Runaway stars masquerading as star formation in galactic outskirts

TL;DR

This paper proposes that runaway stars ejected from stellar clusters can explain the observed star formation in the low-density outskirts of spiral galaxies, challenging traditional theories.

Contribution

It introduces a novel mechanism involving runaway stars to account for star formation in galactic outskirts, supported by hydrodynamic simulations.

Findings

Runaway stars can travel kiloparsecs during their main-sequence lifetime.

Ejection of young stars explains star formation in low-density regions.

Runaway stars are a significant factor in outer galaxy star formation.

Abstract

In the outskirts of nearby spiral galaxies, star formation is observed in extremely low gas surface densities. Star formation in these regions, where the interstellar medium is dominated by diffuse atomic hydrogen, is difficult to explain with classic star formation theories. In this letter, we introduce runaway stars as an explanation for this observation. Runaway stars, produced by collisional dynamics in young stellar clusters, can travel kiloparsecs during their main-sequence lifetime. Using galactic-scale hydrodynamic simulations including a treatment of individual stars, we demonstrate that this mechanism enables the ejection of young massive stars into environments where the gas is not dense enough to trigger star formation. This results in the appearance of star formation in regions where it ought to be impossible. We conclude that runaway stars are a contributing, if not dominant, factor to the observations of star formation in the outskirts of spiral galaxies.