2023MNRAS.525.3439E

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 to estimate the multifrequency angular power spectrum C_ℓ(Δν) from a $24.4\hbox{-} \rm {MHz}$ bandwidth upgraded Giant Metrewave Radio Telescope Band 3 data at $432.8\ \rm {MHz}$. In C_ℓ(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_ℓ(Δν) in this range to model the foregrounds. This foreground model is extrapolated to Δν ≤ [Δν], and subtracted from the measured C_ℓ(Δν). The residual [C_ℓ(Δν)]_res in the range Δν ≤ [Δν] is used to constrain the 21-cm signal, compensating for the signal loss from foreground subtraction. [C_ℓ(Δν)]_res is found to be noise-dominated without any trace of foregrounds. Using [C_ℓ(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.