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The formation of the first galaxies is accompanied by large accretion flows and virialization shocks, during which the gas is shock heated to temperatures of  ∼10 4  K , leading to potentially strong fluxes in the Lyman α line. Indeed, a number of Lyman α blobs have been detected at high redshift. In this Letter, we explore the origin of such Lyman α emission using cosmological hydrodynamical simulations that include a detailed model of atomic hydrogen as a multi‐level atom and the effects of line trapping with the adaptive mesh refinement code  flash . We see that baryons fall into the centre of a halo through cold streams of gas, giving rise to a Lyman α luminosity of at least  10 44  erg s −1   at   z  = 4.7 , similar to the observed Lyman α blobs. We find that a Lyman α flux of  5.0 × 10 −17  erg cm ‐2  s −1   emerges from the envelope of the halo rather than its centre, where the photons are efficiently trapped. Such emission can be probed in detail with the upcoming James Webb Space Telescope (JWST) and will constitute an important probe of gas infall and accretion.

Lyman α emission from the first galaxies: signatures of accretion and infall in the presence of line trapping