The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas (MegaSaura). II. Stacked Spectra

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 < λ rest < 3000Å, with a peak signal-to-noise ratio of 103 per spectral resolution element (∼100 km s −1 ). It is the highest signalto-noise ratio, highest spectral resolution composite spectrum of z ∼ 2-3 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 ∼ 0 COS/HST galaxy spectra. We find remarkable similarity in the velocity profiles at z ∼ 0 and z ∼ 2, suggesting that similar physical processes control the outflows across cosmic time. While the maximum outflow velocity depends strongly on ionization potential, the absorptionweighted 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 −1 . 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 ∼ 2. We publicly release our composite spectra.

Simulating MOS science on the ELT: Lyα forest tomography

“…The blue arrow indicates the spectral range that is simulated in the simulation when m FUV,rest is estimated. For comparison, we also show the stacked spectrum of intermediate resolution by Rigby et al (2018).…”

“…The average spectrum of the Horizon-AGN galaxies is compared to the stacked spectra of Shapley et al (2003) and Rigby et al (2018) in Fig. A.1.…”

“…First, the spectra were smoothed to the desired resolution in the observer frame. We considered three different resolutions: R = 5000 (corresponding to the MOSAIC resolution), 3300 (the average resolution in the Rigby et al (2018) sample), and 560 (the average resolution of the Shapley et al (2003) sample). The stacking was made following the procedure of Rigby et al (2018).…”

“…We considered three different resolutions: R = 5000 (corresponding to the MOSAIC resolution), 3300 (the average resolution in the Rigby et al (2018) sample), and 560 (the average resolution of the Shapley et al (2003) sample). The stacking was made following the procedure of Rigby et al (2018). We shifted the spectra to their rest-frame wavelengths, re-sampled them to a common wavelength grid, normalized them at λ « 1270 Å, and stacked them.…”

Fluorescent C ii* 1335Å emission spectroscopically resolved in a galaxy at z = 5.754

“…redshifted resonant emission and fluorescent emission, which are not observationally separated even when excess emission is seen (e.g. Rivera-Thorsen et al 2015;Rigby et al 2018). These alternative decay channels are however extremely interesting for the study of high-redshift galaxies, as the decays from the excited to the non-excited ground states give rise to far-infrared [O I] 63µm / [O I] 145µm and [C II] 158 µm emission lines, which are crucial to the study of early galaxies.…”