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

Abstract

Neutral hydrogen (${\rm H\, \small {i}}$) 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 (TGE) to estimate the multi-frequency angular power spectrum (MAPS) Cℓ(Δν) from a $24.4\, \rm {MHz}$ bandwidth uGMRT Band 3 data at $432.8\, \rm {MHz}$. In Cℓ(Δν) foregrounds remain correlated across the entire Δν range, whereas the 21-cm signal is localized within Δν ≤ [Δν] (typically $0.5-1\, \rm {MHz}$). Assuming the range Δν > [Δν] to have minimal 21-cm signal, we use Cℓ(Δν) in this range to model the foregrounds. This foreground model is extrapolated to Δν ≤ [Δν], and subtracted from the measured Cℓ(Δν). The residual [Cℓ(Δν)]res in the range Δν ≤ [Δν] is used to constrain the 21-cm signal, compensating for the signal loss from foreground subtraction. [Cℓ(Δν)]res is found to be noise-dominated without any trace of foregrounds. Using [Cℓ(Δν)]res we constrain the 21-cm brightness temperature fluctuations Δ2(k), and obtain the 2σ upper limit $\Delta _{\rm UL}^2(k)\le (18.07)^2\, \rm {mK^2}$ at $k=0.247\, \rm {Mpc}^{-1}$. We further obtain the 2σ upper limit $[\Omega _{{\rm H\, \small {i}}}b_{{\rm H\, \small {i}}}]_{\rm UL}\le 0.022$ where $\Omega _{{\rm H\, \small {i}}}$ and $b_{{\rm H\, \small {i}}}$ are the comoving ${\rm H\, \small {i}}$ 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.