Investigating Cross Correlations between Cosmic Microwave Background Lensing and 21 cm Intensity Mapping

Abstract The neutral hydrogen (H i ) signal is a crucial probe for astrophysics and cosmology, but it is quite challenging to measure from raw data because of bright foreground contaminants at radio wavelengths. Cross-correlating the radio observations with large-scale structure (LSS) tracers could detect faint cosmological signals since they are not correlated with the foreground, but exquisite component separation procedures must be performed to reduce the variance induced by the foreground. In this work, we adopt the lensing of the cosmic microwave background (CMB) as the LSS tracer and investigate the cross correlation of CMB lensing and H I observations at the postreionization epoch. We use simulations to study lensing and H I cross correlations in the context of next-generation CMB and intensity-mapping experiments. We investigate the impact of the component separation based on linear combinations of the H I observations at different frequencies and estimate the signal-to-noise ratios for the cross correlation measurements in different scenarios.