Charles J. Lada
Star Formation Rates and Thresholds in Molecular Clouds of the Milky Way and Beyond: Deciphering the Nature of the Global Scaling Relations.
The star formation rate and its variation in time are intimately connected to the formation and evolution of galaxies from early cosmological epochs to the present era. Ever since the pioneering work of Schmidt a half-century ago there has been great interest in finding an appropriate physical relation that would directly link some physical property of interstellar gas with the rate of star formation in a galaxy. In this review I will describe how observations of nearby Galactic molecular clouds are now providing new insights into this relationship. In particular, wide-field infrared extinction mapping and Spitzer Space Telescope surveys have produced the first reasonably complete accounting of both star formation activity and molecular gas in the local Milky Way. These new observations indicate that the star formation rates in molecular clouds are directly proportional to the amount of gas these clouds contain above a specific (high) density threshold. This correlation between dense gas and star formation likely represents the underlying physical relationship or law that most directly connects star formation activity with interstellar gas both within our Milky Way Galaxy as well as within and between the other galaxies in the universe. Finally, I will conclude this review by showing how these observations enable the unification of the various extragalactic scaling relations (e.g., Schmidt-Kennicutt, Gao-Solomon) into a single framework that connects star formation on local and cosmological scales.