The Evolution of Early-Type Field Galaxies Selected from a NICMOS Map of the Hubble Deep Field North

The redshift distribution of well-defined samples of distant early-typegalaxies offers a means to test the predictions of monolithic and hierarchicalgalaxy formation scenarios. NICMOS maps of the entire Hubble Deep Field Northin the F110W and F160W filters, when combined with the available WFPC2 data,allow us to calculate photometric redshifts and determine the morphologicalappearance of galaxies at rest-frame optical wavelengths out to z ~ 2.5. Herewe report results for two subsamples of early-type galaxies, defined primarilyby their morphologies in the F160W band, which were selected from the NICMOSdata down to H160_{AB} < 24.0. The observed redshift distributions of our twoearly-type samples do not match that predicted by a monolithic collapse model,which shows an overabundance at z > 1.5. A hierarchical formation model bettermatches the redshift distribution of the HDF-N early-types at z > 1.5, butstill does not adequately describe the observed early-types. The hierarchicalmodel predicts significantly bluer colors on average than the observedearly-type colors, and underpredicts the observed number of early-types at z <1. [abridged]Comment: Accepted for publication in the Astronomical Journal; 54 pages, 21 figures. Figures 10 and 11 are included separately in JPEG forma