Lyman-alpha radiation pressure: an analytical exploration

TL;DR

This paper derives analytical expressions for Lyman alpha radiation pressure, demonstrating its dominance over other forces in certain astrophysical environments and emphasizing its importance in galaxy evolution models.

Contribution

It provides new analytical formulas for Lyman alpha radiation pressure as a function of gas properties, including effects of dust and bulk motion, highlighting its dominant role in gas dynamics.

Findings

Lyα pressure dominates over photoionization and dust UV pressure in many conditions.

In static media, Lyα pressure can be 10 to 300 times stronger than other forces.

Lyα pressure significantly influences gas acceleration around luminous sources.

Abstract

We study radiation pressure due to Lyman alpha line photons, obtaining and exploring analytical expressions for the force-multiplier, $M_F(N_H, Z) = F_\alpha/(L_\alpha/c)$, as a function of gas column density, $N_H$, and metallicity, $Z$, for both dust-free and dusty media, employing a WKB approach for the latter case. Solutions for frequency offset emission to emulate non-static media moving with a bulk velocity $v$, have also been obtained. We find that, in static media, Ly$\alpha$ pressure dominates over both photoionization and dust-mediated UV radiation pressure in a very wide parameter range ($16 < \log N_H < 23$; $-4 < \log[Z/Z_\odot] < 0$). For example, it overwhelms the other two forces by 10 (300) times in standard (low-$Z$) star-forming clouds. Thus, in agreement with previous studies, we conclude that Ly$\alpha$ pressure plays a dominant role in the initial acceleration of the gas around luminous sources, and must be implemented in galaxy formation, evolution and outflow models and simulations.