Pushing COS to the {Lyman-}Limit
The newly attained UV sensitivity of COS below 1150 A opens a wavelength domain not available since the end of the FUSE mission in 2007 and enables fundamentally new UV science with HST. We take advantage of this capability and propose COS+G140L spectroscopy of three carefully selected starburst galaxies at cz =13,000 km/s to measure or place stringent limits on the intrinsic Lyman continuum, and constrain the fraction of hydrogen-ionizing photons that escape the galaxies. Our galaxies were selected to have reliable, observed fluxes at 1000 A {based on FUSE archival spectra}, and physical properties that provide the most favorable conditions for measuring the escape of Lyman continuum photons. The opacity to ionizing photons within star-forming galaxies, even in our own Galaxy, remains highly uncertain. Using Starburst99 with our newly developed set of model atmospheres, we will compute significantly improved constraints on the expected Lyman continuum. By measuring the redshifted Lyman break we can derive or set an upper limit to the escape fraction of ionizing radiation. In contrast, prior FUSE observations below 912 A are dominated by systematics due to variable background noise. The proposed observations will also enable studies of outflows, the interstellar reddening law, and stellar population properties, including star-formation rates and the stellar initial mass function. We will compare our results to existing data at higher redshift in an effort to understand the evolution of the escape fraction with redshift, and to shed light on the conditions that must have prevailed if primeval galaxies were responsible for reionizing the intergalactic medium at z >6.
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