The feasibility of weak lensing and 21cm intensity mapping cross-correlation 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 two-point function between HI (at redshift z < 1) and lensing convergence (κ). We also construct HI intensity maps from N-body simulations, and measure two-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 characterize this reduction in signal. We find that after a simple model of foreground removal, the cross-correlation signal is reduced by ∼50-70 per cent; we present the angular and redshift dependence of the effect, which is a weak function of these variables. We then calculate S/N of κHI detection, including cases with cut sky observations, and noise from radio and lensing measurements. We present Fisher forecasts based on the resulting two-point functions; these forecasts show that by measuring κ & Delta;THI correlation functions in a sufficient number of redshift bins, constraints on cosmology and HI bias will be possible.