Polarimetric data from the Effelsberg 2.695 GHz survey have been successfully reduced, extending the Junkes et al. (1987a) work to the latitude limit of the survey (). These data represent a high quality survey of the polarimetric emission throughout much of the first Galactic quadrant. The images reveal a large amount of bright, polarised emission towards higher latitudes, including an intriguing series of patches of bright, polarised emission between longitudes of and which apparently originate from the Sagittarius spiral arm, between distances of approximately 2.5 and 8 kpc. This emission is then depolarised as it passes through regions of denser HI which lie at distances of 2 to 2.5 kpc (a Faraday "screen"), producing an anticorrelation between polarised intensities and HI gas. The depolarisation can be attributed to either an increased density of thermal particles associated with the denser HI regions, or to a significantly more "tangled" magnetic field (on scale-sizes up to pc) within the regions, or both.
The structured, quasi-periodic nature of the polarisation position-angles over this region of the Plane most likely results from a peculiar magnetic field geometry associated with the Sagittarius arm. With scale-sizes of several hundred pc, such magnetic field geometries may be produced by a mechanism similar to the Parker instability.
It would be of considerable interest to see if a similar anticorrelation exists between HI data and the bright, polarised emission over longitudes of . However, no dataset comparable to that of the Dwingeloo HI observations exists over the southern Galactic Plane. The forthcoming narrow-band HI survey with the multi-beam system at Parkes will assist here (Haynes et al. 1999).
In addition to detailed correlations of the polarised and unpolarised emission, more detailed interpretive work will require polarimetric data at other (preferably higher) frequencies; such observations are currently in progress. In the future, we also hope to extend this work (using the observations of Fürst et al. 1990) to cover the entire longitude range of the Galactic Plane.
© European Southern Observatory (ESO) 1999
Online publication: October 4, 1999