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

The post-reionization (z <= 6) neutral hydrogen (HI) 21-cm intensity mapping signal holds the potential to probe the large-scale structures, study the expansion history, and constrain various cosmological parameters. IIere, we apply the Tapered Gridded Estimator (TGE) to estimate P(k(perpendicular to), k(parallel to)) the power spectrum of the z = 2.28 (432.8 MHz) redshifted 21-cm signal using a 24.4 MIIz 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(Delta nu) from which we determine P(k(perpendicular to), k(parallel to)) 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) <= (133.97)(2) mK(2) for the 21-cm brightness temperature fluctuation at k = 0.347 Mpc(-1). This corresponds to [Omega(HI)b(HI)] <= 0.23, where S2HI and b(HI), respectively, denote the cosmic HI mass density and the HI bias parameter. A previous work has analysed 8 MHz of the same data at z = 2.19, and reported Delta(2) (k) <= (61.49)(2) mK(2) and [Omega(HI)b(HI)] <= 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(HI) similar to 10(-3) and b(HI) similar to 2.