Cross-Bispectrum Constraints from HI Intensity Mapping and Galaxy Surveys

Abstract

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 $f$-${\sigma }_8$ degeneracy. Our constraints range from $0.74\%$ to $9.8\%$ on $f$ and $0.073\%$ to $3.5\%$ on ${\sigma }_8$ 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 $b_{\mathrm{HI}}^{(1)}$ and from $2.0\%$ to $29\%$ on $b_{\mathrm{HI}}^{(2)}$, while the galaxy bias constraints range from $2.6\%$ to $11\%$ on $b_{\mathrm{gal}}.$ 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, $r_{\mathrm{HI}\mathrm{HI}{\delta }_g}=r_0+r_{1}k,$ with a break scale of $0.14\,\mbox{Mpc}^{-1}.$ 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 $({\sigma }_{w_0},{\sigma }_{w_a})=(0.036,0.15)$ and $(0.032,0.12)$ to $({\sigma }_{w_0},{\sigma }_{w_a})=(0.014,0.057)$ and $(0.015,0.059)$ when adding the HI-HI-galaxy and HI-HI-shear cross-bispectra, respectively.