Stacked Rest-Frame UV Spectra of Ly$\alpha$-Emitting and Continuum-Selected Galaxies at 2<z<3.5

TL;DR

This study analyzes the rest-UV spectra of star-forming galaxies at redshifts 2 to 3.5, revealing correlations between Lyα emission, UV colors, and interstellar absorption, and providing insights into galactic outflows and gas properties.

Contribution

It presents a detailed comparison of UV spectral features in Lyα-emitting and continuum-selected galaxies at high redshift, highlighting the relationship between Lyα strength, UV colors, and interstellar medium characteristics.

Findings

Galaxies with stronger Lyα emission have bluer UV continua.

Interstellar absorption lines are weaker in galaxies with stronger Lyα emission.

A typical velocity offset of ~600 km/s exists between Lyα emission and low-ionization absorption.

Abstract

We present properties of individual and composite rest-UV spectra of continuum- and narrowband-selected star-forming galaxies (SFGs) at a redshift of 2<z<3.5 discovered by the MUSYC collaboration in the ECDF-S. Among our sample of 81 UV-bright SFGs, 59 have R<25.5, of which 32 have rest-frame equivalent widths W_{Ly{\alpha}}>20 {\AA}, the canonical limit to be classified as a LAE. We divide our dataset into subsamples based on properties we are able to measure for each individual galaxy: Ly{\alpha} equivalent width, rest-frame UV colors, and redshift. Among our subsample of galaxies with R<25.5, those with rest-frame W_{Ly{\alpha}}>20 {\AA} have bluer UV continua, weaker low-ionization interstellar absorption lines, weaker C IV absorption, and stronger Si II* nebular emission than those with W_{Ly{\alpha}}<20 {\AA}. We measure a typical velocity offset of {\Delta}v~600 km s$^{-1}$ between Ly{\alpha} emission and low-ionization absorption among our subsamples. We find that the interstellar component, as opposed to the stellar component, dominates the high-ionization absorption line profiles. We find the low- and high-ionization Si ionization states have similar kinematic properties, yet the low-ionization absorption is correlated with Ly$\alpha$ emission and the high-ionization absorption is not. These trends are consistent with outflowing neutral gas being in the form of neutral clouds embedded in ionized gas as previously suggested by \cite{Steidel2010}. Moreover, our galaxies with bluer UV colors have stronger Ly{\alpha} emission, weaker low-ionization absorption and more prominent nebular emission line profiles. Among our dataset, UV-bright galaxies with W_{Ly{\alpha}}>20 {\AA} exhibit weaker Ly{\alpha} emission at lower redshifts, although we caution that this could be caused by spectroscopic confirmation of low Ly{\alpha} equivalent width galaxies being harder at z~3 than z~2.