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W. H. de Vries, C. P. O'Dea, S. A. Baum (STScI), P. D. Barthel (Groningen U., The Netherlands)
The triggering and subsequent evolution of the most powerful radio emitters in the universe is poorly understood. The radio source size for these objects varies from sub-kpc for the Gigahertz Peaked Spectrum (GPS) sources, to 1 - 15 kpc for the sub-galactic sized Compact Steep Spectrum (CSS) sources, and greater than 15 kpc for the large scale FR-II sources. If these sources are different stages of radio galaxy evolution, their host galaxy properties should be similar since they should not change on the < 108 yr timescale of the radio source lifetime. In order to test whether the host galaxy properties were similar and thus consistent with the evolutionary scenario, we constructed a sample of 60 sources, 20 in each size class, which has been observed with HST and various KPNO facilities. Our HST broad- and narrowband WFPC2 imaging project revealed direct evidence for ongoing interaction between radio plasma and ambient ISM in virtually all CSS galaxies. This suggests that the expanding radio source in its early stages will shock ionize large quantities of ambient gas into (line) emission, which will be aligned with the radio structure as a consequence. Using KPNO near-IR (JHK band) imaging and spectroscopy data needed for accurate redshifts, a detailed comparison of the broadband spectral energy distributions and stellar synthesis modeling yielded no differences in host galaxy properties among the size classes. All hosts were found to be ellipticals with well evolved stellar populations, consistent with the evolutionary scenario. Ongoing analysis of HST NICMOS observations will significantly improve on this.