Origin and powering mechanism of outer Lyα halo emission

Determine how the Lyα emission from diffuse gas in the outer regions of Lyα halos, at radii beyond approximately one virial radius, is formed and identify its principal powering sources.

Background

The paper finds that Lyα emission inside roughly one virial radius correlates with the host galaxy’s recent star-formation rate, whereas emission at larger radii correlates with environmental overdensity and is independent of the central star-formation rate. The authors assess several possible explanations for the outer emission, including individually undetected neighboring LAEs, fluorescence driven by the metagalactic ultraviolet background, and locally enhanced radiation from clustered galaxies or active galactic nuclei. None of these mechanisms is found to provide a sufficient general explanation for the observed environmental correlation.

The authors therefore leave unresolved whether the outer emission is produced primarily by diffuse circumgalactic or intergalactic gas through mechanisms such as collisional excitation and cooling radiation from cold accretion flows, or through another process. They note that spatially resolved Lyα spectral profiles could help distinguish the relevant gas kinematics and emission mechanisms in future work.