Star Formation on Galactic Scales: Empirical Laws

TL;DR

This paper reviews empirical star formation laws over the past 20 years, comparing observations with simulations, highlighting relationships between star formation rates, gas phases, and the influence of gravitational thresholds in galaxies.

Contribution

It provides a comprehensive comparison of empirical laws with simulations, clarifies the role of gas phases and thresholds, and discusses the complexities affecting star formation in galaxies.

Findings

Star formation rate correlates linearly with molecular gas in galaxy disks.

In outer and dwarf galaxies, star formation depends on total gas with long consumption times.

The significance of gravitational instability thresholds remains uncertain due to various physical factors.

Abstract

Empirical star formation laws from the last 20 years are reviewed with a comparison to simulations. The current form in main galaxy disks has a linear relationship between the star formation rate per unit area and the molecular cloud mass per unit area with a timescale for molecular gas conversion of about 2 Gyr. The local ratio of molecular mass to atomic mass scales nearly linearly with pressure, as determined from the weight of the gas layer in the galaxy. In the outer parts of galaxies and in dwarf irregular galaxies, the disk can be dominated by atomic hydrogen and the star formation rate per unit area becomes directly proportional to the total gas mass per unit area, with a consumption time of about 100 Gyr. The importance of a threshold for gravitational instabilities is not clear. Observations suggest such a threshold is not always important, while simulations generally show that it is. The threshold is difficult to evaluate because it is sensitive to magnetic and viscous forces, the presence of spiral waves and other local effects, and the equation of state.