Star formation rates in galaxies

The determination is dealt with of global star formation rates (SFRs) from radio free-free and submm/FIR dust emission. Massive, hot and luminous stars interact with the surrounding interstellar matter (ISM) ionizing the gas and heating the dust. O star formation rates (OSFR) in the galactic disk are estimated with observed Lyman continuum photon production rates. Extrapolation to lower mass stars with a constant initial mass function (IMF), however, yields too high total SFRs. Furthermore the lock-up rate, i.e., the rate at which gas transformed into stars is permanently locked up in low mass and dead stars, can not reproduce the present-day mass distribution of the galactic disk. Agreement betweeen Lyc photon production rate and time integrated lockup rate can be reached by introducing bimodal star formation in the galactic disk. This means that induced star formation in main spiral arms produces only massive stars ≥3mSun, while spontaneous star formation in the interarm region produces stars in the total mass range ≥0.1mSun. Estimates of SFR based on Lyc photon production rates cannot easily be applied to external galaxies because of the difficulty to separate radio synchroton and free-free emission. It is found that similar problems are encountered in separating the emission from warm dust (heated by OB stars) and cold dust (heated by the general interstellar radiation field). The relation between IR luminosity and star formation activity of galaxies is much more complex than previously assumed. (author). 3 figs., 1 tab., 16 refs