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/sq m, but values as high as 8000 rad/sq m are found in small regions. Most of the source has fractional polarization in excess of 10 percent. 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. 14 refs.