Simulated Predictions for HI at $z=3.35$ with the Ooty Wide Field Array (OWFA) -- II: Foreground Avoidance

Abstract

Considering the upcoming OWFA, we use simulations of the foregrounds and the $z=3.35$ HI 21-cm intensity mapping signal to identify the $(k_{\perp },k_{\parallel })$ modes where the expected 21-cm power spectrum $P(k_{\perp },k_{\parallel })$ is substantially larger than the predicted foreground contribution. Only these uncontaminated $k$-modes are used for measuring $P(k_{\perp },k_{\parallel })$ in the "Foreground Avoidance" technique. Though the foregrounds are largely localised within a wedge. we find that the small leakage beyond the wedge surpasses the 21-cm signal across a significant part of the $(k_{\perp },k_{\parallel })$ plane. The extent of foreground leakage is extremely sensitive to the frequency window function used to estimate $P(k_{\perp },k_{\parallel })$. It is possible to reduce the leakage by making the window function narrower, however this comes at the expense of losing a larger fraction of the 21-cm signal. It is necessary to balance these competing effects to identify an optimal window function. Considering a broad class of cosine window functions, we identify a six term window function as optimal for 21-cm power spectrum estimation with OWFA. Considering only the $k$-modes where the expected 21-cm power spectrum exceeds the predicted foregrounds by a factor of $100$ or larger, a $5\sigma$ detection of the binned power spectrum is possible in the $k$ ranges $0.18\le k\le 0.3{\mathrm{Mpc}}^{-1}$ and $0.18\le k\le 0.8{\mathrm{Mpc}}^{-1}$ with $1,000-2,000$ hours and $10^4$ hours of observation respectively.

18 pages, 10+1 figures