Introducing SPHINX-MHD: The Impact of Primordial Magnetic Fields on the
  First Galaxies, Reionization, and the Global 21cm Signal
  We present the first results from SPHINX-MHD, a suite of cosmological
radiation-magnetohydrodynamics simulations designed to study the impact of
primordial magnetic fields (PMFs) on galaxy formation and the evolution of the
intergalactic medium during the epoch of reionization. The simulations are
among the first to employ multi-frequency, on-the-fly radiation transfer and
constrained transport ideal MHD in a cosmological context to simultaneously
model the inhomogeneous process of reionization as well as the growth of PMFs.
We run a series of $(5\,\text{cMpc})^3$ cosmological volumes, varying both the
strength of the seed magnetic field ($B_0$) and its spectral index ($n_B$). We
find that PMFs that have $n_B > -0.562\log_{10}\left(\frac{B_0}{1{\rm
n}G}\right) - 3.35$ produce electron optical depths ($\tau_e$) that are
inconsistent with CMB constraints due to the unrealistically early collapse of
low-mass dwarf galaxies. For $n_B\geq-2.9$, our constraints are considerably
tighter than the $\sim{\rm n}G$ constraints from Planck. PMFs that do not
satisfy our constraints have little impact on the reionization history or the
shape of the UV luminosity function. Likewise, detecting changes in the Lya
forest due to PMFs will be challenging because photoionisation and photoheating
efficiently smooth the density field. However, we find that the first
absorption feature in the global 21cm signal is a sensitive indicator of the
properties of the PMFs, even for those that satisfy our $\tau_e$ constraint.
Furthermore, strong PMFs can marginally increase the escape of LyC photons by
up to 25\% and shrink the effective radii of galaxies by $\sim44\%$ which could
increase the completeness fraction of galaxy surveys. Finally, our simulations
show that surveys with a magnitude limit of ${\rm M_{UV,1500}=-13}$ can probe
the sources that provide the majority of photons for reionization out to
$z=12$.
