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$α$ 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$α$ equivalent width, performing corrections to Ly$α$ 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$α$ 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$α$ equivalent widths, we observe evolution at $z\sim5$ towards greater Ly$α$ emission strength at fixed low-ionization absorption strength. If we interpret the non-evolving relationship of Ly$α$ emission strength and low-ionization line strength at z~2-4 as primarily reflecting the radiative transfer of Ly$α$ photons, this evolution at z~5 suggests a higher intrinsic production rate of Ly$α$ photons than at lower redshift. Our conclusion is supported by the joint evolution of the relationships among Ly$α$ 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$α$ emission evolve towards higher redshift. We conclude that increasing intrinsic Ly$α$ photon production and strong detection of nebular CIV emission (signaling lower metallicity) at z~5 indicate an elevated ionized photon production efficiency ($ξ_{\rm ion}$).

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