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

Context. The cosmological 21 cm line promises to be a formidable tool for cosmology, allowing the investigation of the end of the so-called dark ages, when the first galaxies formed. Aims. Astrophysical foregrounds are expected to be about three orders of magnitude greater than the cosmological signal and therefore represent a serious contamination of the cosmological 21 cm line. Detailed knowledge of both their intensity and polarization structure on the relevant angular scale of 1-30 arcmin will be essential for extracting the cosmological signal from the data. Methods. We present the first results from a series of observations conducted with the Westerbork telescope in the 140-160 MHz range with a 2 arcmin resolution aimed at characterizing the properties of the foregrounds for epoch of reionization experiments. The polarization data were analysed through the rotation measure synthesis technique. We computed total intensity and polarization angular power spectra. Results. For the first time we have detected fluctuations in the Galactic diffuse emission on scales greater than 13 arcmin at 150 MHz, in the low Galactic latitude area known as Fan region, centred at alpha = 3(h)10(m), delta = 65 degrees 30'. Those fluctuations have an rms of 14 K. The total intensity power spectrum shows a power-law behaviour down to l similar to 900 with slope beta(I)(l) = -2.2 +/- 0.3. The detection of diffuse emission at smaller angular scales is limited by residual point sources. We measured an rms confusion noise of similar to 3 mJy beam(-1). Diffuse polarized emission was also detected for the first time at this frequency. The polarized signal shows complex structure both spatially and along the line of sight. The polarization power spectrum is not affected by residual point sources and is only limited by the thermal noise. It shows a power-law behaviour down to l similar to 2700 with slope beta(P)(l) = -1.65 +/- 0.15. The rms of polarization fluctuations is 7.2 K on 4 arcmin scales. Conclusions. The measured total intensity fluctuations are used to estimate the foreground contamination on the cosmological signal. By extrapolating the spectrum of total intensity emission, we find a contamination of delta T = root l(l + 1)C-l(I)/2 pi similar to 5.7 K on 5 arcmin scales and a corresponding rms value of similar to 18.3 K at the same angular scale. The level of the polarization power spectrum is delta T similar to 3.3 K on 5 arcmin scales. However, the Fan region cannot be taken as representative of other sky regions, given its exceptionally bright polarized signal, but is likely to represent an upper limit on the sky brightness at moderate and high Galactic latitude.