On the assembly history of early-type galaxies in the Hubble Deep Field-North

We present deep Keck spectroscopy for a sample of I-814 < 22.5 field early-type galaxies selected morphologically in the redshift range 0.56 ≤ z ≤ 1.02 in the Hubble Deep Field - North (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 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 Δ ln M/L-B = -0.14 +/- 0.13, corresponding to an age difference of only 16% +/- 15% at <(z)over bar> = 0.88. However, we find enhanced Hdelta absorption with an equivalent width of 4.0(-0.5)(+0.9) Angstrom in the mean spectrum of the 10 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. Since the bulk of the stellar mass must have formed at z greater than or similar to 2, our results argue against formation scenarios involving major mergers of gas-rich disk systems at 1 less than or similar to z less than or similar to 1.5, and we conclude that z similar or equal to 1 early-type galaxies were assembled either 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 of early-type galaxies are needed to study in detail the interplay between the evolution of their stellar populations and morphology.