Blue Wings and Bumps via Fermi-like Acceleration of Lyman-alpha photons across Shocks

TL;DR

This paper investigates how Fermi-like acceleration of Lyman-alpha photons across shocks influences spectral line shapes, explaining observed features like blue wings and bumps in galaxies through Monte Carlo simulations.

Contribution

It demonstrates that adding a static hydrogen shell to shell-models explains blue bumps and wings, advancing understanding of Lyα spectral features in galaxies.

Findings

Fermi-like acceleration produces extended blue wings in Lyα spectra.

Static hydrogen shells can generate blue bumps at outflow velocities.

Spectral features reflect evolutionary stages of galactic outflows.

Abstract

We explore the impact of Fermi-like acceleration of Lyman-alpha (Ly{\alpha}) photons across shock fronts on the observed Ly{\alpha} spectral line shape. We first confirm the result of Neufeld & McKee (1988) that this mechanism gives rise to extended blue wings which may have been observed in some radio galaxies. Our Monte-Carlo radiative transfer calculations further show that in a minor modification of the shell-model, in which we add an additional static shell of hydrogen, this process can naturally explain the small blue bumps observed in a subset of Ly{\alpha} emitting galaxies, which have been difficult to explain with conventional shell-models. Blue bumps can be produced with an additional column density of static hydrogen as small as $N_{HI}^{static} \ll N_{HI}^{shell}$, and typically occur at roughly the outflow velocity of the shell. In our model the spectra of so-called 'blue-bump objects' might reflect an evolutionary stage in which the outflows regulating the escape of Ly{\alpha} photons are still engulfed within a static interstellar medium.