HI intensity mapping for clustering-based redshift estimation

Precision cosmology requires accurate galaxy redshifts, but next-generation optical surveys will observe unprecedented numbers of resolved galaxies, placing strain on the amount of spectroscopic follow-up required. We show how useful information can be gained on the redshift distributions of optical galaxy samples from spatial cross-correlations with intensity maps of unresolved HI (21 cm) spectral-line emission. We construct a redshift distribution estimator, which we test using simulations. We utilize the S-3-SAX catalogue that includes HI emission information for each galaxy, which we use to construct H I intensity maps. We also make use of simulated LSST and Euclid-like photometry enabling us to apply the H I clustering calibration to realistic simulated photometric redshifts. While taking into account important limitations to H I intensity mapping such as lost k modes from foreground cleaning and poor angular resolution due to large receiver beams, we show that excellent constraints on redshift distributions can be provided for an optical photometric sample.