The structure of the jet in 3C 15 from multiband polarimetry

Abstract

We investigate the structure of the kpc-scale jet in the nearby (z= 0.073) radio galaxy 3C 15, using new optical Hubble Space Telescope (HST) ACS/F606W polarimetry together with archival multiband HST imaging, Chandra X-ray data and 8.4-GHz VLA radio polarimetry. The new data confirm that synchrotron radiation dominates in the optical. With matched beams, the jet is generally narrower in the optical than in the radio, suggesting a stratified flow. We examine a simple two-component model comprising a highly relativistic spine and lower-velocity sheath. This configuration is broadly consistent with polarization angle differences seen in the optical and radio data. The base of the jet is relatively brighter in the ultraviolet and X-ray than at lower energies, and the radio and optical polarization angles vary significantly as the jet brightens downstream. Further out, the X-ray intensity rises again and the apparent magnetic field becomes simpler, indicating a strong shock. Modelling the synchrotron spectrum of this brightest X-ray knot provides an estimate of its minimum internal pressure, and a comparison with the thermal pressure from X-ray emitting gas shows that the knot is overpressured and likely to be a temporary, expanding feature.