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arXiv · 2308.08284

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

Khandakar Md Asif Elahi·Somnath Bharadwaj·Srijita Pal·Abhik Ghosh·Sk. Saiyad Ali·Samir Choudhuri·Arnab Chakraborty·Abhirup Datta·Nirupam Roy·Madhurima Choudhury·Prasun Dutta
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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) $C_{\ell}(Δν)$ from a $24.4\,\rm{MHz}$ bandwidth uGMRT Band $3$ data at $432.8\,\rm{MHz}$. In $C_{\ell}(Δν)$ foregrounds remain correlated across the entire $Δν$ range, whereas the $21$-cm signal is localized within $Δν\le[Δν]$ (typically $0.5-1\,\rm{MHz}$). Assuming the range $Δν>[Δν]$ to have minimal $21$-cm signal, we use $C_{\ell}(Δν)$ in this range to model the foregrounds. This foreground model is extrapolated to $Δν\leq[Δν]$, and subtracted from the measured $C_{\ell}(Δν)$. The residual $[C_{\ell}(Δν)]_{\rm res}$ in the range $Δν\le[Δν]$ is used to constrain the $21$-cm signal, compensating for the signal loss from foreground subtraction. $[C_{\ell}(Δν)]_{\rm{res}}$ is found to be noise-dominated without any trace of foregrounds. Using $[C_{\ell}(Δν)]_{\rm res}$ we constrain the $21$-cm brightness temperature fluctuations $Δ^2(k)$, and obtain the $2σ$ upper limit $Δ_{\rm UL}^2(k)\leq(18.07)^2\,\rm{mK^2}$ at $k=0.247\,\rm{Mpc}^{-1}$. We further obtain the $2σ$ upper limit $ [Ω_{\rm HI}b_{\rm HI}]_{\rm UL}\leq0.022$ where $Ω_{\rm HI}$ and $b_{\rm HI}$ 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.

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Khandakar Md Asif Elahi, Somnath Bharadwaj, Srijita Pal, Abhik Ghosh, Sk. Saiyad Ali, Samir Choudhuri, Arnab Chakraborty, Abhirup Datta, Nirupam Roy, Madhurima Choudhury, Prasun Dutta. 2023-10-05. Towards $21$-cm intensity mapping at $z=2.28$ with uGMRT using the tapered gridded estimator III: Foreground removal. https://doi.org/10.1093/mnras%2Fstad2495

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