Letter to the Editor

Lyman-α radiation pressure regulates star formation efficiency

Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy

^★ Corresponding author: daniele.manzoni@sns.it

Received: 8 September 2025
Accepted: 27 October 2025

Abstract

Order-unity star formation efficiencies (SFE) in early galaxies may explain the overabundance of bright galaxies observed by JWST at high redshift. Here we show that Lyman-α (Lyα) radiation pressure limits the gas mass converted into stars, particularly in primordial environments. We have developed a shell model including Lyα feedback, and we validate it with one-dimensional hydrodynamical simulations. To account for Lyα resonant scattering, we adopted the most recent force multiplier fits, including the effect of Lyα photon destruction by dust grains. We find that independent of their gas surface density, Σ_g, clouds are disrupted on a timescale shorter than the free-fall time and even before supernova explosions if Σ_g ≳ 10³ M_⊙ pc⁻². At log(Z/Z_⊙) =  − 2, which is relevant for high-redshift galaxies, the SFE is 0.01 ≲ ϵ̂_∗ ≲ 0.66 for 10³ ≲ Σ_g[M_⊙ pc⁻²] ≲ 10⁵. The SFE is even lower for decreasing metallicity. Bursts of star formation with near-unity SFEs are possible only for extreme surface densities, Σ_g ≳ 10⁵ M_⊙ pc⁻², and near-solar metallicities. We conclude that Lyα radiation pressure severely limits a possible extremely efficient, feedback-free phase of star formation in dense metal-poor clouds.