Modeling the Nonlinear 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 1\ Mpc⁻¹), as measured by interferometer arrays like MeerKAT in the low-redshift (z≤ 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≈ 0.44, though with a significantly lower amplitude. We demonstrate how the H i abundance Ω 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.