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We present the first results from a series of observations conducted with theWesterbork telescope in the 140--160 MHz range with a 2 arcmin resolution aimedat characterizing the properties of the foregrounds for epoch of reionizationexperiments. For the first time we have detected fluctuations in the Galacticdiffuse emission on scales greater than 13 arcmin at 150 MHz, in the lowGalactic latitude area known as Fan region. Those fluctuations have an $rms$ of14 K. The total intensity power spectrum shows a power--law behaviour down to$\ell \sim 900$ with slope $\beta^I_\ell = -2.2 \pm 0.3$. The detection ofdiffuse emission at smaller angular scales is limited by residual pointsources. We measured an $rms$ confusion noise of $\sim$3 mJy beam$^{-1}$.Diffuse polarized emission was also detected for the first time at thisfrequency. The polarized signal shows complex structure both spatially andalong the line of sight. The polarization power spectrum shows a power--lawbehaviour down to $\ell \sim 2700$ with slope $\beta^P_\ell = -1.65 \pm 0.15$.The $rms$ of polarization fluctuations is 7.2 K on 4 arcmin scales. Byextrapolating the measured spectrum of total intensity emission, we find acontamination on the cosmological signal of $\delta T= \sqrt{\ell (\ell+1)C^I_\ell / 2\pi} \sim 5.7$ K on 5 arcmin scales and a corresponding $rms$ valueof $\sim$18.3 K at the same angular scale. The level of the polarization powerspectrum is $\delta T \sim 3.3$ K on 5 arcmin scales. Given its exceptionallybright polarized signal, the Fan region is likely to represent an upper limiton the sky brightness at moderate and high Galactic latitude.

astronomy and astrophysics

Foregrounds for observations of the cosmological 21 cm line: I. First  Westerbork measurements of Galactic emission at 150 MHz in a low latitude  field