The morphologies of distant radio galaxies

Abstract

Multicolor line and continuum images of a complete sample of 13 3C radio galaxies at 0.8 < z < 1.3, spanning the range 2500 Å to 1 µm in the rest frame, are presented and analyzed. Quantitative analysis of these images shows that the infrared images are less elongated than those at optical wavelengths and show only a much weaker "alignment effect" with the radio source axis. The quadrupole moments show a progressive reduction from short to long wavelengths, as expected if a symmetric component dominates at infrared wavelengths. A spectral decomposition based on these moments suggests that the aligned component probably has a roughly flat spectral energy distribution (f v≈0 to -1) while the symmetric red component that dominates in the infrared probably has a spectral energy distribution similar to that of a gE galaxy. In two cases where the aligned component is detached from the main galaxy, the spectral energy distribution is directly measured within small apertures and found to have f v ≈ -1. In typical 3C galaxies at z=1, the active aligned component contributes 10% of the infrared light. While more active objects have a larger contamination, these generally modest components are insufficient to perturb significantly either the scatter in, or the continuity of, the observed K-z relation. The conventional interpretation of the K -z diagram in terms of a uniform population of mature host galaxies is thus still likely to be correct, at least at z ≈ 1. As far as can be determined from the data, the scale sizes of the radio galaxy images at infrared wavelengths are consistent with the sizes of the giant ellipticals associated with powerful radio galaxies at low redshift. A model surface fitting analysis of the reddest member of the sample, 3C 65, and of another red galaxy, 3C 437 (z = 1.480), shows that the infrared surface brightness profiles are well fit by a deVaucouleurs r^1/4 law characteristic of giant ellipticals, with characteristic photometric parameters comparable to those of brightest cluster members and low-redshift radio galaxies for standard cosmologies. The small displacement of these galaxies from low-redshift brightest cluster members and radio galaxies on the µe-logre plane suggests that little or no stellar evolution is required in a cosmology with qo = 0.5 (Λ = 0), while a modest degree of stellar evolution is implied in a model with qo = 0 (Λ = 0), or models with Λ > 0. A non-expanding cosmology would require the high-red shift systems to lie at the extreme end of the distribution of properties of local gE galaxies, and the effects of stellar and/or dynamical evolution would be to make the objects more extreme. Several of these radio galaxies are accompanied by small red companion galaxies that are prominent on our infrared images at random position angles relative to the radio axis. These are interpreted as representing a conventional trigger for the radio sources. There is a preference for these companions to be associated with the bluest, most aligned, and generally most active objects.