A systematic study of Lyman-Alpha transfer through outflowing shells: Model parameter estimation

TL;DR

This study investigates the ability of the shell model to accurately recover physical parameters of Ly$ ext{α}$ outflows from simulated spectra, revealing significant degeneracies but overall reliable parameter estimation.

Contribution

The paper systematically assesses parameter degeneracies in the shell model using automated fitting on simulated spectra, advancing understanding of model-physical property relationships.

Findings

Most parameters, especially $N_{HI}$ and $v_{exp}$, are accurately recovered.

Significant degeneracies exist between some shell model parameters.

Simulated spectra and an interactive tool are made publicly available.

Abstract

Outflows promote the escape of Lyman-$\alpha$ (Ly$\alpha$) photons from dusty interstellar media. The process of radiative transfer through interstellar outflows is often modelled by a spherically symmetric, geometrically thin shell of gas that scatters photons emitted by a central Ly$\alpha$ source. Despite its simplified geometry, this `shell model' has been surprisingly successful at reproducing observed Ly$\alpha$ line shapes. In this paper we perform automated line fitting on a set of noisy simulated shell model spectra, in order to determine whether degeneracies exist between the different shell model parameters. While there are some significant degeneracies, we find that most parameters are accurately recovered, especially the HI column density ($N_{\rm HI}$) and outflow velocity ($v_{\rm exp}$). This work represents an important first step in determining how the shell model parameters relate to the actual physical properties of Ly$\alpha$ sources. To aid further exploration of the parameter space, we have made our simulated model spectra available through an interactive online tool.