Optical and radio polarimetry of the M87 jet at 0 ''.2 resolution

We discuss optical (HST/WFPC2 F555W) and radio (15 GHz VLA) polarimetry obs ervations of the M87 jet taken during 1994-1995. The angular resolution of both observations is similar to 0." 2, which at the distance of M87 corresp onds to 15 pc. Many knot regions are very highly polarized (similar to 40%- 50%, approaching the theoretical maximum for optically thin synchrotron rad iation), suggesting highly ordered magnetic fields. High degrees of polariz ation are also observed in interknot regions. The optical and radio polariz ation maps share many similarities, and in both, the magnetic field is larg ely parallel to the jet, except in the "shocklike" knot regions (parts of H ST-1, A, and C), where it becomes perpendicular to the jet. We do observe significant differences between the radio and optical polariz ed structures, particularly for bright knots in the inner jet, giving us im portant insight into the radial structure of the jet. Unlike in the radio, the optical magnetic held position angle becomes perpendicular to the jet a t the upstream ends of knots HST-1, D, E, and F. Moreover, the optical pola rization appears to decrease markedly at the position of the flux maxima in these knots. In contrast, the magnetic held position angle observed in the radio remains parallel to the jet in most of these regions, and the decrea ses in radio polarization are smaller. More minor differences are seen in o ther jet regions. Many of the differences between optical and radio polarim etry results can be explained in terms of a model whereby shocks occur in t he jet interior, where higher energy electrons are concentrated and dominat e both polarized and unpolarized emissions in the optical, while the radio maps show strong contributions from lower energy electrons in regions with B parallel, near the jet surface.