The Sunburst Arc: Direct Lyman {\alpha} escape observed in the brightest known lensed galaxy

TL;DR

This study reports the first clear observation of a triple-peaked Lyman alpha profile in a highly magnified galaxy at z=2.4, revealing direct escape of Lyman alpha photons and validating theoretical models.

Contribution

It provides the first unambiguous detection of a triple-peaked Lyman alpha profile in a lensed galaxy, confirming predictions from radiative transfer models.

Findings

Triple-peaked Lyman alpha profile observed

Profile fits a combination of radiative transfer and direct escape components

Central peak unaffected by radiative transfer effects

Abstract

We present rest-frame ultraviolet and optical spectroscopy of the brightest lensed galaxy yet discovered, at redshift z = 2.4. This source reveals a characteristic, triple-peaked Lyman {\alpha} profile which has been predicted by various theoretical works but to our knowledge has not been unambiguously observed previously. The feature is well fit by a superposition of two components: a double-peak profile emerging from substantial radiative transfer, and a narrow, central component resulting from directly escaping Lyman {\alpha} photons; but is poorly fit by either component alone. We demonstrate that the feature is unlikely to contain contamination from nearby sources, and that the central peak is unaffected by radiative transfer effects apart from very slight absorption. The feature is detected at signal-to-noise ratios exceeding 80 per pixel at line center, and bears strong resemblance to synthetic profiles predicted by numerical models.