Blue optical continuum associated with a radio knot in 3C346

Abstract

We report the discovery of extremely luminous near-UV continuum emission associated with a bright radio knot in the radio galaxy 3C346 (z = 0.162). Photometric measurements from U and r' band images and longslit spectra show a spectral energy distribution that steepens at higher frequencies, with radio and optical spectral indices α_I_ = -0.37+/-0.02 and α_0_ = -1.8+/-0.2, respectively. Based on a comparison of the optical properties of this knot with other known cases of optical emission associated with radio structures, we conclude that the continuum emission is optical synchrotron radiation. Our observations are consistent with the suggestion that 3C346 is a foreshortened FR-II radio galaxy with its radio axis oriented close to the line of sight. The optical and radio emission from the knot appear to be associated with a hotspot (at the end of a jet) on the near side. Finally, our U and r'images of 3C346 provide a striking illustration that the optical morphologies of nearby radio galaxies also depend upon wavelength and that studies of these objects are relevant to the interpretation of the alignment effect seen in high redshift radio galaxies.