Optical polarization of the M 87 jet.

We present optical polarization maps of the M 87 jet at 1.0" and 1.4" resolution (FWHM) from high signal-to-noise ratio electronographic data obtained in the U band. They reveal for the first time the detailed behaviour of the polarization rate and of the magnetic field structure transverse to the jet, free of Faraday effects. This structure is discussed in relation with the radio and optical emission from the jet. The magnetic field appears to & essentially parallel to the jet, except in two knots (A and C), where it is nearly perpendicular, and in knot G where it is oblique. In knot A, the magnetic field is parallel to the oblique sharp linear structure shown by very high resolution radio maps. Beyond knot A, the magnetic field presents a complex structure: it is nearly parallel to the main direction of the jet on the edges while it is oblique with respect to it on the ridge line where a very low polarization rate is observed. This suggests that the magnetic field is wrapped at the surface of a hollow cylinder, whose axis is parallel to the main direction of the jet, right beyond knot A. The mean polarization rates of the knots are high (between 12 and 37%) and vary as FWHM^-0.8^. Comparison with VLA radio data shows that Faraday effect is negligible at 2 cm and that the rotation measure at 6 cm is very constant throughout the brightest part of the jet (~ 190 rad m^-2^), being of opposite sign at knot C. We favour the interpretation that Faraday effect can be attributed to the thermal plasma of the jet.