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 \Delta \log M / L _ { B } \propto - 0.40 z ( \Omega _ { m } = 0.3 , \Omega _ { \Lambda } = 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 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 and \Omega _ { \Lambda } = 0.7 .
At present the limiting factor in constraining z _ { form } and \Omega from the observed luminosity evolution of early-type galaxies is the poor understanding of the IMF .
We find that if \Omega _ { m } = 1 the IMF must be significantly steeper than the Salpeter IMF ( x > 2.6 ) .