Lyman-$\alpha$ radiation pressure regulates star formation efficiency

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-$\alpha$ (Ly$\alpha$) radiation pressure limits the gas mass converted into stars, particularly in primordial environments. We develop a shell model including Ly$\alpha$ feedback, and validate it with one-dimensional hydrodynamical simulations. To account for Ly$\alpha$ resonant scattering, we adopt the most recent force multiplier fits, including the effect of Ly$\alpha$ photon destruction by dust grains. We find that, independently of their gas surface density ${\Sigma }_g$, clouds are disrupted on a timescale shorter than a free-fall time, and even before supernova explosions if ${\Sigma }_g\gtrsim 10^3{M}_{\odot }\mathrm{p}{\mathrm{c}}^{-2}$. At $\log (Z/Z_{\odot })=-2$, relevant for high-redshift galaxies, the SFE is $0.01\lesssim {\hat{\epsilon }}_{\ast }\lesssim 0.66$ for $10^3\lesssim {\Sigma }_g[M_{\odot }\mathrm{p}{\mathrm{c}}^{-2}]\lesssim 10^5$. The SFE is even lower for decreasing metallicity. Near-unity SFEs are possible only for extreme surface densities, ${\Sigma }_g\gtrsim 10^5{M}_{\odot }\mathrm{p}{\mathrm{c}}^{-2}$, and near-solar metallicities. We conclude that Ly$\alpha$ radiation pressure severely limits a possible extremely efficient, feedback-free phase of star formation in dense, metal-poor clouds.

arXiv commentAccepted for publication on A&A Letters

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