The ultraviolet light from galaxies has been used as a beacon for tracing the cosmic star formation history of the Universe, yet we have an incomplete understandingof many characteristics of this light. Most of the UV emission from nearby, normal star--forming galaxies is unresolved and "diffuse", and GALEX has shown that in spiral disks it permeates the inter-arm regions. The nature of this diffuse inter-arm component is under debate. Recent results suggest that it may arise from non-ionizing UV photons which originate in star forming regions in the spiral arms, travel in the plane of the galaxy, and then scatter off of diffusely distributed cold dust grains. Alternatively, an in-situ, unresolved stellar population could produce the observed inter-arm UV emission. This project seeks to establish which of the two competing scenarios is responsible for the bulk of this diffuse emission. We propose to use HST's UV imaging capability (ACS/SBC) to obtain deep observations of selected fields in the nearby spiral galaxy M101, for which available (low angular resolution) data favor the 'scattered light' scenario. Our observations are designed to detect any faint, UV-luminous stellar population down to main sequence B5 stars. With these data, we will establish the nature of the bulk of the diffuse UV light in this spiral galaxy by: (i) quantifying the contribution from dust-scattered light; (ii) measuring the contribution to the ubiquitous diffuse ionized medium from in-situ ionizing stars; and (iii) providing constraints on the observed stellar mass function in the field. Only HST has the UV sensitivity and angular resolution to discriminate in-situ stellar populations from scattered light. The ultimate goal of this project is to re-'calibrate' the UV emission as a star formation rate indicator, which will need to account for any scattered component.

We request ACS/SBC F150LP primary imaging of 2 inter-arm fields in M101, with NICMOS/NIC3 H band parallel imaging. The fields have been chosen such that at the specified Galactocentric radii the disk is not too optically thick (which will hinder the observation of the individual stars) but not too optically thin (since there will then be not sufficient dust to give rise to the putative scattered light component). (2) We choose the two fields just "upstream" and "downstream" of major spiral arms with prominent star-formation complexes, such that the SBC fields extend from the edge of the arms outwards (for a 29 x 26 arcsec aperture) and contain apparently diffuse UV emission on the GALEX images. 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