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Astron. Astrophys. 327, 522-538 (1997)

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The molecular gas content of spiral galaxies
in the Coma/A1367 supercluster *

A. Boselli 1, 2, 3, G. Gavazzi 4, 5, J. Lequeux 3, V. Buat 1, 6, F. Casoli 3, J. Dickey 7 and J. Donas 1

1 Laboratoire d'Astronomie Spatiale du CNRS, BP 8, Traverse du Siphon, F-13376 Marseille Cedex 12, France
2 Max-Planck-Institut für Kernphysik, Postfach 103980, D-69117 Heidelberg, Germany
3 DEMIRM and URA 336 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, F-75014 Paris, France
4 Università degli Studi di Milano, Dipartimento di Fisica, Via Celoria 16, I-20133 Milano, Italy
5 Osservatorio Astronomico di Brera, via Brera 28, I-20121 Milano, Italy
6 Laboratoire des intéractions photons-matières, faculté des Sciences et Techniques de St. Jérôme, F-13397 Marseille Cedex 13
7 Department of Astronomy, University of Minnesota, 116 Church Street S.E., Minneapolis, MN 55455, USA

Received 23 December 1996 / Accepted 7 May 1997

Abstract

We present 12 CO(J=1-0) line observations of 73 spiral galaxies mostly in the Coma/A1367 supercluster. From these data, combined with data available in the literature, we extract the first complete, optically selected sample (mpg [FORMULA] 15.2) of 37 isolated and of 27 cluster galaxies.

Adopting a standard conversion factor X =N(H2)/I(CO), we estimate that the molecular hydrogen content of isolated spiral galaxies is, on average, 20% of the atomic hydrogen reservoir, significantly lower than previous estimates based on samples selected by FIR criteria, thus biased towards CO rich objects.

We show that the frequency distributions of the CO deficiency parameter, defined as the difference between the expected and the observed molecular gas content of a galaxy of given luminosity (or linear diameter), computed separately for cluster and isolated galaxies, are not significantly different, indicating that the environment does not affect the molecular gas content of spiral discs.

A well defined relationship exists between Mi (H2) and the star formation activity in bright galaxies, while it is weaker at lower luminosities. We interpret this finding as indicating that CO emission traces relatively well the H2 mass only in high-mass galaxies, such as the Milky Way. On the other hand, in low-mass spirals the higher far-UV radiation field produced by young O-B stars and the lower metallicity cause the photodissociation of the diffuse molecular gas, weakening the expected relationship between star formation and the CO emission. The conversion factor between the CO line intensity and the amount of molecular hydrogen being ill-determined and variable with the UV flux and abundances, it is difficult to assess the relationship between the star formation and the amount of molecular hydrogen. 1

Key words: Galaxies: general – spiral – ISM – intergalactic medium – star formation – radio lines: galaxies

* based on observations made with the 12-m National Radio Astronomical Observatory, Kitt Peak, Arizona

Send offprint requests to: Alessandro Boselli

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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