Polarization Observations in a Low Synchrotron Emission Field at 1.4 GHz

We present the first observation of the diffuse polarized synchrotron radiation of a patch ($\sim 3^\circ \times 3^\circ$) in the BOOMERanG field, one of the areas with the lowest CMB foreground emission. The work has been carried out with the Australia Telescope Compact Array at 1.4 GHz with 3.4 arcmin resolution and sensitivity of $\sim 0.18$ mJy beam$^{-1}$. The mean polarized signal has been found to be $P_{rms} = \sqrt{(Q_{rms}^2 + U_{rms}^2)} = 11.6 \pm 0.6$ mK, nearly one order of magnitude below than in the Galactic Plane. Extrapolations to frequencies of interest for cosmological investigations suggest that polarized synchrotron foreground noise should allow the detection of the CMB Polarization $E$--mode already at 32 GHz and make us confident that, at 90 GHz, it is accessible with no relevant foreground contamination. Last but not least, even the $B$--mode detection for $T/S > 0.01$ is not ruled out in such a low emission patch.Comment: Uses emulateapj.sty, onecolfloat.sty, 5 pages 4 fig., accepted for publication in ApJ