Gravity or turbulence? VI. The physics behind the Kennicutt-Schmidt relations

TL;DR

This paper proposes a unified, cloud-collapse-based explanation for the variety of star formation laws, emphasizing the role of cold dense gas collapse over turbulence support, and clarifies the influence of different timescales and pressures on observed relations.

Contribution

It introduces a simple, collapse-driven star formation law that accounts for various observed correlations and explains the effects of different timescales and pressures on star formation.

Findings

The star formation rate is proportional to the collapsing mass divided by the free-fall time.

The slope of the SFR correlation changes when orbital time is used instead of free-fall time.

The apparent linear relation between SFR and midplane pressure is due to molecular gas column density variations.

Abstract

We explain the large variety of star formation laws in terms of one single, simple law that can be inferred from the definition of the star formation rate and basic algebra. The resulting equation, $\SFR = \eff\ \Mcollapsing/\tauff$, although it has been presented elsewhere, is interpreted in terms of clouds undergoing collapse { rather than being turbulence-supported, an idea that different groups have pursued this century}. Under such assumption, one can explain the constancy of $\eff$, the different intra-cloud correlations observed in Milky Way's molecular clouds, as well as the resolved and unresolved extragalactic relationships between SFR and a measurement of the mass in CO, HCN, and CO+HI. We also explain why the slope of the correlation changes when the orbital time $\tauorb$ is considered instead of the free-fall time, and why estimations of the free-fall time from extragalactic observations skew the correlation, providing a false sublinear correlation. We furthermore show that the apparent nearly linear correlation between the star formation rate and the dynamical equilibrium pressure in the midplane of the galaxies, $\PDE$, is just a consequence of $\PDE$ values being dominated by the variation of the column density of molecular gas. All in all, we argue that the star formation law is driven by the collapse of cold, dense gas, which happens to be primarily molecular in the present Universe, and that the role of stellar feedback is just to shut down the star formation process, not to shape the star formation law.