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

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$α$ equivalent width (EW), and various galaxy properties, we study the evolutionary trends among Ly$α$, low- and high-ionization interstellar (LIS and HIS) absorption features, and integrated galaxy properties. We also examine the redshift evolution of Ly$α$ and LIS absorption kinematics, and fine-structure emission EWs. The connections among the strengths of Ly$α$, LIS lines, and dust extinction are redshift-independent, as is the decoupling of Ly$α$ and HIS line strengths, and the bulk outflow kinematics as traced by LIS lines. Stronger Ly$α$ emission is observed at higher redshift at fixed UV luminosity, stellar mass, SFR, and age. Much of this variation in average Ly$α$ strength with redshift, and the variation in Ly$α$ 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$α$ and CIII] emission strengths, we additionally find evidence for variations in the intrinsic production rate of Ly$α$ photons at the highest Ly$α$ 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.