Towards 21-cm intensity mapping at z = 2.28 with uGMRT using the tapered gridded estimator - III. Foreground removal

Abstract

Neutral hydrogen () 21-cm intensity mapping (IM) is a promising probe of the large-scale structures in the Universe. However, a few orders of magnitude brighter foregrounds obscure the IM signal. Here, we use the tapered gridded estimator to estimate the multifrequency angular power spectrum C(Δν) from a bandwidth upgraded Giant Metrewave Radio Telescope Band 3 data at. In C(Δν) foregrounds remain correlated across the entire Δν range, whereas the 21-cm signal is localized within Δν ≤ [Δν] (typically, 0.5-1 MHz). Assuming the range Δν > [Δν] to have minimal 21-cm signal, we use C(Δν) in this range to model the foregrounds. This foreground model is extrapolated to Δν ≤ [Δν], and subtracted from the measured C(Δν). The residual [C(Δν)]res in the range Δν ≤ [Δν] is used to constrain the 21-cm signal, compensating for the signal loss from foreground subtraction. [C(Δν)]res is found to be noise-dominated without any trace of foregrounds. Using [C(Δν)]res, we constrain the 21-cm brightness temperature fluctuations Δ2(k), and obtain the 2σ upper limit at. We further obtain the 2σ upper limit, where and are the comoving density and bias parameters, respectively. Although the upper limit is nearly 10 times larger than the expected 21-cm signal, it is 3 times tighter over previous works using foreground avoidance on the same data.