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 is now commonly detected around high redshift
galaxies through stacking and even on individual basis. Despite recent
observational advances, 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 voroILTIS which runs directly on the
unstructured Voronoi tessellation of cosmological hydrodynamical simulations.
We make use of the TNG50 simulation and simulate LAHs from redshift $z=2$ to
$z=5$, focusing on star-forming galaxies with $8.0 <
\log_{10}{(M_\star/\rm{M}_\odot)} < 10.5$. While TNG50 does not directly follow
ionizing radiation, it includes an on-the-fly treatment for active galactic
nuclei and ultraviolet background radiation with self-shielding, which are
important processes impacting the cooling and ionization of the gas. Based on
this model, we present the predictions for the stacked radial surface
brightness profiles of Ly$\alpha$ as a function of galaxy mass and redshift.
Comparison with data from the MUSE UDF at $z>3$ reveals a promising level of
agreement. We measure the correlations of LAH size and central brightness with
galaxy properties, finding that at the masses of $8.5 \leq \log_{10}
\left(M_\star/\rm{M}_\odot\right) \leq 9.5$, physical LAH sizes roughly double
from $z=2$ to $z=5$. Finally, we decompose the profiles into contributions from
diffuse emission 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 halos rather than diffuse emission itself.
