A&A 429, 153-159 (2005)
DOI: 10.1051/0004-6361:20034354

HCN and HCO ⁺ emission in the disk of M 31

N. Brouillet¹, S. Muller^{2, 3}, F. Herpin¹, J. Braine¹ and T. Jacq<sup>1

1</sup>  Observatoire de Bordeaux, Université Bordeaux 1- CNRS, BP 89, 33270 Floirac, France
    e-mail: name@obs.u-bordeaux1.fr
²  IRAM, 300 rue de la Piscine, 38406 Saint-Martin-d'Hères, France
³  Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 106, Taiwan
    e-mail: muller@asiaa.sinica.edu.tw

(Received 19 September 2003 / Accepted 3 September 2004)

Abstract
We report observations made with the IRAM 30 m radiotelescope in the HCN(1-0) and HCO ⁺(1-0) lines towards a sample of molecular complexes (GMCs) in the disk of the Andromeda galaxy ( M 31 ). The targets were identified bright CO GMCs selected from the IRAM 30 m CO survey with various morphologies and environments. The clouds vary in galactocentric distances from 2.4 to 15.5 kpc. The HCN and HCO ⁺ emission is easily detected in almost all observed positions, with line widths generally similar to the CO ones and there is a good correlation between the two dense gas tracers. The HCO ⁺ emission is slightly stronger than the HCN, in particular towards GMCs with a strong star formation activity. However the HCO ⁺ emission is weaker than the HCN towards a quiescent cloud in the inner part of M 31 , which could be due to a lower abundance of HCO ⁺. We derive ratios between 0.008 and 0.03 and ratios between less than 0.003 and 0.04. We study the radial distribution of the dense gas in the disk of M 31 . Unlike our Galaxy the HCO ⁺/CO ratio is lower in the center of M 31 than in the arms, which can be explained by both a lower abundance of HCO ⁺ and different conditions of excitation. Furthermore the HCN/CO and HCO ⁺/CO ratios appear to be higher in the inner spiral arm and weaker in the outer arm.

Key words: ISM: clouds -- ISM: molecules -- galaxies: individual: M 31 -- galaxies: ISM

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© ESO 2004

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