The feasibility of weak lensing and 21cm intensity mapping cross-correlation measurements

TL;DR

This study assesses the potential of cross-correlating weak lensing and 21cm HI intensity maps at low redshift, analyzing foreground removal impacts and forecasting cosmological constraints from such measurements.

Contribution

It provides the first detailed calculation and simulation-based analysis of the HI-lensing cross-correlation signal, including foreground removal effects and signal-to-noise forecasts.

Findings

Foreground removal reduces the cross-correlation signal by 50-70%.

The cross-correlation signal's dependence on angular and redshift variables is weak.

Fisher forecasts indicate potential for cosmological constraints through multi-redshift bin measurements.

Abstract

One of the most promising probes to complement current standard cosmological surveys is the HI intensity map, i.e. the distribution of temperature fluctuations in neutral hydrogen. In this paper we present calculations of the 2-point function between HI (at redshift $z$ < 1) and lensing convergence ($\kappa$). We also construct HI intensity maps from N-body simulations, and measure 2-point functions between HI and lensing convergence. HI intensity mapping requires stringent removal of bright foregrounds, including emission from our galaxy. The removal of large-scale radial modes during this HI foreground removal will reduce the HI-lensing cross-power spectrum signal, as radial modes are integrated to find the convergence; here we wish to characterise this reduction in signal. We find that after a simple model of foreground removal, the cross-correlation signal is reduced by $\sim$50-70\%; we present the angular and redshift dependence of the effect, which is a weak function of these variables. We then calculate S/N of $\kappa$HI detection, including cases with cut sky observations, and noise from radio and lensing measurements. We present Fisher forecasts based on the resulting 2-point functions; these forecasts show that by measuring $\kappa\Delta$$T_\mathrm{HI}$ correlation functions in a sufficient number of redshift bins, constraints on cosmology and HI bias will be possible