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The Evolution of the Intergalactic Medium Transmission to Redshift Six

We have measured the transmission of the Lyman alpha forest produced by neutral hydrogen scattering in the intergalactic medium between redshifts 2 and 6.3 using high signal to noise, high resolution (R > 5000) observations of 50 quasars spread over the redshift range. We use a uniform set of 15 Angstrom intervals covering Lyman alpha, Lyman beta, and Lyman gamma absorption regions to tabulate the forest transmission as a function of redshift. The transmitted fractions show a relatively smooth evolution over the entire range of redshifts, which can be modelled with a smoothly decreasing ionization rate. Previous claims of an abrupt change at a redshift of approximately 6 appear in part to be a consequence of an incorrect conversion of Lyman beta to Lyman alpha optical depths. The tabulated transmissions can be used to calculate the colors of objects with a specified input spectrum as a function of redshift. We calculate the colors of a flat f_nu galaxy with a large intrinsic continuum break, as an important example.