The Lyman Continuum Escape Fraction of Low-Mass Star-Forming Galaxies at z~1

Abstract

To date no direct detection of Lyman continuum emission has been measured for intermediate--redshift z~1 star-forming galaxies . We combine HST grism spectroscopy with GALEX UV and ground--based optical imaging to extend the search for escaping Lyman continuum to a large (~600) sample of z~1 low-mass, moderately star-forming galaxies selected initially on H$\alpha$ emission. The characteristic escape fraction of LyC from SFGs that populate this parameter space remains weakly constrained by previous surveys, but these faint SFGs are assumed to play a significant role in the reionization of neutral hydrogen in the intergalactic medium (IGM) at high redshift (z>6). We do not make an unambiguous detection of escaping LyC radiation from this $z\sim1$ sample, individual non--detections to constrain the absolute Lyman continuum escape fraction, $f_{esc}$<2.1% (3$\sigma$). We measure upper limits of $f_{esc}$<9.6% from a sample of SFGs selected on high H$\alpha$ equivalent width (EW>200\AA), which are thought to be close analogs of high redshift sources of reionization. For reference, we also present an emissivity--weighted escape fraction which is useful as a measurement of the general contribution of the SFGs to the z~1 ionizing UV background. In the discussion, we consider the implications of these intermediate redshift constraints for the reionization of hydrogen in the intergalactic medium at high (z > 6) redshift. If we assume our $z\sim1$ SFGs, for which we measure this emissivity-weighted $f_{esc}$, are analogs to the high redshift sources of reionization, we find is difficult reconcile reionization by faint (M$_{UV}<-13$) SFGs with a low escape fraction ($f_{esc}<$3%), with constraints from independent high redshift observations. If $f_{esc}$ evolves with redshift, reionization by SFGs may be consistent with observations from Planck.