On the Diffuse Lyman-alpha Halo Around Lyman-alpha Emitting Galaxies

TL;DR

This study uses radiative transfer modeling in cosmological simulations to explain the origin of extended Ly$ ext{α}$ halos around high-redshift galaxies, highlighting the significant role of outer halo emission and cooling radiation.

Contribution

The paper introduces a parameter-free radiative transfer model that accurately reproduces observed Ly$ ext{α}$ surface brightness profiles and identifies outer halo emission as the main source of Ly$ ext{α}$ halos.

Findings

Outer halo emission dominates Ly$ ext{α}$ halos.

Cooling radiation significantly contributes to extended Ly$ ext{α}$ emission.

Model matches observed surface brightness profiles without parameter tuning.

Abstract

Ly$\alpha$ photons scattered by neutral hydrogen atoms in the circumgalactic media or produced in the halos of star-forming galaxies are expected to lead to extended Ly$\alpha$ emission around galaxies. Such low surface brightness Ly$\alpha$ halos (LAHs) have been detected by stacking Ly$\alpha$ images of high-redshift star-forming galaxies. We study the origin of LAHs by performing radiative transfer modeling of nine $z=3.1$ Lyman-Alpha Emitters (LAEs) in a high resolution hydrodynamic cosmological galaxy formation simulation. We develop a method of computing the mean Ly$\alpha$ surface brightness profile of each LAE by effectively integrating over many different observing directions. Without adjusting any parameters, our model yields an average Ly$\alpha$ surface brightness profile in remarkable agreement with observations. We find that observed LAHs cannot be accounted for solely by photons originating from the central LAE and scattered to large radii by hydrogen atoms in the circumgalactic gas. Instead, Ly$\alpha$ emission from regions in the outer halo is primarily responsible for producing the extended LAHs seen in observations, which potentially includes both star-forming and cooling radiation. With the limit on the star formation contribution set by the ultra-violet (UV) halo measurement, we find that cooling radiation can play an important role in forming the extended LAHs. We discuss the implications and caveats of such a picture.