Star Formation Thresholds

TL;DR

This paper compares the Toomre criterion and thermo-gravitational instability hypothesis to explain star formation thresholds, arguing that the latter better accounts for observed phenomena in galaxy outer regions.

Contribution

It challenges the traditional Toomre criterion and advocates for the thermo-gravitational instability model as a more accurate explanation for star formation thresholds.

Findings

Toomre criterion assumptions break down in galaxy outskirts.

Thermo-gravitational instability predicts a nearly constant surface density threshold.

The hypothesis explains phenomena previously thought unrelated to star formation thresholds.

Abstract

To make predictions for the existence of ``dark galaxies'', it is necessary to understand what determines whether a gas cloud will form stars. Star formation thresholds are generally explained in terms of the Toomre criterion for gravitational instability. I contrast this theory with the thermo-gravitational instability hypothesis of Schaye (2004), in which star formation is triggered by the formation of a cold gas phase and which predicts a nearly constant surface density threshold. I argue that although the Toomre analysis is useful for the global stability of disc galaxies, it relies on assumptions that break down in the outer regions, where star formation thresholds are observed. The thermo-gravitational instability hypothesis can account for a number of observed phenomena, some of which were thought to be unrelated to star formation thresholds.