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

Abstract

Strong low-ionization transitions like the Mg IIλ2796, 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 < zMg II < 7.42 (zMg II,median = 6.47). The correlation function exhibits strong small-scale clustering and a pronounced peak at the doublet velocity (Δv = 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] < zMg II < 6.47; zMg II,median = 6.235) and high-z (6.47 < zMg II < 7.42; zMg 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.