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

We present deep near-IR images of high redshift radio galaxies obtained withNIRC on the Keck I telescope. In most cases, the near-IR data sample restwavelengths at ~4000 Angstroms, free of strong emission lines. At z > 3, therest frame optical morphologies generally have faint, large-scale emissionsurrounding multiple components of ~10 kpc size. The brightest of the smallknots are often aligned with the radio structures. At z < 3, the morphologieschange dramatically, showing single, compact structures without radio-alignedfeatures. The sizes and luminosities of the individual components in the z > 3radio galaxies are similar to those of the radio-quiet star-forming galaxiesdiscovered at z ~ 3 by the Lyman dropout technique. The rest frame opticalcolors of the z > 3 radio galaxies are consistent with models in which recentstar formation dominates the observed IR light, and in one case (4C 41.17) wehave direct spectroscopic evidence for massive star formation (Dey et al.1997a). Our results suggest that the z > 3 radio galaxies evolve into verymassive elliptical galaxies at 2 < z < 3, in qualitative agreement with thehierarchical model of galaxy formation. We also discuss the Hubble diagram ofradio galaxies, the possibility of a radio power dependence in the K-zrelation, and the implications for radio galaxy formation.Comment: Accepted for publication in The Astrophysical Journal; 34 pages including 8 postscript figures; plates 1-4 may be obtained from ftp://bigz.berkeley.edu/pub/nirc