The cosmic web in Lyman-alpha emission

Abstract

We develop a comprehensive theoretical model for Lyman-alpha emission, from the scale of individual Lyman-alpha emitters (LAEs) to Lyman-alpha halos (LAHs), Lyman-alpha blobs (LABs), and Lyman-alpha filaments (LAFs) of the diffuse cosmic web itself. To do so, we post-process the high-resolution TNG50 cosmological magnetohydrodynamical simulation with a Monte Carlo radiative transfer method to capture the resonant scattering process of Lyman-alpha photons. We build an emission model incorporating recombinations and collisions in diffuse gas, including radiative effects from nearby AGN, as well as emission sourced by stellar populations. Our treatment includes a physically motivated dust model, which we empirically calibrate to the observed LAE luminosity function. We then focus on the observability, and physical origin, of the $z=2$ Lyman-alpha cosmic web, studying the dominant emission mechanisms and spatial origins. We find that diffuse Lyman-alpha filaments are, in fact, illuminated by photons which originate, not from the intergalactic medium itself, but from within galaxies and their gaseous halos. In our model, this emission is primarily sourced by intermediate mass halos ($10^{10} - 10^{11}\,$M$_{\odot}$), principally due to collisional excitations in their circumgalactic media as well as central, young stellar populations. Observationally, we make predictions for the abundance, area, linear size, and embedded halo/emitter populations within filaments. Adopting an isophotal surface brightness threshold of $10^{-20}\,$erg$\,$s$^{-1}\,$cm$^{-2}\,$arcsec$^{-2}$, we predict a volume abundance of Lyman-alpha filaments of ${\sim}10^{-3}$ cMpc$^{-3}\,$ for lengths above $400\,$pkpc. Given sufficiently large survey footprints, detection of the Lyman-alpha cosmic web is within reach of modern integral field spectrographs, including MUSE, VIRUS, and KCWI.