A direct precision measurement of the intergalactic Lyman-α opacity at 2 ≤ z ≤ 4.2

We directly measure the evolution of the intergalactic Lyman-α effective optical depth, τ_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 Lya forest obtained by spline fitting are systematically biased low, but that this bias can be accounted for using mock spectra. The mean fractional error 〈ΔC/Ct _rue〉 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 Lya 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.