The cosmic web in Lyman-alpha emission

Abstract

We develop a comprehensive theoretical model for Lyman-alpha (Lyα) emission, from the scale of individual Lyα emitters to Lyα haloes, blobs, filaments, and the diffuse cosmic web itself. We post-process the high-resolution TNG50 cosmological magnetohydrodynamical simulation with Monte Carlo radiative transfer to capture the resonant scattering of Lyα photons. Our emission model incorporates recombinations and collisions in diffuse gas, radiative effects from nearby AGN, emission sourced by stellar populations, and a physically motivated dust model calibrated to the observed Lyα-emitter luminosity function. We focus on the observability and physical origin of the z = 2 Lyα cosmic web. We find that diffuse Lyα filaments are illuminated by photons originating not only from the intergalactic medium itself but also from within galaxies and their gaseous haloes. This emission is primarily sourced by intermediate-mass haloes (10^10-10^11 M☉), principally through collisional excitations in their circumgalactic media and central young stellar populations. At a surface-brightness threshold of 10^-20 erg s^-1 cm^-2 arcsec^-2, we predict a volume abundance of Lyα filaments of approximately 10^-3 cMpc^-3 for lengths above 400 pkpc. Given sufficiently large survey footprints, detection of the Lyα cosmic web is within reach of modern integral-field spectrographs, including MUSE, VIRUS, and KCWI.