Star formation in galaxies: the role of spiral arms

TL;DR

This paper reviews recent simulations and observations of star formation in spiral arms, highlighting how GMC evolution, cloud collisions, and gas dynamics inform our understanding of spiral structure and galaxy evolution.

Contribution

It synthesizes recent findings on GMC lifecycle, cloud interactions, and gas flow dynamics to elucidate the nature of spiral arms in galaxies.

Findings

Massive GMCs are predominantly found in spiral arms.

Cloud-cloud collisions are likely responsible for forming massive clouds.

Gas flow dynamics differ between long-lived and transient spiral arms.

Abstract

Studying star formation in spiral arms tells us not only about the evolution of star formation, and molecular clouds, but can also tell us about the nature of spiral structure in galaxies. I will address both these topics using the results of recent simulations and observations. Galactic scale simulations are beginning to examine in detail the evolution of GMCs as they form in spiral arms, and then disperse by stellar feedback or shear. The overall timescale for this process appears comparable to the crossing time of the GMCs, a few Myrs for $10^5$ M$_{\odot}$ clouds, 20 Myr or so for more massive GMCs. Both simulations and observations show that the massive clouds are found in the spiral arms, likely as a result of cloud-cloud collisions. Simulations including stars should also tell us about the stellar age distribution in GMCs, and across spiral arms. More generally, recent work on spiral galaxies suggests that the dynamics of gas flows in spiral arms are different in longlived and transient spiral arms, resulting in different age patterns in the stars. Such results could be used to help establish the main driver of spiral structure in the Milky Way (Toomre instabilities, the bar, or nearby companion galaxies) in conjunction with future surveys.