Astron. Astrophys. 339, 398-404 (1998)

Detection of neutral carbon in the M 31 dark cloud D 478

F.P. Israel ¹, R.P.J. Tilanus ^{2, 3} and F. Baas <sup>1, 2

1</sup> Sterrewacht Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands
² Joint Astronomy Centre, 660 N. A'ohoku Pl., Hilo, Hawaii, 96720, USA
³ Netherlands Foundation for Research in Astronomy, P.O. Box 2, 7990 AA Dwingeloo, The Netherlands

Received 17 April 1998 / Accepted 7 July 1998

Abstract

Emission from the transition at 492 GHz has been detected towards the dark cloud D 478 in the Local Group galaxy M 31. Using literature detections of the lower and transitions, models for the gas distribution in D 478 are discussed. The observed CO and C line ratios can be explained by two-component models (high density cores and low-density envelopes); single-density models appear less likely. The models indicate kinetic temperatures of the order of 10 K. The beam-averaged column density of neutral carbon is 0.3-0.8 times that of CO, whereas the total carbon to hydrogen ratio / is 5-3 10^-4. The resulting CO to conversion factor X is about half that of the Solar Neighbourhood. With temperatures of about 10 K and projected mass-densities of 5-10 pc^-2 there appears to be no need to invoke the presence of very cold and very massive clouds. Rather, D 478 appears to be comparable to Milky Way dark cloud complexes at the higher metallicity expected from its central location in M 31. In particular, several similarities between D 478 and the Galactic Taurus-Auriga dark cloud complex may be noted.

Key words: galaxies: individual: M 31 galaxies: ISM ISM: individual objects: D 478 ISM: molecules radio lines: galaxies

Send offprint requests to: F.P. Israel

© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998

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