Elevated ionizing photon production efficiency in faint
  high-equivalent-width Lyman-alpha emitters
  While low-luminosity galaxies dominate number counts at all redshifts, their
contribution to cosmic Reionization is poorly understood due to a lack of
knowledge of their physical properties. We isolate a sample of 35 z~4-5
continuum-faint Lyman-alpha emitters from deep VLT/MUSE spectroscopy and
directly measure their Halpha emission using stacked Spitzer/IRAC Ch. 1
photometry. Based on Hubble Space Telescope imaging, we determine that the
average UV continuum magnitude is fainter than -16 (~0.01 L_star), implying a
median Lyman-alpha equivalent width of 249 Angstroms. By combining the Halpha
measurement with the UV magnitude we determine the ionizing photon production
efficiency, xi_ion, a first for such faint galaxies. The measurement of log
(xi_ion [Hz/erg]) = 26.28 (+0.28; -0.40) is in excess of literature
measurements of both continuum- and emission line-selected samples, implying a
more efficient production of ionizing photons in these lower-luminosity,
Lyman-alpha-selected systems. We conclude that this elevated efficiency can be
explained by stellar populations with metallicities between 4e-4 and 0.008,
with light-weighted ages less than 3 Myr.
