Massive Ellipticals at High Redshift: Nicmos Imaging of Z ∼ 1 Radio Galaxies

We present deep, ≈ 1.6μm, continuum images of eleven high-redshift (0.811 < z < 1.875) 3CR radio galaxies observed with NICMOS on-board the Hubble Space Telescope. Our NICMOS images probe the rest-frame optical light where stars are expected to dominate the galaxy luminosity. The rest-frame ultraviolet light of eight of these galaxies demonstrates the well-known “alignment effect”, with extended and often complex morphologies elongated along an axis close to that of the FRII radio source. As has been previously noted from ground-based near-infrared imaging, most of the radio galaxies have rounder, more symmetric morphologies at rest-frame optical wavelengths. Here we show the most direct evidence that in most cases the stellar hosts are normal elliptical galaxies with r law light profiles. For a few galaxies very faint traces (less than 4% of the total H-band light) of the UV-bright aligned component are also visible in the infrared images. We derive both the effective radius and surface brightness for nine of eleven sample galaxies by fitting oneand two-dimensional surface-brightness models to them. We compare the high-redshift radio galaxies to lower redshift counterparts. We find their sizes are similar to those of local FRII radio source hosts and are in general larger than other local galaxies. The derived host galaxy luminosities are very high and lie at the bright end of luminosity functions constructed at similar redshifts. This indicates that the high-redshift radio galaxies are likely rare, massive sources. The galaxies in our sample are also brighter than the rest-frame size–surface-brightness locus defined by the low-redshift sources. Passive evolution roughly aligns the z ≈ 1 galaxies with the low-redshift samples with a slope equal to 4.7. This value falls intermediate between the canonical Kormendy relation (≈ 3.5) and a constant luminosity line (= 5). The optical host is sometimes centered on a local minimum in the rest-frame UV emission, suggesting the presence of substantial dust obscuration. We also see good evidence of nuclear point sources (no brighter than 5% of the total H-band light) in three galaxies. Overall, our results are consistent with the hypothesis that these galaxies have already formed the bulk of their stars at redshifts greater than z ∼ > 2, and that the AGN phenomenon takes place within otherwise normal, perhaps passively evolving, galaxies. Subject headings: galaxies: formation — galaxies: active — infrared: galaxies