  We present deep Keck spectroscopy for a sample of I<22.5 field early-type
galaxies selected morphologically in the redshift range 0.56<z<1.02 in the
HDF-N. Using velocity dispersions determined from the Keck spectra in
conjunction with structural parameters measured from the deep WFPC2 images we
study the evolution of the M/L_B ratio and the fundamental plane (FP) with
redshift. For the majority of galaxies the trends observed are very similar to
those determined earlier for rich clusters. The systematic offset between HDF-N
galaxies and cluster galaxies is d(ln M/L_B) = -0.14 +- 0.13, corresponding to
an age difference of only 16 +- 15 % at <z>=0.88. However, we find enhanced H
delta absorption of equivalent width 4.0 Angstrom in the mean spectrum of the
ten galaxies, indicating the presence of young stars. We infer that the
galaxies have composite stellar populations, consisting of a low mass young
component in addition to a dominating old component. As the bulk of the stellar
mass must have formed at z >~ 2 our results argue against formation scenarios
involving major mergers of gas-rich disk systems at 1<z<1.5, and we conclude
that z~1 early-type galaxies were either assembled at higher redshift or in
mergers involving little gas. The ubiquitous enhanced Balmer lines and the
presence of tidal features in two of the galaxies lend some support to the
latter hypothesis. The main uncertainty in the analysis is the small sample;
larger samples are needed to study the interplay between the evolution of
stellar populations and morphology in detail.
