SIMBAD references

Neutral hydrogen (H I) 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 multifrequency angular power spectrum C_ℓ(Δν). We expect this to mitigate several effects like noise bias, calibration errors, etc., which affect the 'Total' TGE that combines the two polarizations. Here, we apply the Cross TGE on $24.4 \text{-} \rm {MHz}$-bandwidth uGMRT (upgraded Giant Metrewave Radio Telescope) Band 3 data centred at $432.8 \, \rm {MHz}$ aiming H I 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 $\Delta _{\mathrm{ UL}}^{2}(k) \le (58.67)^{2} \, {\rm mK}^{2}$ at $k=0.804 \, {\rm Mpc}^{-1}$, which is a factor of 5.2 improvement on our previous estimate based on the Total TGE. Assuming that the H I traces the underlying matter distribution, we have modelled C_ℓ(Δν) to simultaneously estimate the foregrounds and $[\Omega _{\rm{ H}\, {\small {I}}} b_{\rm{ H}\, {\small {I}}}]$, where $\Omega _{\rm{ H}\, {\small {I}}}$ and $b_{\rm{ H}\, {\small {I}}}$ are the H I density and linear bias parameters, respectively. We obtain a best-fitting value of $[\Omega _{\rm{ H}\, {\small {I}}}b_{\rm{ H}\, {\small {I}}}]^2 = 7.51\times 10^{-4} \pm 1.47\times 10^{-3}$ that is consistent with noise. Although the 2σ upper limit $[\Omega _{\rm{ H}\, {\small {I}}}b_{\rm{ H}\, {\small {I}}}]_{\mathrm{ UL}} \le 0.061$ is ∼50 times larger than the expected value, this is a considerable improvement over earlier works at this redshift.