Cross-Bispectrum Constraints from HI Intensity Mapping and Galaxy Surveys

Future neutral hydrogen (HI) intensity mapping (IM) and optical surveys probe large-scale structure over unprecedented volumes, offering powerful constraints on cosmological models. We study the cross-correlation between the HI IM field and the galaxy density and cosmic shear fields from optical surveys. By formulating the HI-HI-galaxy and HI-HI-shear cross-bispectra, we recover sensitivity to long-wavelength radial modes otherwise removed by foreground subtraction in HI maps, evaluating their detectability for HIRAX and Rubin-LSST. Combining the probes breaks degeneracies in redshift-dependent quantities, including the f - σ 8 degeneracy. Our constraints range from 0.74% to 9.8% on f and 0.073% to 3.5% on σ 8 across the six redshift bins that we consider. Our constraints on the first- and second-order HI bias parameters range from 0.42% to 5.6% on b HI(1) and from 2.0% to 29% on b HI(2), while the galaxy bias constraints range from 2.6% to 11% on b gal. The HI-HI-galaxy density cross-bispectrum measurement allows us to constrain a linear scale-dependent model for the stochastic cross-correlation coefficient between HI and stars, r HI,HI, δ g = r 0 + r 1 k, with a break scale of 0.14, Mpc-1. We obtain r 0 constraints that range from 3.8% to 13.5% and r 1 constraints that range from 7.5% to 19.9%. For cosmology, adding cross-bispectra enhances Lambda CDM constraints, notably via the HI-HI-shear cross-bispectrum. For the w 0, w a CDM model, the power-spectrum-only constraints on w 0 and w a, including Planck priors, improve from (σ w 0,σ w a)=(0.036,0.15) and (0.032,0.12) to (σ w 0,σ w a)=(0.014,0.057) and (0.015,0.059) when adding the HI-HI-galaxy and HI-HI-shear cross-bispectra, respectively.