Morphological Evolution in High Redshift Radio Galaxies and the Formation of Giant Elliptical Galaxies

We present deep near–infrared images of high redshift radio galaxies (HzRGs) obtained with the Near Infrared Camera (NIRC) on the Keck I telescope. In most cases, the near–IR data sample rest wavelengths free of contamination from strong emission lines and at λrest > 4000Å, where older stellar populations, if present, might dominate the observed flux. At z > 3, the rest–frame optical morphologies generally have faint, large–scale (∼50 kpc) emission surrounding multiple, ∼ 10 kpc size components. The brightest of these components are often aligned with the radio structures. These morphologies change dramatically at 2 < z < 3, where the K images show single, compact structures without bright, radio–aligned features. The linear sizes (∼ 10 kpc) and luminosities (M(Brest) ∼ −20 to −22) of the individual components in the z > 3 HzRGs are similar to the total sizes and luminosities of normal, radio–quiet, star forming galaxies at z = 3 − 4 (Steidel et al. 1996; Lowenthal et al. 1997). For objects where such data are available, our observations show that the line–free, near–IR colors of the z > 3 galaxies are very blue, consistent with models in which recent star formation dominates the observed light. Direct, spectroscopic evidence for massive star formation in one of the z > 3 HzRGs exists (4C41.17, Dey et al. 1997a). Our results suggest that the z > 3 HzRGs evolve into much more massive systems than the radio–quiet galaxies and that they are qualitatively consistent with models in which massive galaxies form in hierarchical fashion through the merging of smaller star–forming systems. The presence of relatively luminous sub–components along the radio axes of the z > 3 galaxies suggests a causal connection with the AGN. We compare the radio and near–IR sizes as a function of redshift and suggest that this parameter may be a measure of the degree to which the radio sources have induced star formation in the parent objects. We also discuss the Hubble diagram of radio galaxies, the possibility of a radio power dependence in the K − z relation, and its implications for radio galaxy formation. Finally, we present for the first time in published format basic radio and optical information on 3C 257 (z = 2.474), the highest redshift galaxy in the 3C sample and among the most powerful radio sources known. Subject headings: galaxies: active — galaxies: galaxies — elliptical: high redshift — radio continuum: galaxies