Probing reionization with the cross power spectrum of 21 cm and
  near-infrared radiation backgrounds
  The cross-correlation between the 21 cm emission from the high-redshift
intergalactic medium and the near-infrared (NIR) background light from the
high-redshift galaxies promises to be a powerful probe of cosmic reionization.
In this paper, we investigate the cross power spectrum during the epoch of
reionization. We employ an improved halo approach to derive the distribution of
the density field and consider two stellar populations in the star formation
model: metal-free stars and metal-poor stars. The reionization history is
further generated to be consistent with the electron-scattering optical depth
from cosmic microwave background measurements. Then the intensity of NIR
background is estimated by collecting emission from stars in the first-light
galaxies. On large scales, we find the 21 cm and NIR radiation backgrounds are
positively correlated during the very early stages of reionization. However,
these two radiation backgrounds quickly become anti-correlated as reionization
proceeds. The maximum absolute value of the cross power spectrum is
$|\Delta^2_{\rm{21,NIR}}|\sim10^{-4}$ mK nW $\rm{m}^{-2}$ $\rm{sr}^{-1}$
reached at $\ell\sim1000$, when the mean fraction of ionized hydrogen is
$\bar{x}_{i}\sim0.9$. We find that SKA can measure the 21 cm-NIR cross power
spectrum in conjunction with mild extensions to the existing CIBER survey,
provided that the integration time independently adds up to 1000 and 1 hours
for 21 cm and NIR observations, and that the sky coverage fraction of CIBER
survey is extended from $4\times10^{-4}$ to 0.1. Measuring the
cross-correlation signal as a function of redshift provides valuable
information on reionization and helps confirm the origin of the "missing" NIR
background.
