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Astron. Astrophys. 356, L13-L16 (2000)

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2. Distribution of polarized intensity

In Fig. 1 we show a gray-scale representation of the polarized intensity in a 5 MHz wide frequency band centered at 349 MHz. The map shows a region of [FORMULA] centered at [FORMULA] at an angular resolution of about 4´. It is one of 8 frequency bands observed simultaneously. Three of those have strong interference, but we obtained good data at 341, 349, 355, 360 and 375 MHz. All 5 maps were made combining mosaics of 7[FORMULA]5 pointing centres. This yields constant sensitivity over a large area (see e.g. Rengelink et al. 1997). The observations were made with the WSRT in January and February 1996, largely at night, and ionospheric Faraday rotation was therefore well-behaved. No corrections were applied.

[FIGURE] Fig. 1. Linearly polarized intensity P at 349 MHz in a [FORMULA] field centered at [FORMULA]. The resolution is [FORMULA] 4´, the maximum brightness temperature is [FORMULA] 10 K. The generally `cloudy' distribution contains long narrow `canals' of low P. The white box shows the area displayed in Fig. 2.

The region in Fig. 1 is rather special because [FORMULA] goes up to 10 K, and because it contains large, almost linear structures in P. Our attention was drawn to this field by the panoramic view of galactic polarization produced in the WENSS survey (de Bruyn & Katgert 2000). However, this field is not unique, and there are other regions with similarly high [FORMULA]. Over a very large fraction of the map the P-signal is quite significant, with a noise [FORMULA] 0.5 K. With S/N-ratios of generally more than 3 and going up to 30, polarization angles are well-defined. Note that in this region, the upper limit to structure in Stokes I (total intensity) on small scales ([FORMULA] 30´) is about 1 K, or less than 2% of the total I.

There appear to be at least two distinct components in the polarized intensity distribution. The first one is a fairly smooth, `cloudy' component, pervading the entire map, with intensity variations on typical scales of (several) tens of arcminutes. In addition, there are conspicuous, very narrow and often quite long and wiggly structures, which we will refer to as `canals', in which the polarized intensity is considerably lower than in the immediate surroundings. In this Letter we focus on the nature and implications of the narrow `canals'; we will discuss the `cloudy' component in more detail in another paper (Haverkorn et al. 2000).

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© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
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