The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas (MEGaSaURA) II: Stacked Spectra

TL;DR

This paper presents a high signal-to-noise composite ultraviolet spectrum of 14 gravitationally lensed galaxies at redshifts 1.6 to 3.6, revealing new spectral diagnostics and comparing outflow features across cosmic time.

Contribution

It provides the highest quality composite spectrum for $z extasciitilde2$--3 galaxies and analyzes outflow velocities, stellar wind features, and photospheric lines, with implications for JWST observations.

Findings

Remarkable similarity in outflow velocity profiles at $z extasciitilde0$ and $z extasciitilde2$

High-ionization absorption traces low-ionization gas and stellar winds

Starburst99 models partially reproduce high-velocity tails but poorly match photospheric features

Abstract

We stack the rest-frame ultraviolet spectra of N=14 highly magnified gravitationally lensed galaxies at redshifts 1.6<z<3.6. The resulting new composite spans $900< \lambda_{rest} < 3000$ \AA, with a peak signal-to-noise ratio of 103 per spectral resolution element ($\sim$100 km/s). It is the highest signal-to-noise ratio, highest spectral resolution composite spectrum of $z\sim2$--3 galaxies yet published. The composite reveals numerous weak nebular emission lines and stellar photospheric absorption lines that can serve as new physical diagnostics, particularly at high redshift with the James Webb Space Telescope (JWST). We report equivalent widths to aid in proposing for and interpreting JWST spectra. We examine the velocity profiles of strong absorption features in the composite, and in a matched composite of $z\sim0$ COS/HST galaxy spectra. We find remarkable similarity in the velocity profiles at $z\sim 0$ and $z\sim2$, suggesting that similar physical processes control the outflows across cosmic time. While the maximum outflow velocity depends strongly on ionization potential, the absorption-weighted mean velocity does not. As such, the bulk of the high-ionization absorption traces the low-ionization gas, with an additional blueshifted absorption tail extending to at least $-2000$ km/s . We interpret this tail as arising from the stellar wind and photospheres of massive stars. Starburst99 models are able to replicate this high-velocity absorption tail. However, these theoretical models poorly reproduce several of the photospheric absorption features, indicating that improvements are needed to match observational constraints on the massive stellar content of star-forming galaxies at $z \sim 2$. We publicly release our composite spectra.