Characterizing Lyman alpha emission from high-redshift galaxies

TL;DR

This paper develops an empirical model for the emergent Lyman alpha spectra from high-redshift galaxies, improving understanding of Ly$ extalpha$ transmission during the Epoch of Reionization and aiding interpretation of observational data.

Contribution

It introduces a flexible, empirical model for Ly$ extalpha$ emission properties conditioned on galaxy UV magnitude, constrained by recent high-redshift galaxy observations.

Findings

Better match to Subaru Ly$ extalpha$ equivalent width data

Extended distributions of Ly$ extalpha$ properties facilitate EoR studies

Model can identify outliers like GN-z11 potentially from mergers

Abstract

The Lyman $\alpha$ (Ly$\alpha$) line from high-redshift galaxies is a powerful probe of the Epoch of Reionization (EoR). Neutral hydrogen in the intergalactic medium (IGM) can significantly attenuate the emergent Ly$\alpha$ line, even in the damping wing of the cross-section. However, interpreting this damping wing imprint relies on our prior knowledge of the spectrum that escapes from the galaxy and its environs into the IGM. This emergent spectrum is highly sensitive to the composition and geometry of the interstellar and circumgalactic media, and so exhibits a large galaxy to galaxy scatter. Characterizing this scatter is further complicated by non-trivial selection effects introduced by observational surveys. Here we build a flexible, empirical model for the emergent Ly$\alpha$ spectra. Our model characterizes the emergent Ly$\alpha$ luminosity, the velocity offset of the Ly$\alpha$ line with respect to the systemic redshift, and the H$\alpha$ luminosity, with multivariate probability distributions conditioned on the UV magnitude. We constrain these distributions using $z\sim5-6$ galaxy observations with VLT MUSE and JWST NIRCam, forward-modeling observational selection functions together with galaxy parameters. Our model results in Ly$\alpha$ equivalent width distributions that are a better match to (independent) Subaru observations than previous empirical models. The extended distributions of Ly$\alpha$ equivalent widths and velocity offsets we obtain could facilitate Ly$\alpha$ transmission during the early stages of the EoR. We also illustrate how our model can be used to identify GN-z11-like outliers, potentially originating from merging systems. We publish fitting functions and make our model publicly available.