We present evidence that 'bona fide' disks and starburst systems occupy
distinct regions in the gas mass versus star formation (SF) rate plane, both
for the integrated quantities and for the respective surface densities. This
result is based on CO observations of galaxy populations at low and high
redshifts, and on the current consensus for the CO luminosity to gas mass
conversion factors. The data suggest the existence of two different star
formation regimes: a long-lasting mode for disks and a more rapid mode for
starbursts, the latter probably occurring during major mergers or in dense
nuclear SF regions. Both modes are observable over a large range of SF rates.
The detection of CO emission from distant near-IR selected galaxies reveals
such bimodal behavior for the first time, as they allow us to probe gas in disk
galaxies with much higher SF rates than are seen locally. The different regimes
can potentially be interpreted as the effect of a top-heavy IMF in starbursts.
However, we favor a different physical origin related to the fraction of
molecular gas in dense clouds. The IR luminosity to gas mass ratio (i.e., the
SF efficiency) appears to be inversely proportional to the dynamical (rotation)
timescale. Only when accounting for the dynamical timescale, a universal SF law
is obtained, suggesting a direct link between global galaxy properties and the
local SF rate.Comment: 5 pages, 4 figures. ApJ Letters in pres
