HST POLARIZATION MAP OF THE ULTRAVIOLET EMISSION FROM THE OUTER JET IN M87 AND A COMPARISON WITH THE 2-CM RADIO-EMISSION

We present the first high-resolution polarization map of the ultraviol et emission from the outer jet in M87. The data were obtained by the F aint Object Camera (FOG) on the Hobble Space Telescope. The polarizati on map has a resolution of 0.2 arcsec and was derived using data from three linearly polarized images combined with the best available calib ration data. The ultraviolet emission is highly polarized (similar to 40 per cent) with the magnetic vector aligned roughly with the jet axi s, except in the region of the brightest knot (Knot A) where the posit ion angle changes abruptly and the magnetic vector becomes perpendicul ar to the jet axis. A similar behaviour is seen in the 2-cm VLA radio polarization map. By aligning the FOC and VLA data, we present ultravi olet-2 cm spectral index, depolarization and rotation measure maps. We identify a region of high depolarization adjacent to Knot A. This is the first direct observational evidence that indicates the presence of a cloud or filament of dense thermal material which is mixed with the synchrotron-emitting plasma of the jet. The interaction of the jet wi th this cloud is likely to be responsible for the sudden increase in t he brightness of the jet at Knot A due to an induced shock. We suggest that the dark line seen in the 2-cm radio data between Knot A and Kno t C could be the shadow or magnetotail of the depolarizing cloud in th e jet.