A kiloparsec-scale polarized plume associated with the Galactic Centre

Abstract

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° in Galactic longitude, centred near l = -3°, and probably covers 15° or more in latitude (a linear extent of ≈2 kpc, assuming a distance to the GC of 8.5 kpc). Large 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 extend to angular scales of several arcmin. The lack of correlation between polarized-intensity and total-intensity images (for both VLA and Parkes data) indicates that the polarization detected is the result of Faraday rotation, on small angular scales, of a uniform, polarized background. Recent γ-ray results show the presence of a similar plume of electrons and positrons lying to the north of the GC. Because of similarities in the location and size, an association between the radio and γ-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 within the plume is 'clumpy'. The VLA data also reveal large RMs in the fields observed, up to approximately ±300 rad m-2, and clearly show 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°, we estimate a thermal electron (positron) density of ≈3 cm -3 and a magnetic field strength of several μG. The Parkes data also show some evidence for a radio plume to the south of the GC.