ACA [CI] observations of the starburst galaxy NGC 253

¹ IRAM, Domaine Universitaire, 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
e-mail: krips@iram.fr
² European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
³ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁴ Academia Sinica, Institute of Astronomy and Astrophysics, Taipei, Taiwan
⁵ Department of Earth and Space Sciences, Onsala Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden
⁶ Max-Planck-Institut für Extraterrestrische Physik, Giessenbacherstr. 1, 85748 Garching, Germany
⁷ Department of Astronomy, University of Florida, Gainsville, FL 32611, USA
⁸ Department of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
⁹ University of Cologne, I. Physikalisches Institut, Zülpicher Str. 77, 50937 Köln, Germany
¹⁰ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹¹ Observatorio de Madrid (OAN-IGN), Alfonso XII, 3, 28014 Madrid, Spain
¹² Centre for Astrophysics Research, Science & Technology Research Institute, University of Hertfordshire, Hatfield AL109 AB, UK
¹³ Department of Physics and Astronomy, UCL, Gower St., London, WC1E 6BT, UK
¹⁴ Department of Astronomy, University of Massachusetts-Amherst, Amherst, MA, USA
¹⁵ NRAO, 520 Edgemont Rd., Charlottesville, VA 22903, USA
¹⁶ SKA Organziation, Lower Withington Macclesfield, Cheshire SK11 9DL, UK

Received: 9 May 2016
Accepted: 24 June 2016

Abstract

Context. Carbon monoxide (CO) is widely used as a tracer of the molecular gas in almost all types of environments. However, several shortcomings of CO complicate usaging it as H₂ tracer, such as its optical depth effects, the dependence of its abundance on metallicity, or its susceptibility to dissociation in highly irradiated regions. Neutral carbon emission has been proposed to overcome some of these shortcomings and hence to help revealing the limits of CO as a measure of the molecular gas.

Aims. We aim to study the general characteristics of the spatially and spectrally resolved carbon line emission in a variety of extragalactic sources and evaluate its potential as complementary H₂ tracer to CO.

Methods. We used the Atacama Compact Array to map the [CI](³P₁–³P₀) line emission in the nearby starburst galaxy NGC 253 at unprecedented angular resolution (~3′′). This is the first well-resolved interferometric [CI] map of an extragalactic source.

Results. We have detected the [CI] line emission at high significance levels along the central disk of NGC 253 and its edges where expanding shells have previously been found in CO. Globally, the distribution of the [CI] line emission strongly resembles that of CO, confirming the results of previous Galactic surveys that [CI] traces the same molecular gas as CO. However, we also identify a significant increase of [CI] line emission with respect to CO in (some of) the outflow or shocked regions of NGC 253, namely the bipolar outflow emerging from the nucleus. A first-order estimate of the [CI] column densities indicates abundances of [CI] that are very similar to the abundance of CO in NGC 253. Interestingly, we find that the [CI] line is marginally optically thick within the disk.

Conclusions. The enhancement of the [CI]/CO line ratios (~0.4−0.6) with respect to Galactic values (≤0.1), especially in the shocked regions of NGC 253, clearly indicates that mechanical perturbation such as shocks and the strong radiation fields in this starburst galaxy have a marked effect on the carbon excitation and/or abundance.