Beyond power spectrum to unveil systematics on HI intensity maps

Abstract

HI intensity mapping is a promising technique to probe large-scale structure, traditionally analyzed via two-point statistics such as the angular power spectrum. This technique has proven very powerful but may miss key non-Gaussian information present in the signal. We extend the starlet l1-norm, a multi-scale higher-order statistic previously applied to weak lensing maps, to the brightness temperature fluctuations of the HI density field. The HI signal is highly non-Gaussian at late times (z < 1) due to nonlinear structure growth, motivating the use of advanced summary statistics. We simulated full-sky HI lognormal brightness temperature maps using CAMB and GLASS, generating 10,000 realizations with associated cosmological parameters. We extracted both the starlet l1-norm and angular power spectrum from these maps. Using the JaxILI framework, we performed neural density estimation for implicit likelihood inference. The analysis considered simulated maps incorporating realistic noise and telescope beam, capturing the impact of observational effects on parameter inference. In this work, we focus on the redshift range 0.4 < z < 0.45, chosen to match the interval already targeted by existing MeerKLASS observations. The starlet l1-norm significantly outperforms the angular power spectrum in constraining cosmological parameters, achieving almost a 3x improvement in the figure of merit relative to the angular power spectrum by capturing non-Gaussian features missed by two-point statistics. Moreover, our results suggest that the starlet l1-norm is robust to several of the systematic effects included in our simulations. Our findings highlight the potential of multi-scale higher-order statistics such as the starlet l1-norm to enhance cosmological inference from future HI intensity mapping surveys.