CO, HI and cold dust in a sample of IRAS galaxies

Abstract

Using IRAM 30m, SEST 15m, and Nancay radiotelescopes, we have gathered the 1mm continuum emission, spectra of the J=1--0 CO line and of the HI line at 21cm for two samples of IRAS galaxies. Using these data, we have estimated the HI masses, the H$_2$ masses and the dust masses. We find that for far-infrared selected galaxies: (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. (2) the $H2$ surface density (s.d.) is better correlated with the cold dust s.d. than the HI s.d., but the correlation of HI with dust is not negligible. Globally in these galaxies the cold dust emission is associated with both the molecular and atomic phases. (3) the FIR surface brightness increases as the third power of the S60um/S100um ratio, it shows a tight correlation with both the H$_2$ and dust surface densities and a weaker one with the HI surface density: a large part of the far-infrared emission of these galaxies originates in the molecular medium. (4) the gas-to-dust ratio 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, a result which can be explained in the framework of an enhancement of metallicity in galaxy discs having a higher star formation rate.