Abstract. I review some recent results about the molecular content of galaxies, obtained essentially from the CO lines, but also dense tracers, or the dust continuum emission. New results have been obtained on molecular cloud physics, and their efficiency to form stars, shedding light on the Kennicutt-Schmidt law as a function of surface density and galaxy type. Large progress has been made on galaxy at moderate and high redshifts, allowing to interprete the star formation history and star formation efficiency as a function of gas content, or galaxy evolution. In massive galaxies, the gas fraction was higher in the past, and galaxy disks were more unstable and more turbulent. ALMA observations will allow the study of more normal galaxies at high z with higher spatial resolution and sensitivity.Keywords. galaxies: general, galaxies: high-redshift, galaxies: evolution, ISM: molecule, galaxies: ISM, galaxies: spiral, galaxies: starburst, galaxies: structure
Nearby galaxiesA new survey (HERACLES) has been completed with the IRAM receiver array in the CO(2-1) line, allowing extended maps of nearby galaxies, at 12" resolution (Leroy et al. 2009). The survey contains an atlas of 18 nearby galaxies, observed at multi-wavelengths, and in particular in the HI line, and in the mid infrared by Spitzer. Among the results, it is interesting to note a very good correlation between CO and HI kinematics. The excitation of the molecular gas, as traced by the first two rotational lines of CO, is usually low in the disk (R = CO(2-1)/CO(1-0) = 0.6) while it is higher in nuclei (R = 1), indicating denser gas. The CO emission is compatible with optically thick clouds at a kinetic temperature of T = 10K.A more refined view of the star formation law in galaxies has been obtained by Bigiel et al. (2008). The Schmidt-Kennicutt law relating star formation and gas density has a different slope n, according to the gas surface density. At high surface density, when the gas is molecular, the gas forms stars at a constant efficiency (n = 1), and the time-scale for star formation is 2 10 9 yrs. While, at low surface density, when the gas is atomic, the slope is much higher n = 2 or more. At sub-kpc scale, the star formation rate is not strongly correlated with HI surface density. The transition between HI and H 2 occurs when the surface density is > 9 M pc −2 .In order to better determine the change across the spiral arms of the molecular gas physical properties, Schinnerer et al. (2010) have mapped at interferometric resolution several lines of CO and its isotopes, together with dense gas tracers, such as HCN and HCO + , in two selected regions across M51 spiral arms. They find no change across the arms, and the GMC population in the spiral arms of M51 is similar to those of the Milky Way, even if the star formation rate is much higher.On the contrary, the low surface brightness dwarf spiral M33 reveals different conditions than in the Milky Way. In the center, the lower metallicity is the cause of a higher conversion factor, as will be describ...