Outer Disk Star Formation in HI selected Galaxies

TL;DR

This study shows that outer disks of HI-selected galaxies frequently host low-level star formation, with UV and H-alpha observations revealing extended star-forming regions beyond the optical galaxy boundaries.

Contribution

It provides the first large-scale confirmation that outer disks are actively forming stars, and introduces a model linking star formation efficiency to disk stability and dark matter distribution.

Findings

Outer disks commonly contain HII regions and UV-bright areas.

Star formation efficiency remains nearly constant across different surface brightness levels.

HI traces the distribution of dark matter in galaxy outskirts.

Abstract

The HI in galaxies often extends past their conventionally defined optical extent. I report results from our team which has been probing low intensity star formation in outer disks using imaging in H-alpha and ultraviolet. Using a sample of hundreds of HI selected galaxies, we confirm that outer disk HII regions and extended UV disks are common. Hence outer disks are not dormant but are dimly forming stars. Although the ultraviolet light in galaxies is more centrally concentrated than the HI, the UV/HI ratio (the Star Formation Efficiency) is nearly constant, with a slight dependency on surface brightness. This result is well accounted for in a model where disks maintain a constant stability parameter Q. This model also accounts for how the ISM and star formation are distributed in the bright parts of galaxies, and how HI appears to trace the distribution of dark matter in galaxy outskirts.