Light cone effect on the reionization 21cm signal  II. Evolution, anisotropies and observational implications
Abstract
Measurements of the H I 21cm power spectra from the reionization epoch will be influenced by the evolution of the signal along the lineofsight direction of any observed volume. We use numerical as well as seminumerical simulations of reionization in a cubic volume of 607 Mpc across to study this socalled lightcone effect on the H I 21cm power spectrum. We find that the lightcone effect has the largest impact at two different stages of reionization: one when reionization is ∼20 per cent and other when it is ∼80 per cent completed. We find a factor of ∼4 amplification of the power spectrum at the largest scale available in our simulations. We do not find any significant anisotropy in the 21cm power spectrum due to the lightcone effect. We argue that for the power spectrum to become anisotropic, the lightcone effect would have to make the ionized bubbles significantly elongated or compressed along the line of sight, which would require extreme reionization scenarios. We also calculate the twopoint correlation functions parallel and perpendicular to the line of sight and find them to differ. Finally, we calculate an optimum frequency bandwidth below which the lightcone effect can be neglected when extracting power spectra from observations. We find that if one is willing to accept a 10 per cent error due to the lightcone effect, the optimum frequency bandwidth for k = 0.056 Mpc^{1} is ∼7.5 MHz. For k = 0.15 and 0.41 Mpc^{1}, the optimum bandwidth is ∼11 and ∼16 MHz, respectively.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 August 2014
 DOI:
 10.1093/mnras/stu927
 arXiv:
 arXiv:1402.0508
 Bibcode:
 2014MNRAS.442.1491D
 Keywords:

 methods: numerical;
 methods: statistical;
 cosmology: theory;
 dark ages;
 reionization;
 first stars;
 diffuse radiation;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 17 pages, accepted for publication in MNRAS, minor changes