Tightly Correlated HI and FUV Emission in the Outskirts of M83

TL;DR

This study reveals a strong spatial correlation between HI gas and FUV emission in M83's outer disk, indicating widespread low-level star formation and a slow gas consumption rate that influences galaxy evolution.

Contribution

It demonstrates a detailed spatial correlation between HI and FUV emissions in the outer disk of M83, highlighting the role of HI as a star formation reservoir at large radii.

Findings

HI and FUV emissions are spatially correlated out to 4 r25

Most of the outer disk's HI mass is in low column densities (<2 M_solar/pc^2)

Star formation in the outer disk has very long gas consumption times (~100 Gyr)

Abstract

We compare sensitive HI data from The HI Nearby Galaxy Survey (THINGS) and deep far UV (FUV) data from GALEX in the outer disk of M83. The FUV and HI maps show a stunning spatial correlation out to almost 4 optical radii (r25), roughly the extent of our maps. This underscores that HI traces the gas reservoir for outer disk star formation and it implies that massive (at least low level) star formation proceeds almost everywhere HI is observed. Whereas the average FUV intensity decreases steadily with increasing radius before leveling off at ~1.7 r25, the decline in HI surface density is more subtle. Low HI columns (<2 M_solar/pc^2) contribute most of the mass in the outer disk, which is not the case within r25. The time for star formation to consume the available HI, inferred from the ratio of HI to FUV intensity, rises with increasing radius before leveling off at ~100 Gyr, i.e., many Hubble times, near ~1.7 r25. Assuming the relatively short H2 depletion times observed in the inner parts of galaxies hold in outer disks, the conversion of HI into bound, molecular clouds seems to limit star formation in outer galaxy disks. The long consumption times suggest that most of the extended HI observed in M83 will not be consumed by in situ star formation. However, even these low star formation rates are enough to expect moderate chemical enrichment in a closed outer disk.