First measurement of the Mg ii forest correlation function in the Epoch of Reionization.

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. [ABSTRACT FROM AUTHOR]