The effect of peculiar velocities on the epoch of reionization 21-cm signal

We have used seminumerical simulations of reionization to study the behaviour of the power spectrum of the epoch of reionization 21-cm signal in redshift space. We have considered two models of reionization, one which has homogeneous recombination (HR) and the other incorporating inhomogeneous recombination (IR). We have estimated the observable quantities - quadrupole and monopole moments of H I power spectrum at redshift space from our simulated data. We find that the magnitude and nature of the ratio between the quadrupole and monopole moments of the power spectrum (P^s_2 /P^s_0) can be a possible probe for the epoch of reionization. We observe that this ratio becomes negative at large scales for bar{x}_{{H riptsc{I}}}le 0.7 irrespective of the reionization model, which is a direct signature of an inside-out reionization at large scales. It is possible to qualitatively interpret the results of the simulations in terms of the fluctuations in the matter distribution and the fluctuations in the neutral fraction which have power spectra and cross-correlation P_ΔΔ(k), P_xx(k) and P_Δx(k), respectively. We find that at large scales the fluctuations in matter density and neutral fraction are exactly anticorrelated through all stages of reionization. This provides a simple picture where we are able to qualitatively interpret the behaviour of the redshift-space power spectra at large scales with varying bar{x}_{{H riptsc{I}}} entirely in terms of a just two quantities, namely bar{x}_{{H riptsc{I}}} and the ratio P_xx/P_ΔΔ. The nature of P_Δx becomes different for HR and IR scenarios at intermediate and small scales. We further find that it is possible to distinguish between an inside-out and an outside-in reionization scenario from the nature of the ratio P^s_2 /P^s_0 at intermediate length scales.