Feedback-regulated star formation in molecular clouds and galactic discs

We present a two-zone theory for feedback-regulated star formation in galactic discs, consistently connecting the galaxy-averaged star formation law with star formation proceeding in giant molecular clouds (GMCs). Our focus is on galaxies with gas surface density Σg & 100 M pc−2, where the interstellar medium (ISM) can be assumed to be fully molecular. This regime includes most star formation in the Universe and our basic framework can be extended to other galaxies. In our theory, the galactic disc consists of Toomre-mass GMCs embedded in a volume-filling ISM. Radiation pressure on dust disperses GMCs and most supernovae explode in the volume-filling medium. A galaxy-averaged star formation law is derived by balancing the momentum input from supernova feedback with the vertical gravitational weight of the disc gas. This star formation law is in good agreement with observations for a CO conversion factor depending continuously on Σg. We argue that the galaxy-averaged star formation efficiency per free fall time, galff, is only a weak function of the efficiency with which GMCs convert their gas into stars, GMCint. This is possible because the rate limitin