A direct precision measurement of the intergalactic Lyα opacity at 2 ≤ z ≤ 4.21,2

We measure the evolution of the intergalactic Ly alpha effective optical depth, tau(eff), over the redshift range 2 <= z <= 4: 2 from a sample of 86 high-resolution, high-S/N 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 Ly alpha forest from high-resolution spectra in this redshift interval of a factor of 2 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 Ly alpha 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 <Delta C/C-true > is < 1% at z = 2, 4% at z = 3, and 12% at z = 4. Previous measurements of tau(eff) at z greater than or similar to 3 based on directly fitting the quasar continua in the Ly alpha 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 Ly alpha forest based on both direct and statistical metal removal results in the literature, finding that this contribution is approximate to 6%-9% at z = 3 and decreases monotonically with redshift. The high precision of our measurement, attaining 3% in redshift bins of width Delta z = 0.2 around z = 3, indicates significant departures from the best-fit power-law redshift evolution [tau(eff) = 0.0018(1 + z)(3.92), 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 SDSS spectra by Bernardi and coworkers using an independent approach.