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We present new measurements of the mean transmitted flux in the Ly-alphaforest over 2 < z < 5 made using 6065 quasar spectra from the Sloan Digital SkySurvey DR7. We exploit the general lack of evolution in the mean quasarcontinuum to avoid the bias introduced by continuum fitting over the Ly-alphaforest at high redshifts, which has been the primary systematic uncertainty inprevious measurements of the mean Ly-alpha transmission. The individual spectraare first combined into twenty-six composites with mean redshifts spanning 2.25< z_comp < 5.08. The flux ratios of separate composites at the same restwavelength are then used, without continuum fitting, to infer the meantransmitted flux, F(z), as a fraction of its value at z~2. Absolute values forF(z) are found by scaling our relative values to measurements made fromhigh-resolution data by Faucher-Giguere et al. (2008) at z < 2.5, wherecontinuum uncertainties are minimal. We find that F(z) evolves smoothly withredshift, with no evidence of a previously reported feature at z~3.2. Thistrend is consistent with a gradual evolution of the ionization and thermalstate of the intergalactic medium over 2 < z < 5. Our results generally agreewith the most careful measurements to date made from high-resolution data, butoffer much greater precision and extend to higher redshifts. This work alsoimproves upon previous efforts using SDSS spectra by significantly reducing thelevel of systematic error.

A Refined Measurement of the Mean Transmitted Flux in the Ly-alpha Forest over 2 < z < 5 Using Composite Quasar Spectra