A Technique for Foreground Subtraction in Redshifted 21 cm Observations
  One of the main challenges for future 21 cm observations is to remove
foregrounds which are several orders of magnitude more intense than the HI
signal. We propose a new technique for removing foregrounds of the redshifted
21 cm observations. We consider multi-frequency interferometer observations. We
assume that the 21 cm signals in different frequency channels are uncorrelated
and the foreground signals change slowly as a function of frequency. When we
add the visibilities of all channels, the foreground signals increase roughly
by a factor of ~N because they are highly correlated. However, the 21 cm
signals increase by a factor of ~\sqrt{N} because the signals in different
channels contribute randomly. This enables us to obtain an accurate shape of
the foreground angular power spectrum. Then, we obtain the 21-cm power spectrum
by subtracting the foreground power spectrum obtained this way. We describe how
to obtain the average power spectrum of the 21 cm signal.
