Modelling the non-linear power spectrum in low-redshift H i intensity mapping.

Neutral hydrogen (H  i) serves as a competitive tracer of the large scale structures, especially with the advent of more intensity mapping H  i surveys. In this work, we present a simulation-based framework to forecast the H  i power spectrum on non-linear scales (⁠|$k\gtrsim 1\ {\rm Mpc^{-1}}$|⁠), as measured by interferometer arrays like MeerKAT in the low-redshift (⁠|$z\le 1.0$|⁠) Universe. Building on a galaxy-based H  i mock catalogue, we meticulously consider various factors, including the emission line profiles of H  i discs and some observational settings, and explore their impacts on the H  i power spectrum. We find that the H  i power spectrum is relatively insensitive to the profile shape of H  i emission line at these scales, while showing a strong correlation with the profile width. We propose an empirical model to simulate the emission line profile width for each H  i source. The resulting H  i power spectrum is consistent with the results from the IllustrisTNG hydrodynamical simulation and follows the trend of the measurements obtained by MeerKAT at |$z\approx 0.44$|⁠ , though with a significantly lower amplitude. We demonstrate how the H  i abundance |$\Omega _{\rm HI}$| and the amplitude parameter in our width model can be constrained with the MeerKAT measurements, though a strong degeneracy is uncovered. Our work shows the potential to constrain statistical properties of H  i emission line profiles with future H  i intensity mapping experiments. [ABSTRACT FROM AUTHOR]