Star Formation in Spiral Arms

TL;DR

This paper reviews how different types of spiral arms in galaxies influence star formation, highlighting the roles of gas dynamics, gravitational instabilities, and galaxy interactions in forming molecular clouds and stars.

Contribution

It provides a comprehensive overview of the mechanisms linking spiral arm structures to star formation processes in galaxies.

Findings

Flocculent arms result from transient gas instabilities promoting star formation.

Global spiral arms can be driven by galaxy interactions and evolve over several rotations.

Giant molecular clouds form in high-density regions within spiral arms, leading to star formation.

Abstract

The origin and types of spiral arms are reviewed with an emphasis on the connections between these arms and star formation. Flocculent spiral arms are most likely the result of transient instabilities in the gas that promote dense cloud formation, star formation, and generate turbulence. Long irregular spiral arms are usually initiated by gravitational instabilities in the stars, with the gas contributing to and following these instabilities, and star formation in the gas. Global spiral arms triggered by global perturbations, such as a galaxy interaction, can be wavemodes with wave reflection in the inner regions. They might grow and dominate the disk for several rotations before degenerating into higher-order modes by non-linear effects. Interstellar gas flows through these global arms, and through the more transient stellar spiral arms as well, where it can reach a high density and low shear, thereby promoting self-gravitational instabilities. The result is the formation of giant spiral arm cloud complexes, in which dense molecular clouds form and turn into stars. The molecular envelops and debris from these clouds appear to survive and drift through the interarm regions for a long time, possibly 100 Myr or more, with lingering spontaneous star formation and triggered star formation in the pieces that are still at high-pressure edges near older HII regions.