Luminosity Evolution of Early-type Galaxies to Z = 0.83: Constraints on Formation Epoch and Ω

We present deep spectroscopy with the Keck telescope of eight galaxies in the luminous 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 data to measure their structural parameters. Six galaxies have early-type spectra, and two have “E+A” spectra. The galaxies with early-type spectra define a tight Fundamental Plane (FP) relation. The evolution of the mass-to-light ratio is derived from the FP. The M⁄ L ratio evolves as ∆ logM⁄ LB ∝ −0.40z (Ωm = 0.3, ΩΛ = 0). The observed evolution of the M⁄ L ratio provides a combined constraint on the formation redshift of the stars, the IMF, and cosmological parameters. For a Salpeter IMF (x = 2.35) we find that zform > 2.8 and Ωm < 0.86 with 95 % confidence. The constraint on the formation redshift is weaker if ΩΛ > 0: zform > 1.7 if Ωm = 0.3 and ΩΛ = 0.7. At present the limiting factor in constraining zform and Ω from the observed luminosity evolution of early-type galaxies is the poor understanding of the IMF. We find that if Ωm = 1 the IMF must be significantly steeper than the Salpeter IMF (x > 2.6). Subject headings: galaxies: evolution, galaxies: elliptical and lenticular, cD, galaxies: structure of, galaxies: clusters: individual (MS 1054−03)