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Published May 1, 2010 | Published
Journal Article Open

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 distinct regions in the gas mass versus star formation rate (SFR) plane, both for the integrated quantities and for the respective surface densities. This result is based on carbon monoxide (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 SF 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 SFRs. 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 SFRs than are seen locally. The different regimes can potentially be interpreted as the effect of a top-heavy initial mass function 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 SFR.

Additional Information

© 2010 The American Astronomical Society. Received 2009 December 17; accepted 2010 March 22; published 2010 April 5. We thank Adam Leroy for discussions and help with Figure 2 and Padelis Papadopoulos for discussions. We acknowledge the funding support of the ERC-StG grant UPGAL-240039, ANR-07-BLAN-0228, and ANR-08-JCJC-0008. D.R. acknowledges NASA Hubble Fellowship grant HST-HF-51235.01 awarded by the STScI, operated by AURA for NASA, contract NAS-5-26555.

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