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

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 1D 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 similar to 0.05 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.