21 cm signal from cosmic dawn – II. Imprints of the light-cone effects

Abstract

Details of various unknown physical processes during the cosmic dawn and the epoch of reionization can be extracted from observations of the redshifted 21-cm signal. These observations, however, will be affected by the evolution of the signal along the line-of-sight which is known as the "light-cone effect". We model this effect by post-processing a dark matter $N-$body simulation with an 1-D radiative transfer code. We find that the effect is much stronger and dramatic in presence of inhomogeneous heating and Ly$\alpha$ coupling compared to the case where these processes are not accounted for. One finds increase (decrease) in the spherically averaged power spectrum up to a factor of 3 (0.6) at large scales ($k \sim 0.05\, \rm Mpc^{-1}$) when the light-cone effect is included, though these numbers are highly dependent on the source model. The effect is particularly significant near the peak and dip-like features seen in the power spectrum. The peaks and dips are suppressed and thus the power spectrum can be smoothed out to a large extent if the width of the frequency band used in the experiment is large. We argue that it is important to account for the light-cone effect for any 21-cm signal prediction during cosmic dawn.