The Star Formation Law in Atomic and Molecular Gas
  We propose a simple theoretical model for star formation in which the local
star formation rate in a galaxy is determined by three factors. First, the
interplay between the interstellar radiation field and molecular self-shielding
determines what fraction of the gas is in molecular form and thus eligible to
form stars. Second, internal feedback determines the properties of the
molecular clouds that form, which are nearly independent of galaxy properties
until the galactic ISM pressure becomes comparable to the internal GMC
pressure. Above this limit, galactic ISM pressure determines molecular gas
properties. Third, the turbulence driven by feedback processes in GMCs makes
star formation slow, allowing a small fraction of the gas to be converted to
stars per free-fall time within the molecular clouds. We combine analytic
estimates for each of these steps to formulate a single star formation law, and
show that the predicted correlation between star formation rate, metallicity,
and surface densities of atomic, molecular, and total gas agree well with
observations.
