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

Strong low-ionization transitions like the Mg iiλ 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 ≥ 6.80 quasar spectra probing the redshift range 5.96 < z_ Mg\, II < 7.42 (z_ Mg\, II, median = 6.47). The correlation function exhibits strong small-scale clustering and a pronounced peak at the doublet velocity (Δ v = 768~ 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 [ Mg/H] < -3.73 at z_ Mg\, II,median = 6.47, assuming uninformative priors on [Mg/H] and the IGM neutral fraction x_ H\, I. Splitting the data into low-z (5.96 < z_ Mg\, II < 6.47; z_ Mg\, II,median = 6.235) and high-z (6.47 < z_ Mg\, II < 7.42; z_ Mg\, II,median = 6.72) subsamples again yields null detections and 95 per cent upper limits of [ Mg/H] < -3.75 and < -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.