The Effects of Stellar Population and Gas Covering Fraction on the
  Emergent Lyman Alpha Emission of High-Redshift Galaxies
  We perform joint modeling of the composite rest-frame far-UV (FUV) and
optical spectra of redshift 1.85<z<3.49 star-forming galaxies to deduce key
properties of the massive stars, ionized ISM, and neutral ISM, with the aim of
investigating the principal factors affecting the production and escape of
Ly-alpha (Lya) photons. Our sample consists of 136 galaxies with deep Keck/LRIS
and MOSFIRE spectra covering, respectively, Ly-beta through CIII] 1907, 1909;
and [OII], [NeIII], H-beta, [OIII], H-alpha, [NII], and [SII]. Spectral and
photoionization modeling indicate that the galaxies are uniformly consistent
with stellar population synthesis models that include the effects of stellar
binarity. Over the dynamic range of our sample, there is little variation in
stellar and nebular abundance with Lya equivalent width, W(Lya), and only a
marginal anti-correlation between age and W(Lya). The inferred range of
ionizing spectral shapes is insufficient to solely account for the variation in
W(Lya). Rather, the covering fraction of optically-thick HI appears to be the
principal factor modulating the escape of Lya, with most of the Lya photons in
down-the-barrel observations of galaxies escaping through low-column-density or
ionized channels in the ISM. Our analysis shows that a high star-formation-rate
surface density, Sigma_SFR, particularly when coupled with a low galaxy
potential (i.e., low stellar mass), can aid in reducing the covering fraction
and ease the escape of Lya photons. We conclude with a discussion of the
implications of our results for the escape of ionizing radiation at high
redshift.
