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Issue A&A
Volume 429, Number 1, January I 2005
Page(s) 153 - 159
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20034354



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. Muller2, 3, F. Herpin1, J. Braine1 and T. Jacq1

1  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 $I_{\rm HCN}/I_{\rm CO}$ ratios between 0.008 and 0.03 and $I_{\rm HCO^{+}}/I_{\rm CO}$ 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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