Different star formation laws for disks versus starbursts at low and high redshifts

摘要

We present evidence that 'bona fide' disks and starburst systems occupydistinct regions in the gas mass versus star formation (SF) rate plane, bothfor the integrated quantities and for the respective surface densities. Thisresult is based on CO observations of galaxy populations at low and highredshifts, and on the current consensus for the CO luminosity to gas massconversion factors. The data suggest the existence of two different starformation regimes: a long-lasting mode for disks and a more rapid mode forstarbursts, the latter probably occurring during major mergers or in densenuclear SF regions. Both modes are observable over a large range of SF rates.The detection of CO emission from distant near-IR selected galaxies revealssuch bimodal behavior for the first time, as they allow us to probe gas in diskgalaxies with much higher SF rates than are seen locally. The different regimescan potentially be interpreted as the effect of a top-heavy IMF in starbursts.However, we favor a different physical origin related to the fraction ofmolecular gas in dense clouds. The IR luminosity to gas mass ratio (i.e., theSF efficiency) appears to be inversely proportional to the dynamical (rotation)timescale. Only when accounting for the dynamical timescale, a universal SF lawis obtained, suggesting a direct link between global galaxy properties and thelocal SF rate.