Galaxies 09: Assembly, Gas Content and Star Formation History of Galaxies
Mark Krumholz
UC Santa Cruz
Star Formation Laws for Atomic and Molecular Gas
I discuss 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. Combining analytic estimates for each of these leads to a single star formation law, which can be formulated in terms of either the atomic or molecular content of a galaxy, and agrees very well with observations to date. I discuss how this model can be tested and extended using the capabilities of ALMA and other future observatories.
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