Foregrounds for observations of the cosmological 21 cm line: I. First
  Westerbork measurements of Galactic emission at 150 MHz in a low latitude
  field
  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. 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. Those fluctuations have an $rms$ of
14 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 of
diffuse emission at smaller angular scales is limited by residual point
sources. We measured an $rms$ confusion noise of $\sim$3 mJy beam$^{-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 shows a power--law
behaviour 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. By
extrapolating the measured spectrum of total intensity emission, we find a
contamination 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 $rms$ value
of $\sim$18.3 K at the same angular scale. The level of the polarization power
spectrum is $\delta T \sim 3.3$ K on 5 arcmin scales. Given its exceptionally
bright polarized signal, the Fan region is likely to represent an upper limit
on the sky brightness at moderate and high Galactic latitude.
