The redshift evolution of rest-UV spectroscopic properties in Lyman Break Galaxies at z ~ 2-4

TL;DR

This study analyzes how rest-UV spectroscopic features of star-forming galaxies evolve from redshift 2 to 4, revealing consistent relationships among spectral properties and increased Ly$ ext{alpha}$ emission at higher redshifts.

Contribution

It provides the first comprehensive analysis of UV spectral evolution in galaxies at z~2-4, linking spectral features to galaxy properties and gas/dust content across cosmic time.

Findings

Stronger Ly$ ext{alpha}$ emission at higher redshift for fixed galaxy properties.

Relationships among Ly$ ext{alpha}$, LIS lines, and dust are redshift-independent.

Variation in Ly$ ext{alpha}$ strength linked to neutral gas and dust content.

Abstract

We present the first comprehensive evolutionary analysis of the rest-frame UV spectroscopic properties of star-forming galaxies at z ~ 2-4. We match samples at different redshifts in UV luminosity and stellar mass, and perform systematic measurements of spectral features and stellar population modeling. By creating composite spectra grouped according to Ly$\alpha$ equivalent width (EW), and various galaxy properties, we study the evolutionary trends among Ly$\alpha$, low- and high-ionization interstellar (LIS and HIS) absorption features, and integrated galaxy properties. We also examine the redshift evolution of Ly$\alpha$ and LIS absorption kinematics, and fine-structure emission EWs. The connections among the strengths of Ly$\alpha$, LIS lines, and dust extinction are redshift-independent, as is the decoupling of Ly$\alpha$ and HIS line strengths, and the bulk outflow kinematics as traced by LIS lines. Stronger Ly$\alpha$ emission is observed at higher redshift at fixed UV luminosity, stellar mass, SFR, and age. Much of this variation in average Ly$\alpha$ strength with redshift, and the variation in Ly$\alpha$ strength at fixed redshift, can be explained in terms of variations in neutral gas covering fraction and/or dust content in the ISM and CGM. However, based on the connection between Ly$\alpha$ and CIII] emission strengths, we additionally find evidence for variations in the intrinsic production rate of Ly$\alpha$ photons at the highest Ly$\alpha$ EWs. The challenge now is to understand the observed evolution in neutral gas covering fraction and dust extinction within a coherent model for galaxy formation, and make robust predictions for the escape of ionizing radiation at z > 6.