A KILOPARSEC-SCALE POLARIZED PLUME ASSOCIATED WITH THE GALACTIC-CENTER

Polarimetric data from the Parkes radio telescope at frequencies of 1. 4, 2.4 and 4.8 GHz reveal the presence of a large 'plume' of polarized emission to the north of the Galactic Centre (GC). This plume extends at least 15 degrees in Galactic longitude, centred near l = -3 degree s, and probably covers 15 degrees or more in latitude (a linear extent of approximate to 2 kpc, assuming a distance to the GC of 8.5 kpc). L arge rotation measures (RMs) and depolarizations are observed over the plume. Higher resolution observations at a frequency of 1.4 GHz with the Very Large Array (VLA) show this patchy polarized emission to exte nd to angular scales of several arcmin. The lack of correlation betwee n polarized-intensity and total-intensity images (for both VLA and Par kes data) indicates that the polarization detected is the result of Fa raday rotation, on small angular scales, of a uniform, polarized backg round. Recent gamma-ray results show the presence of a similar plume o f electrons and positrons lying to the north of the GC. Because of sim ilarities in the location and size, an association between the radio a nd gamma-ray plumes seems highly probable. The detection of large RMs in the presence of both electrons and positrons is rather unexpected. It is possible that the plume carries an excess of one charge carrier over the other, or that the distribution of electrons and positrons wi thin the plume is 'clumpy'. The VLA data also reveal large RMs in the fields observed, up to approximately +/-300 rad m(-2), and clearly sho w the presence of Faraday depolarization. The patches of large RM seen in the VLA images are approximately 35 pc in diameter. Assuming that electrons (positrons) dominate in the observed regions, which Lie near b = +4 degrees, we estimate a thermal electron (positron) density of approximate to 3 cm(-3) and a magnetic field strength of several mu G The Parkes data also show some evidence for a radio plume to the south of the GC.