Imaging neutral hydrogen on large-scales during the Epoch of
  Reionization with LOFAR
  The first generation of redshifted 21 cm detection experiments, carried out
with arrays like LOFAR, MWA and GMRT, will have a very low signal-to-noise
ratio per resolution element (\sim 0.2). In addition, whereas the variance of
the cosmological signal decreases on scales larger than the typical size of
ionization bubbles, the variance of the formidable galactic foregrounds
increases, making it hard to disentangle the two on such large scales. The poor
sensitivity on small scales on the one hand, and the foregrounds effect on
large scales on the other hand, make direct imaging of the Epoch of
Reionization of the Universe very difficult, and detection of the signal
therefore is expected to be statistical.Despite these hurdles, in this paper we
argue that for many reionization scenarios low resolution images could be
obtained from the expected data. This is because at the later stages of the
process one still finds very large pockets of neutral regions in the IGM,
reflecting the clustering of the large-scale structure, which stays strong up
to scales of \sim 120 comoving Mpc/h (\sim 1 degree). The coherence of the
emission on those scales allows us to reach sufficient S/N (\sim 3) so as to
obtain reionization 21 cm images. Such images will be extremely valuable for
answering many cosmological questions but above all they will be a very
powerful tool to test our control of the systematics in the data. The existence
of this typical scale (\sim 120 comoving Mpc/h) also argues for designing
future EoR experiments, e.g., with SKA, with a field of view of at least 4
degree.
