Synergies between intensity maps of hydrogen lines
Abstract
We study synergies between Hi 21cm and Hα intensity map observations, focusing on SKA1-
like and SPHEREx-like surveys. We forecast how well such a combination can measure features
in the angular power spectrum on the largest scales, that arise from primordial non-
Gaussianity and from general relativistic effects. For the first time we consider Doppler,
Sachs-Wolfe and integrated SW effects separately. We confirm that the single-tracer surveys
on their own cannot detect general relativistic effects and can constrain the non-Gaussianity
parameter fNL only slightly better than Planck. Using the multi-tracer technique, constraints
on fNL can be pushed down to ~ 1. Amongst the general relativistic effects, the Doppler term
is detectable with the multi-tracer. The Sachs-Wolfe terms and the integrated SW effect are
still not detectable.