A SPECTROSCOPIC SEARCH FOR LEAKING LYMAN CONTINUUM AT z ∼ 0.7

We present the results of rest-frame, UV slitless spectroscopic observations of a sample of 32 z similar to 0.7 Lyman break galaxy (LBG) analogs in the COSMOS field. The spectroscopic search was performed with the Solar Blind Channel on the Hubble Space Telescope. We report the detection of leaking Lyman continuum (LyC) radiation from an active galactic nucleus-starburst composite. While we find no direct detections of LyC emission in the remainder of our sample, we achieve individual lower limits (3 sigma) of the observed non-ionizing UV-to-LyC flux density ratios, f(v), (1500 angstrom)/f(v) (830 angstrom) of 20 to 204 (median of 73.5) and 378.7 for the stack. Assuming an intrinsic Lyman break of 3.4 and an intergalactic medium transmission of LyC photons along the line of sight to the galaxy of 85%, we report an upper limit for the relative escape fraction in individual galaxies of 0.02-0.19 and a stacked 3 sigma upper limit of 0.01. We find no indication of a relative escape fraction near unity as seen in some LBGs at z similar to 3. Our UV spectra achieve the deepest limits to date at any redshift for the escape fraction in individual sources. The contrast between these z similar to 0.7 low escape fraction LBG analogs with z similar to 3 LBGs suggests that either the processes conducive to high f(esc) are not being selected for in the z less than or similar to 1 samples or the average escape fraction is decreasing from z similar to 3 to z similar to 1. We discuss possible mechanisms that could affect the escape of LyC photons.