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

TL;DR

This paper demonstrates that disk and starburst galaxies follow distinct star formation laws across different redshifts, highlighting different physical regimes and efficiencies linked to galaxy dynamics and molecular gas properties.

Contribution

It provides observational evidence for two separate star formation regimes in galaxies, supported by CO data at various redshifts, and proposes a universal law when accounting for dynamical timescales.

Findings

Disks and starbursts occupy distinct regions in gas mass vs. SF rate plane.

Distant IR-selected galaxies show bimodal star formation behavior.

Star formation efficiency correlates inversely with dynamical timescale.

Abstract

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.