Origin of the Diffuse, Far Ultraviolet Emission in the Interarm Regions of M101

TL;DR

This study uses HST imaging to analyze the origin of diffuse FUV emission in M101's interarm regions, revealing that it results from a combination of faint stars and scattered light, with recent star formation history influencing the emission.

Contribution

It provides new insights into the sources of diffuse FUV emission in galaxy interarm regions, highlighting the roles of faint stars and scattering, based on high-resolution HST observations.

Findings

Diffuse FUV emission is partly due to undetected faint stars.

Scattered FUV light significantly contributes near spiral arms.

Recent decline in star formation rate explains observed stellar luminosity functions.

Abstract

We present images from the Solar Blind Channel on HST that resolve hundreds of far ultraviolet (FUV) emitting stars in two ~1 kpc$^2$ interarm regions of the grand-design spiral M101. The luminosity functions of these stars are compared with predicted distributions from simple star formation histories, and are best reproduced when the star formation rate has declined recently (past 10-50 Myr). This pattern is consistent with stars forming within spiral arms and then streaming into the interarm regions. We measure the diffuse FUV surface brightness after subtracting all of the detected stars, clusters and background galaxies. A residual flux is found for both regions which can be explained by a mix of stars below our detection limit and scattered FUV light. The amount of scattered light required is much larger for the region immediately adjacent to a spiral arm, a bright source of FUV photons.