On the Star Formation Law for Spiral and Irregular Galaxies

TL;DR

This paper presents a dynamical model explaining star formation rates in spiral and irregular galaxies, showing how they depend on gas density and galaxy region, with implications for galaxy evolution.

Contribution

It introduces a unified dynamical framework for star formation laws, accounting for variations across galaxy types and regions, including molecular and total gas dependencies.

Findings

Star formation rate scales with gas density as 1.5 power in main regions.

In outer regions and dwarf irregulars, the rate scales as the square of gas density.

Star formation duration can exceed 100 Gyr in outer galaxy regions.

Abstract

A dynamical model for star formation on a galactic scale is proposed in which the interstellar medium is constantly condensing to star-forming clouds on the dynamical time of the average midplane density, and the clouds are constantly being disrupted on the dynamical time scale appropriate for their higher density. In this model, the areal star formation rate scales with the 1.5 power of the total gas column density throughout the main regions of spiral galaxies, and with a steeper power, 2, in the far outer regions and in dwarf irregular galaxies because of the flaring disks. At the same time, there is a molecular star formation law that is linear in the main and outer parts of disks and in dIrrs because the duration of individual structures in the molecular phase is also the dynamical time scale, canceling the additional 0.5 power of surface density. The total gas consumption time scales directly with the midplane dynamical time, quenching star formation in the inner regions if there is no accretion, and sustaining star formation for ~100 Gyr or more in the outer regions with no qualitative change in gas stability or molecular cloud properties. The ULIRG track follows from high densities in galaxy collisions.