1995A&A...300...43A

Using the IRAM 30m, SEST 15m, and Nancay radiotelescopes, we have gathered the 1mm continuum emission, the intensities of the J=1-0 line of the CO molecule and of the atomic hydrogen line at 21cm for two samples of IRAS galaxies. The southern sample was selected from the IRAS Catalogue and is complete at the limiting flux of 2Jy at 60µm; of the 10 northern objects 7 belong to the Smith et al. complete sample (1987) and 3 are isolated objects. Using these data, we have estimated the atomic hydrogen masses from the 21cm emission, the molecular gas masses from the CO(1-0)line brightness, and the dust and gas masses from the mm continuum emission using two "extreme" dust models. The main conclusions of this work for far-infrared selected galaxies can be summarized in the following points: (1) the median value of M_H2/M_HI is 0.5, meaning that the atomic phase dominates in these galaxies. The fraction of gas in molecular form increases with increasing FIR luminosity but does not show any obvious trend with other galaxy properties, in particular with the FIR surface brightness. (2) the H₂ surface density derived from CO(1-0)emission is better correlated with the cold dust surface density than the HI surface density, but the correlation of HI with dust is not negligible (we found a correlation coefficient of 0.5, while the correlation coefficient with σ_H2 is 0.70). Thus, globally in these galaxies, the cold dust emission is likely associated with both the molecular and atomic phases. Indeed, the dust surface density is also correlated with the total gas surface density. (3) the FIR surface brightness increases as the third power of the S(60µm)/S(100µm) ratio. It shows a tight correlation with both the H₂ and dust surface densities and a weaker one with the HI surface density. This suggests that a large part of the far-infrared emission of these galaxies originates in the molecular medium. (4) the gas-to-dust ratio, (M_H2+ M_HI)/M_d ranges between 100 and 1000 and its average value is 230, close to the Galactic value. There is indeed a clear trend: this ratio decreases as the FIR surface density increases. This result can be explained in the framework of an enhancement of metallicity in galaxy discs having a higher star formation rate.