Spiral-Induced Star Formation in the Outer Disks of Galaxies

TL;DR

This paper uses simulations to show that spiral structures can induce star formation in the outer regions of galactic disks, explaining observed extended UV disks, though not all types.

Contribution

It demonstrates that spiral-induced over-densities can trigger star formation in outer disks, providing a physical explanation for Type I XUV disks.

Findings

Spiral structures can induce star formation in extended gas disks.

Simulated galaxies match observed Type I XUV disks.

Unable to reproduce Type II XUV disks with current models.

Abstract

The outer regions of galactic disks have received increased attention since ultraviolet observations with GALEX demonstrated that nearly 30% of galaxies have UV emission beyond their optical extents, indicating star formation activity. These galaxies have been termed extended UV (XUV) disks. Here, we address whether these observations contradict the gas surface density threshold for star formation inferred from Halpha radial profiles of galaxies. We run smoothed particle hydrodynamics simulations of isolated disk galaxies with fiducial star formation prescriptions and show that over-densities owing to the presence of spiral structure can induce star formation in extended gas disks. For direct comparison with observations, we use the 3-D radiative transfer code Sunrise to create simulated FUV and K_s band images. We find that galaxies classified as Type I XUV disks are a natural consequence of spiral patterns, but we are unable to reproduce Type II XUV disks. We also compare our results to studies of the Kennicutt-Schmidt relation in outer disks.