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From: p098wre@sun42 (Wolfgang Reich)
To: hfalcke@mpifr-bonn.mpg.de

% ftp://ftp.physics.uq.edu.au/pub/GCPlume



\documentstyle [referee] {laa}

\begin{document}
\title{A kiloparsec-scale polarized plume associated with the Galactic Centre}
\author{A.R.~Duncan\inst{1} \and R.F.~Haynes\inst{2} \and
W.~Reich\inst{3} \and P. Reich\inst{3} \and A.D.~Gray\inst{4}}

\institute{Physics Department, The University Of Queensland, 4072, Australia
\and Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping, NSW 
2121, Australia
\and Max-Planck-Institut f\"ur Radioastronomie, Auf dem H\"ugel 69, 53121 Bonn, 
Germany
\and National Research Council Canada, P.O. Box 248, Penticton, B.C., V2A 6K3, 
Canada}
\maketitle
\begin{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 $15^{\circ}$ in Galactic longitude, centred near $l 
= -3^{\circ}$,
and probably covers $15^{\circ}$ or more in latitude (a linear extent of 
$\approx 2$~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) 
indicate
that the polarization detected is the result of Faraday rotation, on 
small angular scales, of a uniform, polarized background.

Recent $\gamma$-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 $\gamma$-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 within the plume is ``clumpy''. 
The VLA data also reveal
large RMs in the fields observed, up to approximately $\pm 300$~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 $b = +4^{\circ}$, we estimate a thermal
electron (positron) density of $\approx 3$~cm~$^{-3}$
and a magnetic field strength of several $\mu$G.

\end{abstract}
\end{document}
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