The impact of gas bulk rotation on the lyman-alpha line

TL;DR

This study investigates how gas bulk rotation in distant galaxies affects the shape and properties of the Lyman-alpha emission line, revealing viewing-angle dependence and the absence of spatial anisotropy in integrated flux.

Contribution

It provides a detailed radiative transfer model showing the impact of solid-body rotation on Lyman-alpha line morphology and offers an analytic approximation for the viewing-angle dependence.

Findings

Rotation causes viewing-angle dependent line morphology.

Line width and intensity increase for lines of sight perpendicular to rotation axis.

No spatial anisotropy in total line flux or escape fraction due to rotation.

Abstract

We present results of radiative transfer calculations to measure the impact of gas bulk rotation on the morphology of the Lyman $\alpha$ emission line in distant galaxies. We model a galaxy as a sphere with an homogeneous mixture of dust and hydrogen at a constant temperature. These spheres undergo solid-body rotation with maximum velocities in the range 0-300 \kms and neutral hydrogen optical depths in the range $\tau_{\rm H}=10^{5}-10^{7}$. We consider two types of source distributions in the sphere: central and homogeneous. Our main result is that rotation introduces a dependence of the line morphology with viewing angle and rotational velocity. Observations with a line of sight parallel to the rotation axis yield line morphologies similar to the static case. For lines of sight perpendicular to the rotation axis both the intensity at the line center and the line width increase with rotational velocity. Along the same line of sight, the line becomes single peaked at rotational velocities close to half the line width in the static case. Notably, we find that rotation does not induce any spatial anisotropy in the integrated line flux, the escape fraction or the average number of scatterings. This is because Lyman {\alpha} scattering through a rotating solid-body proceeds identical as in the static case. The only difference is the doppler shift from the different regions in the sphere that move with respect to the observer. This allows us to derive an analytic approximation for the viewing-angle dependence of the emerging spectrum, as a function of rotational velocity.