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

Abstract

Neutral hydrogen (HI) $21$-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) $C_{\ell }(\Delta \nu )$ from a $24.4\mathrm{MHz}$ bandwidth uGMRT Band $3$ data at $432.8\mathrm{MHz}$. In $C_{\ell }(\Delta \nu )$ foregrounds remain correlated across the entire $\Delta \nu$ range, whereas the $21$-cm signal is localized within $\Delta \nu \le [\Delta \nu ]$ (typically $0.5-1\mathrm{MHz}$). Assuming the range $\Delta \nu >[\Delta \nu ]$ to have minimal $21$-cm signal, we use $C_{\ell }(\Delta \nu )$ in this range to model the foregrounds. This foreground model is extrapolated to $\Delta \nu \le [\Delta \nu ]$, and subtracted from the measured $C_{\ell }(\Delta \nu )$. The residual $[C_{\ell }(\Delta \nu ){]}_{\mathrm{res}}$ in the range $\Delta \nu \le [\Delta \nu ]$ is used to constrain the $21$-cm signal, compensating for the signal loss from foreground subtraction. $[C_{\ell }(\Delta \nu ){]}_{\mathrm{res}}$ is found to be noise-dominated without any trace of foregrounds. Using $[C_{\ell }(\Delta \nu ){]}_{\mathrm{res}}$ we constrain the $21$-cm brightness temperature fluctuations ${\Delta }^2(k)$, and obtain the $2\sigma$ upper limit ${\Delta }_{\mathrm{UL}}^2(k)\le (18.07{)}^2\mathrm{m}{\mathrm{K}}^2$ at $k=0.247{\mathrm{Mpc}}^{-1}$. We further obtain the $2\sigma$ upper limit $[{\Omega }_{\mathrm{HI}}{b}_{\mathrm{HI}}{]}_{\mathrm{UL}}\le 0.022$ where ${\Omega }_{\mathrm{HI}}$ and $b_{\mathrm{HI}}$ are the comoving HI 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.

Accepted for publication in MNRAS. 16 pages (including Appendix), 8 figures (plus 8 in Appendix), 5 Tables. In version 2, the HI symbol is changed in the arxiv abstract; there are no changes in the manuscript

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