The evolution of the intergalactic medium transmission to redshift 6

We have measured the transmission of the Lyalpha forest produced by neutral hydrogen scattering in the intergalactic medium between redshifts 2 and 6.3 using high signal-to-noise, high-resolution ( R greater than or equal to 5000) observations of 50 quasars spread over the redshift range. We use a uniform set of 15 Angstrom intervals covering Lyalpha, Lybeta, and Lygamma 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 modeled with a smoothly decreasing ionization rate. Previous claims of an abrupt change at z similar to 6 appear in part to be a consequence of an incorrect conversion of Lybeta to Lyalpha 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.