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

We measure the evolution of the intergalactic Ly alpha effective optical depth, [image], over the redshift range [image] 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 [image] is