Probing Reionization and Early Cosmic Enrichment with the MgII Forest
  Because the same massive stars that reionized the intergalactic medium (IGM)
inevitably exploded as supernovae that polluted the Universe with metals, the
history of cosmic reionization and enrichment are intimately intertwined. While
the overly sensitive Ly-alpha transition completely saturates in a neutral IGM,
strong low-ionization metal lines like the MgII 2796,2804 doublet will give
rise to a detectable `metal-line forest' if the metals produced during
reionization (Z ~ 10^{-3}Z_sol) permeate the neutral IGM. We simulate the MgII
forest for the first time by combining a large hydrodynamical simulation with a
semi-numerical reionization topology, assuming a simple enrichment model where
the IGM is uniformly suffused with metals. In contrast to the traditional
approach of identifying discrete absorbers, we treat the absorption as a
continuous random field and measure its two-point correlation function,
leveraging techniques from precision cosmology. We show that a realistic mock
dataset of 10 JWST spectra can simultaneously determine the Mg abundance,
[Mg/H], with a 1sigma precision of 0.02 dex and measure the global neutral
fraction <x_HI> to 5% for a Universe with <x_HI> = 0.74 and [Mg/H] = -3.7.
Alternatively, if the IGM is pristine, a null-detection of the MgII forest
would set a stringent upper limit on the IGM metallicity of [Mg/H] < -4.4 at
95% credibility, assuming <x_HI> > 0.5 from another probe. Concentrations of
metals in the circumgalactic environs of galaxies can significantly contaminate
the IGM signal, but we demonstrate how these discrete absorbers can be easily
identified and masked such that their impact on the correlation function is
negligible. The MgII forest thus has tremendous potential to precisely
constrain the reionization and enrichment history of the Universe.
