Stacking Redshifted 21cm Images of HII Regions Around High Redshift
  Galaxies as a Probe of Early Reionization
  A number of current and future experiments aim to detect the reionization of
neutral hydrogen by the first stars and galaxies in the Universe via the
redshifted 21cm line. Using the \textsc{BlueTides} simulation, we investigate
the measurement of an \textit{average} ionised region towards the beginning of
reionization by stacking redshifted 21cm images around optically identified
bright galaxies using mock observations. We find that with an SKA 1000 hour
observation, assuming perfect foreground subtraction, a $5\sigma$ detection of
a stacked HII region can be made with 30 images around some of the brightest
galaxies in \textsc{bluetides} (brighter than $M_{UV} < -22.75$) at $z=9$
(corresponding to a neutral fraction of 90.1 \% in our model). We present
simulated relationships between the UV magnitude of galaxies, the sizes of the
ionised regions they reside in, and the shape of the stacked profiles. These
mock observations can also distinguish between scenarios where the IGM is in
net emission or absorption of 21cm photons. Once 21cm foreground contamination
is included, we find that even with up to 200 images around these rare, bright
galaxies, only a tentative $> 1\sigma$ detection will be possible. However,
partial foreground subtraction substantially improves signal-to-noise. For
example, we predict that reducing the area of Fourier space dominated by
foregrounds by 50 (80) percent will allow $> 3\sigma$ ($> 5\sigma$) detections
of ionised regions at $z=9$.
