Cosmological Implications of Cross-Correlation between Galaxy Clustering and 21 cm Line Intensity Mapping

Abstract

The apparent anisotropies of galaxy clustering and 21 cm mapping in redshift space offer a unique opportunity to simultaneously probe cosmic expansion and gravity on cosmological scales through the Alcock–Paczynski effect and redshift-space distortions (RSD). Although improved theoretical models exist for anisotropic clustering, their applicability is limited by the nonperturbative smearing effect caused by the randomness of relative velocities. Here, we consider an alternative approach using the statistical power of the cross-correlation between galaxy clustering and 21 cm line intensity mapping (LIM). Based on a Fisher matrix analysis, fully incorporating nonlinear RSDs, we estimate the benefit of combining both observables. We find that, for spectroscopic surveys like the Dark Energy Spectroscopic Instrument combined with 21 cm LIM surveys, constraints on the growth of structure and the cosmic expansion rate are improved by a factor of 2 relative to those from the galaxy autocorrelation. Crucially, such observations can strongly constrain the neutral hydrogen (H I) content, ΩHI, to a subpercent level. This level of precision unlocks the potential of this method to probe post-reionization astrophysics with enhanced precision. It would far surpass existing constraints from stacked 21 cm emission and break the degeneracy between ΩHI and the H I bias, bHI, inherent in linear-regime power spectrum analysis. This cross-correlation approach effectively compensates for the loss of constraining power when using galaxy clustering alone.