2022MNRAS.516.2851P

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 $z = 2.28\, (432.8\, {\rm MHz})$ redshifted 21-cm signal using a $24.4\, {\rm MHz}$ 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 multifrequency angular power spectrum (MAPS) C_ℓ(Δν) from which we determine P(k_⊥, k_∥) using maximum likelihood that naturally overcomes the issue of missing frequency channels (55 per cent here). The entire methodology is validated using simulations. For the data, using the foreground avoidance technique, we obtain a $2\, \sigma$ upper limit of $\Delta ^2(k) \le (133.97)^2 \, {\rm mK}^{2}$ for the 21-cm brightness temperature fluctuation at $k = 0.347 \, \textrm {Mpc}^{-1}$. 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 analysed $8 \, {\rm MHz}$ of the same data at z = 2.19, and reported $\Delta ^{2}(k) \le (61.49)^{2} \, {\rm mK}^{2}$ and $[\Omega _{\rm H\, {\small I}~} b_{\rm H\, {\small I}~}] \le 0.11$ at $k=1 \, {\rm Mpc}^{-1}$. 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 $b_{\rm H\, {\small I}~} \sim 2$.