The Redshift Evolution of Rest-UV Spectroscopic Properties to z~5

TL;DR

This study investigates how the rest-UV spectral features of star-forming galaxies evolve up to redshift 5, revealing increased intrinsic Lyα photon production and changes in interstellar medium properties at high redshift.

Contribution

It provides new measurements of UV spectral features at z~5 and demonstrates the evolution of Lyα emission and interstellar absorption, linking these to galaxy properties and ionized photon production.

Findings

Decreasing low-ionization absorption correlates with increasing Lyα emission from z~2 to 5.

At z~5, galaxies show higher Lyα emission strength at fixed absorption, indicating higher intrinsic Lyα production.

Elevated ionized photon production efficiency ($\xi_{ion}$) at z~5 suggests more efficient ionizing photon output in early galaxies.

Abstract

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 ($\xi_{\rm ion}$).