  The correlation between 21cm fluctuations and galaxies is sensitive to the
astrophysical properties of the galaxies that drove reionization. Thus,
detailed measurements of the cross-power spectrum and its evolution could
provide a powerful measurement both of the properties of early galaxies and the
process of reionization. In this paper, we study the evolution of the
cross-power spectrum between 21cm emission and galaxies using a model which
combines the hierarchical galaxy formation model GALFORM implemented within the
Millennium-II dark matter simulation, with a semi-numerical scheme to describe
the resulting ionization structure. We find that inclusion of different
feedback processes changes the cross-power spectrum shape and amplitude. In
particular, the feature in the cross-power spectrum corresponding to the size
of ionized regions is significantly affected by supernovae feedback. We
calculate predicted observational uncertainties of the cross-correlation
coefficient based on specifications of the Murchison Widefield Array (MWA)
combined with galaxy surveys of varying area and depth. We find that the
cross-power spectrum could be detected over several square degrees of galaxy
survey with galaxy redshift errors less than 0.1.
