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 \ion Mgii \lambda 2796 , \lambda 2804 doublet will give rise to a detectable ‘ metal-line forest ’ if the metals produced during reionization ( Z \sim 10 ^ { -3 } Z _ { \odot } ) permeate the neutral IGM .
We simulate the \ion 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 1 \sigma precision of 0.02 dex and measure the global neutral fraction \langle x _ { \ion { H } { i } } \rangle to 5 % for a Universe with \langle x _ { \ion { H } { i } } \rangle = 0.74 and [ { Mg } / { H } ] = -3.7 .
Alternatively , if the IGM is pristine , a null-detection of the \ion Mgii forest would set a stringent upper limit on the IGM metallicity of [ { Mg } / { H } ] < -4.4 at 95 % credibility , assuming \langle x _ { \ion { H } { i } } \rangle > 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 \ion Mgii forest thus has tremendous potential to precisely constrain the reionization and enrichment history of the Universe .