGC 253, respectively, and ³²S/³⁴S  >  16 in NGC 253. The derived CCH fractional abundances toward these galaxies of  ≲ 1.1 × 10^-8 agree well with those of molecular clouds in the Galactic disk.

Conclusions. Our lower limits to the ¹²C/¹³C ratio from CCH are a factor of 2−3 larger than previous limits. The results are discussed in the context of molecular and nucleo-chemical evolution. The high ¹²C/¹³C isotopic ratio of the molecular ISM in these starburst galaxies suggest that the gas has been recently accreted toward their nuclear regions.