The large-scale 21-cm power spectrum from reionization
  Radio interferometers, such as the Low-Frequency Array and the future Square
Kilometre Array, are attempting to measure the spherically averaged 21-cm power
spectrum from the Epoch of Reionization. Understanding of the dominant physical
processes which influence the power spectrum at each length-scale is therefore
crucial for interpreting any future detection. We study a decomposition of the
21-cm power spectrum and quantify the evolution of its constituent terms for a
set of numerical and semi-numerical simulations of a volume of
$(714~\mathrm{Mpc})^3$, focusing on large scales with $k\lesssim
0.3$~Mpc$^{-1}$. We find that after $\sim 10$ per cent of the Universe has been
ionized, the 21-cm power spectrum follows the power spectrum of neutral
hydrogen fluctuations, which itself beyond a certain scale follows the matter
power spectrum. Hence the signal has a two-regime form where the large-scale
signal is a biased version of the cosmological density field, and the
small-scale power spectrum is determined by the astrophysics of reionization.
We construct a bias parameter to investigate the relation between the
large-scale 21-cm signal and the cosmological density field. We find that the
transition scale between the scale-independent and scale-dependent bias regimes
is directly related to the value of the mean free path of ionizing photons
($\lambda_{\mathrm{MFP}}$), and is characterised by the empirical formula
$k_{\mathrm{trans}} \approx 2/\lambda_{\mathrm{MFP}}$. Furthermore, we show
that the numerical implementation of the mean free path effect has a
significant impact on the shape of this transition. Most notably, the
transition is more gradual if the mean free path effect is implemented as an
absorption process rather than as a barrier.
