Identifying Ionized Regions in Noisy Redshifted 21 cm Data Sets
  One of the most promising approaches for studying reionization is to use the
redshifted 21 cm line. Early generations of redshifted 21 cm surveys will not,
however, have the sensitivity to make detailed maps of the reionization
process, and will instead focus on statistical measurements. Here we show that
it may nonetheless be possible to {\em directly identify ionized regions} in
upcoming data sets by applying suitable filters to the noisy data. The
locations of prominent minima in the filtered data correspond well with the
positions of ionized regions. In particular, we corrupt semi-numeric
simulations of the redshifted 21 cm signal during reionization with thermal
noise at the level expected for a 500 antenna tile version of the Murchison
Widefield Array (MWA), and mimic the degrading effects of foreground cleaning.
Using a matched filter technique, we find that the MWA should be able to
directly identify ionized regions despite the large thermal noise. In a
plausible fiducial model in which ~20% of the volume of the Universe is neutral
at z ~ 7, we find that a 500-tile MWA may directly identify as many as ~150
ionized regions in a 6 MHz portion of its survey volume and roughly determine
the size of each of these regions. This may, in turn, allow interesting
multi-wavelength follow-up observations, comparing galaxy properties inside and
outside of ionized regions. We discuss how the optimal configuration of radio
antenna tiles for detecting ionized regions with a matched filter technique
differs from the optimal design for measuring power spectra. These
considerations have potentially important implications for the design of future
redshifted 21 cm surveys.
