Towards $21$-cm intensity mapping at $z=2.28$ with uGMRT using the tapered gridded estimator II: Cross-polarization power spectrum

Abstract

Neutral hydrogen (H ) 21-cm intensity mapping (IM) offers an efficient technique for mapping the large-scale structures in the universe. We introduce the 'Cross' Tapered Gridded Estimator (Cross TGE), which cross-correlates two cross-polarizations (RR and LL) to estimate the multi-frequency angular power spectrum (MAPS) 𝐶 ℓ (Δ𝜈). We expect this to mitigate several effects like noise bias, calibration errors etc., which affect the 'Total' TGE which combines the two polarizations. Here we apply the Cross TGE on a 24.4 MHz bandwidth uGMRT Band 3 data centred at 432.8 MHz aiming H IM at 𝑧 = 2.28. The measured 𝐶 ℓ (Δ𝜈) is modelled to yield maximum likelihood estimates of the foregrounds and the spherical power spectrum 𝑃(𝑘) in several 𝑘 bins. Considering the mean squared brightness temperature fluctuations, we report a 2𝜎 upper limit Δ 2 𝑈 𝐿 (𝑘) ≤ (58.67) 2 mK 2 at 𝑘 = 0.804 Mpc −1 which is a factor of 5.2 improvement on our previous estimate based on the Total TGE. Assuming that the H traces the underlying matter distribution, we have modelled 𝐶 ℓ (Δ𝜈) to simultaneously estimate the foregrounds and [Ω H 𝑏 H ] where Ω H and 𝑏 H are the H density and linear bias parameters respectively. We obtain a best fit value of [Ω H 𝑏 H ] 2 = 7.51 × 10 −4 ± 1.47 × 10 −3 which is consistent with noise. Although the 2𝜎 upper limit [Ω H 𝑏 H ] 𝑈 𝐿 ≤ 0.061 is ∼ 50 times larger than the expected value, this is a considerable improvement over earlier works at this redshift.