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.
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Fonseca, J. (Creator), Maartens, R. (Creator) & Santos, M. G. (Creator), Oxford University Press, 28 Jun 2018