A Century of Cosmology: A Direct Precision Measurement of the Intergalactic Lyman-alpha Opacity at 2<z<4.2

TL;DR

This study provides a precise measurement of the evolution of intergalactic Lyman-alpha opacity from redshift 2 to 4.2, revealing deviations from previous models and accounting for metal absorption contributions.

Contribution

It offers the first high-precision, direct measurement of tau_eff over this redshift range, correcting for continuum bias and metal contamination, improving understanding of intergalactic medium evolution.

Findings

Detected significant deviations from power-law evolution near z=3.2

Quantified metal absorption contribution as 6-9% at z=3

Achieved <1% continuum bias at z=2, increasing to 12% at z=4

Abstract

We directly measure the evolution of the intergalactic Lyman-alpha effective optical depth, tau_eff, over the redshift range 2<z<4.2 from a sample of 86 high-resolution, high-signal-to-noise quasar spectra obtained with Keck/ESI, Keck/HIRES, and Magellan/MIKE. We find that our estimates of the quasar continuum levels in the Ly-alpha forest obtained by spline fitting are systematically biased low, but that this bias can be accounted for using mock spectra. The mean fractional error <Delta C/C_true> is <1% at z=2, 4% at z=3, and 12% at z=4. We provide estimates of the level of absorption arising from metals in the Ly-alpha forest based on both direct and statistical metal removal results in the literature, finding that this contribution is ~6-9% at z=3 and decreases monotonically with redshift. The high precision of our measurement indicates significant departures from the best-fit power-law redshift evolution, particularly near z=3.2.