Luminosity evolution of early-type galaxies to z=0.83: constraints on formation epoch and Omega

We present deep Keck telescope spectroscopy of eight galaxies in the lumino us X-ray cluster MS 1054-03 at z = 0.83, The data are combined with imaging observations from the Hubble Space Telescope (HST). The spectroscopic data are used to measure the internal kinematics of the galaxies, and the HST d ata are used to measure their structural parameters. Six galaxies have earl y-type spectra, and two have "E + A" spectra. The galaxies with early-type spectra define a tight fundamental plane (FP) relation. The evolution of th e mass-to-light ratio is derived from the FP. The M/L ratio evolves as Delt a log M/L-B proportional to -0.40z (Omega(m) = 0.3, Omega(Lambda) = 0). The observed evolution of the MIL ratio provides a combined constraint on the formation redshift of the stars, the initial mass function (IMF), and cosmo logical parameters. For a Salpeter IMF (x = 2.35), we find that z(form) > 2 .8 and Omega(m) < 0.86 with 95% confidence. The constraint on the formation redshift is weaker if Omega(Lambda) > 0: z(form) > 1.7 if Omega(m) = 0.3 a nd Omega(Lambda) = 0.7. At present, the limiting factor in constraining z(f orm) and Omega from the observed luminosity evolution of early-type galaxie s is the poor understanding of the IMF We find that if Omega(m) = 1, the IM F must be significantly steerer than the Salpeter IMF (x > 2.6).