Towards 21-cm intensity mapping at z = 2.28 with uGMRT using the tapered gridded estimator I: Foreground avoidance

Abstract

The post-reionization (z ≤ 6) neutral hydrogen (H i ) 21-cm intensity mapping signal holds the potential to probe the large scale structures, study the expansion history and constrain various cosmological parameters. Here we apply the Tapered Gridded Estimator (TGE) to estimate P(k⊥, k∥) the power spectrum of the z = 2.28 (432.8 MHz) redshifted 21-cm signal using a 24.4 MHz sub-band drawn from uGMRT Band 3 observations of European Large-Area ISO Survey-North 1 (ELAIS-N1). The TGE allows us to taper the sky response which suppresses the foreground contribution from sources in the periphery of the telescope’s field of view. We apply the TGE on the measured visibility data to estimate the multi-frequency angular power spectrum (MAPS) Cℓ(Δν) from which we determine P(k⊥, k∥) using maximum-likelihood which naturally overcomes the issue of missing frequency channels (55 % here). The entire methodology is validated using simulations. For the data, using the foreground avoidance technique, we obtain a 2 σ upper limit of Δ2(k) ≤ (133.97)2 mK2 for the 21-cm brightness temperature fluctuation at k = 0.347 Mpc−1. This corresponds to $[\Omega _{\rm H\, {\small I}~}b_{\rm H\, {\small I}~}] \le 0.23$, where $\Omega _{\rm H\, {\small I}~}$ and $b_{\rm H\, {\small I}~}$ respectively denote the cosmic H i mass density and the H i bias parameter. A previous work has analyzed 8 MHz of the same data at z = 2.19, and reported Δ2(k) ≤ (61.49)2 mK2 and $[\Omega _{\rm H\, {\small I}~} b_{\rm H\, {\small I}~}] \le 0.11$ at k = 1 Mpc−1. The upper limits presented here are still orders of magnitude larger than the expected signal corresponding to $\Omega _{\rm H\, {\small I}~} \sim 10^{-3}$ and $b_{\rm H\, {\small I}~} \sim 2$.

Calibration requirement for Epoch of Reionization 21-cm signal observation. Part III. Bias and variance in uGMRT ELAIS-N1 field power spectrum

“…However, the gain characteristics of different observation days differ significantly, suggesting that in a real scenario, the actual observation time required to detect the 21-cm signal can be higher than quoted here. Pal et al(2022) [57] use a variant of TGE to estimate the 21-cm cylindrically averaged power spectrum and find a significant presence of systematics. [58] discuss the cross-TGE that can reduce much of the systematics by cross correlating visibilities from RR with LL polarisation.…”

“…These upper limits constrain the product of neutral H i mass density (Ω HI ) and H i bias (b HI ) to the underlying dark matter density field as 0.09, 0.11, 0.12, and 0.24 at the corresponding redshifts for k ∼ 1 Mpc −1 . Pal et al (2022) [57] use foreground avoidance technique to estimate the power spectrum from uGMRT Band 3 observations of ELAIS-N1 field at z = 2.28 and quote 2 − σ upper limit of ∆ 2 (k) ≤ (133.97 mK) 2 for the 21-cm brightness temperature fluctuation at k = 0.347 Mpc −1 . Elahi et al (2023a) [58] investigate 'Cross' Tapered Gridded Estimator (Cross-TGE), to estimate multi-frequency angular power JCAP05(2024)068 spectrum (MAPS).…”

Towards $21$-cm intensity mapping at $z=2.28$ with uGMRT using the tapered gridded estimator II: Cross-polarization power spectrum

“…However, the wide-field point sources introduce oscillatory patterns along Δ𝜈 in the estimated 𝐶 ℓ (Δ𝜈) due to the inherent frequency response of the radio interferometers (Ghosh et al 2011a,b). These oscillations, whose frequency increases at larger ℓ due to baseline migration, also manifest themselves as the 'foreground wedge' (Datta et al 2010;Morales et al 2012) structure in the estimated cylindrical PS 𝑃(𝑘 ⊥ , 𝑘 ) (Pal et al 2022;hereafter, Paper I). The frequency structures in the 𝐶 ℓ (Δ𝜈), or the foreground wedge, jeopardise faithful foreground removal and recovery of the 21-cm signal from the measured visibility data.…”