Measuring ultra-large scale effects in the presence of 21cm intensity mapping foregrounds

TL;DR

This paper assesses how foreground contamination impacts the measurement of large-scale effects like primordial non-Gaussianity and gravity tests in 21cm intensity mapping, finding that joint analysis can still yield precise constraints if foregrounds are accurately modeled.

Contribution

It provides a framework to quantify the degeneracy between large-scale cosmological effects and foreground residuals, demonstrating that joint analysis mitigates degradation of constraints.

Findings

Joint analysis does not cause catastrophic degradation of large-scale effect constraints.

Foreground properties can be measured with high precision.

Accurate foreground modeling is crucial to avoid bias in cosmological parameters.

Abstract

\textsc{Hi} intensity mapping will provide maps of the large-scale distribution of neutral hydrogen (\textsc{Hi}) in the universe. These are prime candidates to be used to constrain primordial non-Gaussianity using the Large Scale Structure of the Universe as well as to provide further tests of Einstein's theory of Gravity (GR). But \textsc{Hi} maps are contaminated by foregrounds, which can be several orders of magnitude above the cosmological signal. Here we quantify how degenerated are the large-scale effects ($f_{\rm NL}$ and GR effects) with the residual foregrounds. We conclude that a joint analysis does not provide a catastrophic degradation of constraints and provides a framework to determine the marginal errors of large scale-effects in the presence of foregrounds. Similarly, we conclude that the macroscopical properties of the foregrounds can be measured with high precision. Notwithstanding, such results are highly dependent on accurate forward modelling of the foregrounds, which incorrectly done catastrophically bias the best fit values of cosmological parameters, foreground parameterisations, and large-scale effects.