The Redshift Evolution of Rest-UV Spectroscopic Properties to z~5
  We perform a comprehensive analysis of the redshift evolution of the rest-UV
spectra of star-forming galaxies out to z~5. We combine new z~5 measurements of
HI Ly$\alpha$ and low- and high-ionization interstellar metal absorption
features with comparable measurements at z~2-4. We measure the equivalent
widths of interstellar absorption features using stacked spectra in bins of
Ly$\alpha$ equivalent width, performing corrections to Ly$\alpha$ strengths
based on a model for the transmission of the intergalactic medium. We find a
strong correlation between decreasing low-ionization absorption strength and
increasing Ly$\alpha$ emission strength over the redshift range z~2-5,
suggesting that both of these quantities are fundamentally linked to neutral
gas covering fraction. At the highest Ly$\alpha$ equivalent widths, we observe
evolution at $z\sim5$ towards greater Ly$\alpha$ emission strength at fixed
low-ionization absorption strength. If we interpret the non-evolving
relationship of Ly$\alpha$ emission strength and low-ionization line strength
at z~2-4 as primarily reflecting the radiative transfer of Ly$\alpha$ photons,
this evolution at z~5 suggests a higher intrinsic production rate of Ly$\alpha$
photons than at lower redshift. Our conclusion is supported by the joint
evolution of the relationships among Ly$\alpha$ emission strength, interstellar
absorption strength, and dust reddening. We perform additional analysis in bins
of stellar mass, star-formation rate, UV luminosity, and age, examining how the
relationships between galaxy properties and Ly$\alpha$ emission evolve towards
higher redshift. We conclude that increasing intrinsic Ly$\alpha$ photon
production and strong detection of nebular CIV emission (signaling lower
metallicity) at z~5 indicate an elevated ionized photon production efficiency
($\xi_{\rm ion}$).
