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, τeff , over the redshift range 2 ≤ z ≤ 4.2 from a sample of 84 high-resolution, high-signal-to-noise quasar spectra obtained with the ESI and HIRES spectrographs on Keck, and with the MIKE spectrograph on Magellan. This represents an improvement over previous analyses of the Lyα forest from highresolution spectra in this redshift interval of a factor of two in the size of the data set alone. We pay particular attention to robust error estimation and extensively test for systematic effects. We find that our estimates of the quasar continuum levels in the Lyα forest obtained by spline fitting are systematically biased low, with the magnitude of the bias increasing with redshift, but that this bias can be accounted for using mock spectra. The mean fractional error 〈∆C/Ctrue〉 is < 1% at z = 2, 4% at z = 3, and 12% at z = 4. Previous measurements of τeff at z & 3 based on directly fitting the quasar continua in the Lyα forest, which have generally neglected this effect, are therefore likely biased low. We provide estimates of the level of absorption arising from metals in the Lyα 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, attaining 3% in redshift bins of width ∆z =0.2 around z = 3, indicates significant departures from the best-fit power-law redshift evolution (τeff = 0.0018(1 + z) , when metals are left in), particularly near z = 3.2. The observed downward departure is statistically consistent with a similar feature detected in a precision statistical measurement using Sloan Digital Sky Survey spectra by Bernardi and coworkers using an independent approach. Subject headings: Cosmology: observations, theory — methods: data analysis, statistical, numerical — quasars: absorption lines