Lightcone effect on the reionization 21cm power spectrum
Abstract
Observations of redshifted 21cm radiation from neutral hydrogen during the epoch of reionization are considered to constitute the most promising tool to probe that epoch. One of the major goals of the first generation of lowfrequency radio telescopes is to measure the 3D 21cm power spectrum. However, the 21cm signal could evolve substantially along the lineofsight (LOS) direction of an observed 3D volume, since the received signal from different planes transverses to the LOS originated from different lookback times and could therefore be statistically different. Using numerical simulations we investigate this socalled lightcone effect on the spherically averaged 3D 21cm power spectrum. For this version of the power spectrum, we find that the effect mostly 'averages out' and observe a smaller change in the power spectrum compared to the amount of evolution in the mean 21cm signal and its rms variations along the LOS direction. Nevertheless, changes up to ∼50 per cent at large scales are possible. In general, the power is enhanced/suppressed at large/small scales when the effect is included. The crossover mode below/above which the power is enhanced/suppressed moves towards larger scales as reionization proceeds. When considering the 3D power spectrum we find it to be anisotropic at the late stages of reionization and on large scales. The effect is dominated by the evolution of the ionized fraction of hydrogen during reionization and including peculiar velocities hardly changes these conclusions. We present simple analytical models which explain qualitatively all the features we see in the simulations.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 August 2012
 DOI:
 10.1111/j.13652966.2012.21293.x
 arXiv:
 arXiv:1109.1284
 Bibcode:
 2012MNRAS.424.1877D
 Keywords:

 methods: numerical;
 methods: statistical;
 cosmology: theory;
 dark ages;
 reionization;
 first stars;
 diffuse radiation;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 15 pages, 15 figures, 4 tables, moderate revision, added results on anisotropies in the power spectra arising from the light cone effect and a discussion on the foreground subtraction effect. MNRAS (in press)