Magnetogenesis around the first galaxies: the impact of different field
  seeding processes on galaxy formation
  We study the evolution of magnetic fields generated by charge segregation
ahead of ionization fronts during the Epoch of Reionization, and their effects
on galaxy formation. We compare this magnetic seeding process with the Biermann
battery, injection from supernovae, and an imposed seed field at redshift
$z\gtrsim127$. Using a suite of self-consistent cosmological and zoom-in
simulations based on the Auriga galaxy-formation model, we determine that all
mechanisms produce galactic magnetic fields that equally affect galaxy
formation, and are nearly indistinguishable at $z\lesssim1.5$. The former is
compatible with observed values, while the latter is correlated with the gas
metallicity below a seed-dependent redshift. Low-density gas and haloes below a
seed-dependent mass threshold retain memory of the initial magnetic field. We
produce synthetic Faraday rotation measure maps, showing that they have the
potential to constrain the seeding process, although current observations are
not yet sensitive enough. Our results imply that the ad-hoc assumption of a
primordial seed field - widely used in galaxy formation simulations but of
uncertain physical origin - can be replaced by physically-motivated mechanisms
for magnetogenesis with negligible impact on galactic properties. Additionally,
magnetic fields generated ahead of ionization fronts appear very similar but
weaker than those produced by the Biermann battery. Hence, in a realistic
scenario where both mechanisms are active, the former will be negligible
compared to the latter. Finally, our results highlight that the high-redshift
Universe is a fruitful testing ground for our understanding of magnetic fields
generation.
