Contribution of cross-correlations to the 21-cm angular power spectrum in the epoch of re-ionization

Measurement of the 21-cm hyperfine transition of neutral hydrogen provides a unique probe of the epoch of re-ionization 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 minihaloes 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 21-cm 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 21-cm 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 21-cm power spectrum during the transition of 21-cm radiation from emission-dominated phase to absorption-dominated phase at redshift z approximate to 20. A significant suppression in the 21-cm 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 21-cm power spectrum should take the cross-correlation into account especially at the transition phase.