Probing reionization and early cosmic enrichment with the Mg ii forest

Abstract

ABSTRACT 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 is intimately intertwined. While the overly sensitive Ly α transition completely saturates in a neutral IGM, strong low-ionization metal lines like the $\rm{Mg\, \small {II}}$ λ2796, λ2804 doublet will give rise to a detectable ‘metal-line forest’ if the metals produced during reionization ($Z\sim 10^{-3}\, \mathrm{Z}_{\odot }$) permeate the neutral IGM. We simulate the $\rm{Mg\,\small {II}}$ forest for the first time by combining a large hydrodynamical simulation with a seminumerical 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 data set of 10 James Webb Space Telescope spectra can simultaneously determine the Mg abundance, $[{\rm Mg}/{\rm H}]$, with a 1σ precision of 0.02 dex and measure the global neutral fraction $\langle x_{\rm{H\,\small {I}}}\rangle$ to 5 per cent for a Universe with $\langle x_{\rm{H\,\small {I}}}\rangle = 0.74$ and $[{\rm Mg}/ {\rm H}] = -3.7$. Alternatively, if the IGM is pristine, a null detection of the $\rm{Mg\,\small {II}}$ forest would set a stringent upper limit on the IGM metallicity of $[{\rm Mg}/ {\rm H}] \lt -4.4$ at 95 per cent credibility, assuming $\langle x_{\rm{H\,\small {I}}}\rangle \gt 0.5$ 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 $\rm{Mg\,\small {II}}$ forest thus has tremendous potential to precisely constrain the reionization and enrichment history of the Universe.