Synergizing 21 cm and submillimetre surveys during reionization: new empirical insights

We use the latest results from Atacama Large Millimetre/submillimetre Array (ALMA) surveys targeting the ionized carbon [C II] 158 μm and oxygen [O III] 88 μm lines, in combination with data-driven predictions for the evolution of neutral hydrogen (H I), to illustrate the prospects for intensity mapping cross-correlations between 21 cm and submillimetre surveys over z ~ 5 - 7. We work with a data set including the ALPINE and REBELS surveys for [C II] over z ~ 4.5 - 7, and ALMA [O III] detections over z ~ 6 - 9. The resultant evolution of the [C II] luminosity-halo mass relation is well described by a double power law at high redshifts, with the best-fitting parameters in good agreement with the results of simulations. The data favour secure detections of the autopower spectrum of [C II] at all redshifts with an enhanced Fred Young Submillimetre Telescope (FYST)-like configuration. Such an experiment, along with the Murchison Widefield Array (MWA) will be able to measure the 21 cm - [C II] cross-correlation power with a signal-to-noise ratio of a few tens to a few hundreds. We find that a balloon-borne experiment improving upon the Experiment for Cryogenic Large-Aperture Intensity Mapping (EXCLAIM) should be able to detect the 21 cm - [O III] cross-correlation with the MWA and the Square Kilometre Array (SKA)-LOW out to z ~ 7. Our results have implications for constraining the evolution of luminous sources during the mid-to-end stages of reionization.