On the Assembly History of Early-Type Galaxies in the Hubble Deep Field North

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.