Lyman Continuum Escape Fraction of Star-forming Dwarf 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 Hubble Space Telescope 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 log(M)≃ 9.3 M_☉), moderately star-forming (ψ ≾ 10M_☉ yr^(−1)) galaxies selected initially on Hα emission. The characteristic escape fraction of LyC from star-forming galaxies (SFGs) that populate this parameter space remains weakly constrained by previous surveys, but these faint (sub-Lsstarf) 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 ~ 1 sample, individual non-detections to constrain the absolute Lyman continuum escape fraction, f_(esc) < 2.1% (3σ). We measure an upper limit of f_(esc) < 9.6% from a sample of SFGs selected on high Hα equivalent width EW > 200Å), which are thought to be close analogs of high redshift sources of reionization. For reference, we also present an emissivity-weighted escape fraction that is useful for measuring the general contribution SFGs to the ionizing UV background. In the discussion, we consider the implications of these intermediate redshift constraints for the reionization of hydrogen in the IGM at high (z > 6) redshift. If we assume our z ~ 1 SFGs, for which we measure this emissivity-weighted f_(esc), are analogs to the high redshift sources of reionization, we find it is difficult to 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.

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