Fonseca, JoseMaartens, RoySantos, Mario G.2018-09-042018-09-042018Fonseca, J. et al. (2018). Synergies between intensity maps of hydrogen lines. Monthly Notices of the Royal Astronomical Society, 479: 34900035-8711http://dx.doi.org/10.1093/mnras/sty1702http://hdl.handle.net/10566/4004We 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.enThis is the pre-print version of the article published online at: http://dx.doi.org/10.1093/mnras/sty1702CosmologyLarge-scale structure of the UniverseCosmological parametersMiscellaneous.Article