The physical origins and dominant emission mechanisms of Lyman-alpha halos: results from the TNG50 simulation in comparison to MUSE observations

Extended Lyman-alpha emission around galaxies and quasars at high redshift is now commonly detected through stacking as well as surrounding individual galaxies. Despite the increasing samples and the extensive characterization through surface brightness profiles, the physical origin of these Lyman-alpha halos (LAHs), as well as their relationships to galaxies, quasars, circumgalactic gas, and other environmental factors remains unclear. We present results from our new Lyman-alpha full radiative transfer code \textsc{voroILTIS} which runs directly on the unstructured Voronoi tessellation of cosmological hydrodynamical simulations. We use TNG50, the highest resolution run of the IllustrisTNG suite, and simulate LAHs from redshift $z=2$ to $z=5$, focusing on star-forming galaxies with $8.0 3$ reveals a promising level of agreement. We measure the correlations of LAH size and central brightness with galaxy properties, finding that at intermediate masses $8.5 \leq \log_{10} \left(M_\star/\rm{M}_\odot\right) \leq 9.5$, LAH sizes roughly double from $z=2$ to $z=5$. Finally, we decompose the profiles into contributions from different sources, namely diffuse emission from recombinations and de-excitations, and scattered photons from star-forming regions. In our simulations, we find rescattered photons from star-forming regions to be the major source in observed LAHs. Unexpectedly, we find that the flattening of LAH profiles at large radii becomes dominated by photons originating from other nearby, more massive halos, rather than diffuse emission itself.