DIFFERENT STAR FORMATION LAWS FOR DISKS VERSUS STARBURSTS AT LOW AND HIGH REDSHIFTS

Abstract

We present evidence that 'bona fide' disks and starburst systems occupy\ndistinct regions in the gas mass versus star formation (SF) rate plane, both\nfor the integrated quantities and for the respective surface densities. This\nresult is based on CO observations of galaxy populations at low and high\nredshifts, and on the current consensus for the CO luminosity to gas mass\nconversion factors. The data suggest the existence of two different star\nformation regimes: a long-lasting mode for disks and a more rapid mode for\nstarbursts, the latter probably occurring during major mergers or in dense\nnuclear SF regions. Both modes are observable over a large range of SF rates.\nThe detection of CO emission from distant near-IR selected galaxies reveals\nsuch bimodal behavior for the first time, as they allow us to probe gas in disk\ngalaxies with much higher SF rates than are seen locally. The different regimes\ncan potentially be interpreted as the effect of a top-heavy IMF in starbursts.\nHowever, we favor a different physical origin related to the fraction of\nmolecular gas in dense clouds. The IR luminosity to gas mass ratio (i.e., the\nSF efficiency) appears to be inversely proportional to the dynamical (rotation)\ntimescale. Only when accounting for the dynamical timescale, a universal SF law\nis obtained, suggesting a direct link between global galaxy properties and the\nlocal SF rate.\n