2024MNRAS.535..223T

Strong low-ionization transitions like the Mg II$\lambda 2796,2804$ Å doublet are believed to produce a detectable 'metal-line forest', if metals pollute the neutral intergalatic medium (IGM). We measure the autocorrelation of the Mg II forest transmission using 10 ground-based $z \ge 6.80$ quasar spectra probing the redshift range $5.96 \lt z_{\rm Mg\, {\small II}} \lt 7.42$ ($z_{\rm Mg\, {\small II}, median} = 6.47$). The correlation function exhibits strong small-scale clustering and a pronounced peak at the doublet velocity ($\Delta v = 768~{\rm km\, s^{-1}}$) arising from discrete absorbers in the circumgalactic medium of galaxies. After these strong absorbers are identified and masked the signal is consistent with noise. Our measurements are compared to a suite of models generated by combining a large hydrodynamical simulation with a seminumerical reionization topology, assuming a simple uniform enrichment model. We obtain a 95 per cent credibility upper limit of $[{\rm Mg/H}] \lt -3.73$ at $z_{\rm Mg\, {\small II},median} = 6.47$, assuming uninformative priors on [Mg/H] and the IGM neutral fraction $x_{\rm {H\, {\small I}}}$. Splitting the data into low-z ($5.96 \lt z_{\rm Mg\, {\small II}} \lt 6.47$; $z_{\rm Mg\, {\small II},median} = 6.235$) and high-z ($6.47 \lt z_{\rm Mg\, {\small II}} \lt 7.42$; $z_{\rm Mg\, {\small II},median} = 6.72$) subsamples again yields null detections and 95 per cent upper limits of $[{\rm Mg/H}] \lt -3.75$ and $\lt -3.45$, respectively. These first measurements set the stage for making the Mg II forest an emerging tool to precisely constrain the Universe reionization and enrichment history.