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


Extended Lyman-alpha emission around galaxies and quasars at high redshift – Lyman-alpha halos (LAHs) – are 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 LAHs as well as their relationships to galaxies, quasars, circumgalactic gas, and other environmental factors remains unclear. Here we present results from our new Lyman-alpha full radiative transfer code voroILTIS which runs directly on the global, unstructured Voronoi tessellation of cosmological hydrodynamical simulations, self-consistently treating attenuation from the intergalactic medium. We use TNG50, the highest resolution run of the IllustrisTNG suite, and simulate Lyman-alpha halos from redshift $z=2$ to $z=5$, focusing on star-forming galaxies with $7.0 < \log {10}{(M {\star}/\rm{M {\odot}})} < 10.5$. We present the stacked, median predictions for the radial surface brightness profiles of Ly$\alpha$ from TNG as a function of galaxy mass and redshift. Comparison of stacked and individual Ly$\alpha$ profiles with data from the MUSE UDF at $z>3$ reveals a promising level of agreement. We measure the correlations of Ly$\alpha$ halo 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 increase by a factor of $\sim$2 from $z=2$ to $z=5$. Finally, we decompose profiles into contributions from different physical sources, namely diffuse emission from both 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 ($ > $ 10s of pkpc) becomes dominated by rescattered photons originating from other nearby, more massive halos, rather than diffuse emission itself.

arXiv e-prints