Molecular gas in normal late-type galaxies^*

¹ Laboratoire d'Astrophysique de Marseille, BP 8, Traverse du Siphon, 13376 Marseille Cedex 12, France
² DEMIRM and URA 336 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France e-mail: James.Lequeux@obspm.fr
³ Università degli Studi di Milano-Bicocca, Dipartimento di Fisica, Piazza dell'Ateneo Nuovo 1, 20126 Milano, Italy e-mail: Giuseppe.Gavazzi@mib.infn.it

Corresponding author: A. Boselli, Alessandro.Boselli@astrsp-mrs.fr

Received: 7 September 2001
Accepted: 28 November 2001

Abstract

We present ¹²CO(–0) line observations of 22 low-luminosity spiral galaxies in the Virgo cluster. These data, together with 244 others available in the literature, allow us to build a large sample that we use to study the molecular gas properties of galaxies spanning a large range of morphological types and luminosities and belonging to different environments (clusters – field). The molecular gas content of the target galaxies is estimated using a luminosity-dependent H/I(CO) conversion factor that has been calibrated on a sample of nearby galaxies. X spans from ~10²⁰ mol cm^-2 (K km s^-1)^-1 in giant spirals to ~10²¹ mol cm^-2 (K km s^-1)^-1 in dwarf irregulars. The value of the X conversion factor is found consistent with a value derived independently from dust masses estimated from FIR fluxes, with a metallicity-dependent dust to gas ratio. The relationships between X and the UV radiation field (as traced by the HNII] EW), the metallicity and the H band luminosity are analysed. We show that the molecular gas contained in molecular clouds or complexes is of the order of 15% of the total gas on average whatever the luminosity or the Hubble type of the galaxies. We discuss the relation between the star formation rate and the molecular gas content and estimate the average star formation efficiency of late-type galaxies.