Detection of large Faraday rotation in the inner 2 kiloparsecs of M87

VLA polarization maps of M87 at four frequencies between 4.635 and 4.935 GHz reveal large Faraday rotation measures in the direction of the 2 kpc radio lobes. Over most of the source the typical rotation measure is 1000 rad m^-2^ but values as high as 8000 rad m^-2^ are found in small regions. Most of the source has fractional polarization in excess of 10%. This fact coupled with the high rotation measures suggests that the rotation is taking place in a Faraday screen in front of the radio-emitting plasma rather than mixed with it. The observed scale sizes coupled with the X-ray density profile suggest that most of the rotation is taking place close to the scale size of the lobes. Over most of the source the rotation measures are smooth on scales of 500 pc to 1 kpc and the values are mostly positive, requiring a large ordered field component. Based on the radio structure of the lobes we estimate a magnitude of ~ 40 μG for this component of the B field. The pressure corresponding to this field strength is interestingly close to the thermal pressure estimated from X-ray observations. Emission-line limits for the region of highest rotation measure show that this particular structure is either magnetically supported or confined, or must be overpressured and transient. The smallest rotation measures we see are in the direction of the jet. This suggests either that the jet probably lies mostly in front of the Faraday screen. We suggest that the overall pattern is most consistent with the Faraday screen lying in a moderately thick boundary layer enveloping the 2 kpc radio lobes.