Theoretical and observational constraints on the HI intensity power spectrum

Mapping of the neutral hydrogen (HI) 21-cm intensity fluctuations across redshifts promises a novel and powerful probe of cosmology. The neutral hydrogen gas mass density, $\Omega_{\rm HI}$ and bias parameter, $b_{\rm HI}$ are key astrophysical inputs to the HI intensity fluctuation power spectrum. We compile the latest theoretical and observational constraints on $\Omega_{\rm HI}$ and $b_{\rm HI}$ at various redshifts in the post-reionization universe. Constraints are incorporated from galaxy surveys, HI intensity mapping experiments, damped Lyman-$\alpha$ system observations, theoretical prescriptions for assigning HI to dark matter halos, and the results of numerical simulations. Using a minimum variance interpolation scheme, we obtain the predicted uncertainties on the HI intensity fluctuation power spectrum across redshifts 0-3.5 for three different confidence scenarios. We provide a convenient tabular form for the interpolated values of $\Omega_{\rm HI}$, $b_{\rm HI}$ and the HI power spectrum amplitude and their uncertainties. We discuss the consequences for the measurement of the power spectrum by current and future intensity mapping experiments.