Morphological Evolution in High-Redshift Radio Galaxies and the Formation of Giant Elliptical Galaxies
- Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, Livermore, CA 94550 (United States)
- Astronomy Department, University of California, Berkeley, CA 94720 (United States)
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 that are free of contamination from strong emission lines and at {lambda}{sub rest} {gt} 4000 {Angstrom}, where older stellar populations, if present, might dominate the observed flux. At {ital z} {gt} 3, the rest-frame optical morphologies generally have faint, large-scale ({approximately}50 kpc) emission surrounding multiple, {approximately}10 kpc components. The brightest of these components are often aligned with the radio structures. These morphologies change dramatically at 2 {lt} {ital z} {lt} 3, where the {ital K}-band images show single, compact structures without bright, radio-aligned features. The linear sizes ({approximately}10 kpc) and luminosities [{ital M}({ital B}{sub rest}) {approximately} {minus}20 to {minus}22] of the {ital individual} components in the {ital z} {gt} 3 HzRGs are similar to the {ital total} sizes and luminosities of normal radio-quiet star forming galaxies at {ital z} = 3{endash}4. For objects where such data are available, our observations show that the line-free, near-IR colors of the {ital z} {gt} 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 {ital z} {gt} 3 HzRGs exists (4C 41.17). Our results suggest that the {ital z} {gt} 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 subcomponents along the radio axes of the {ital z} {gt} 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 {ital K}-{ital 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 ({ital z} = 2.474), the highest redshift galaxy in the 3C sample and among the most powerful radio sources known. {copyright} {ital {copyright} 1998.} {ital The American Astronomical Society}
- OSTI ID:
- 666547
- Journal Information:
- Astrophysical Journal, Vol. 502, Issue 2; Other Information: PBD: Aug 1998
- Country of Publication:
- United States
- Language:
- English
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