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We present the first results from SPHINX-MHD, a suite of cosmologicalradiation-magnetohydrodynamics simulations designed to study the impact ofprimordial magnetic fields (PMFs) on galaxy formation and the evolution of theintergalactic medium during the epoch of reionization. The simulations areamong the first to employ multi-frequency, on-the-fly radiation transfer andconstrained transport ideal MHD in a cosmological context to simultaneouslymodel 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 thestrength of the seed magnetic field ($B_0$) and its spectral index ($n_B$). Wefind that PMFs that have $n_B > -0.562\log_{10}\left(\frac{B_0}{1{\rmn}G}\right) - 3.35$ produce electron optical depths ($\tau_e$) that areinconsistent with CMB constraints due to the unrealistically early collapse oflow-mass dwarf galaxies. For $n_B\geq-2.9$, our constraints are considerablytighter than the $\sim{\rm n}G$ constraints from Planck. PMFs that do notsatisfy our constraints have little impact on the reionization history or theshape of the UV luminosity function. Likewise, detecting changes in the Lyaforest due to PMFs will be challenging because photoionisation and photoheatingefficiently smooth the density field. However, we find that the firstabsorption feature in the global 21cm signal is a sensitive indicator of theproperties of the PMFs, even for those that satisfy our $\tau_e$ constraint.Furthermore, strong PMFs can marginally increase the escape of LyC photons byup to 25\% and shrink the effective radii of galaxies by $\sim44\%$ which couldincrease the completeness fraction of galaxy surveys. Finally, our simulationsshow that surveys with a magnitude limit of ${\rm M_{UV,1500}=-13}$ can probethe sources that provide the majority of photons for reionization out to$z=12$.

Introducing SPHINX-MHD: The Impact of Primordial Magnetic Fields on the First Galaxies, Reionization, and the Global 21cm Signal