  Measurement of the 21cm hyperfine transition of neutral hydrogen provides a
unique probe of the epoch of reionization and the Dark Ages. Three major
mechanisms are believed to dominate the radiation process: emission from
neutral hydrogen surrounding the ionized bubbles of first galaxies and/or
quasars, emission from neutral hydrogen inside minihalos, and absorption of
diffuse neutral hydrogen against the cosmic microwave background. In the
present work, by simply combining the existing analytic models for the three
mechanisms, we investigate the contribution of cross-correlation between these
three components to the total 21cm angular power spectrum, in the sense that
neutral hydrogen associated with different radiation processes traces the
large-scale structures of underlying density perturbations. While the overall
21cm power spectrum remains almost unchanged with the inclusion of the
cross-correlations, the cross-correlation may play a key role in the
determination of the 21cm power spectrum during the transition of 21cm
radiation from emission-dominated phase to absorption-dominated phase at
redshift z~20. A significant suppression in the 21cm angular power spectrum
during this transition is anticipated as the result of negative contribution of
the cross-correlation between the absorption of diffuse neutral hydrogen and
the emission components. Therefore, an accurate prediction of the cosmic 21cm
power spectrum should take the cross-correlation into account especially at the
transition phase.
