A Direct Precision Measurement of the Intergalactic Lyα Opacity at 2 <= z <= 4.2

We measure the evolution of the intergalactic Lyα effective optical depth, τ_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 Lyα 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 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 langle Δ C/C_truerangle is <1% at z = 2, 4% at z = 3, and 12% at z = 4. Previous measurements of τ_eff at zgtrsim 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 Δ z = 0.2 around z = 3, indicates significant departures from the best-fit power-law redshift evolution [τ_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.