Origin of the diffuse, far ultraviolet emission in the Interarm regions of m101

Abstract

We present images from the Solar Blind Channel on the Hubble Space Telescope that resolve hundreds of far- ultraviolet (FUV) emitting stars in two ∼1 kpc2 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 recently declined (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.
Key words: dust, extinction – galaxies: individual (M101) – galaxies: spiral – galaxies: star clusters: general –
galaxies: star formation – scattering