Measuring cosmic velocities with 21 cm intensity mapping and galaxy redshift survey cross-correlation dipoles

Abstract

We investigate the feasibility of measuring the effects of peculiar velocities in large-scale structure using the dipole of the redshift-space cross-correlation function. We combine number counts of galaxies with brightness-temperature fluctuations from 21 cm intensity mapping, demonstrating that the dipole may be measured at modest significance ($\ensuremath{\lesssim}2\ensuremath{\sigma}$) by combining the upcoming radio survey Canadian Hydrogen Intensity Mapping Experiment with the future redshift surveys of Dark Energy Spectroscopic Instrument (DESI) and Euclid. More significant measurements ($\ensuremath{\lesssim}10\ensuremath{\sigma}$) will be possible by combining intensity maps from the Square Kilometre Array (SKA) with these of DESI or Euclid, and an even higher significance measurement ($\ensuremath{\lesssim}100\ensuremath{\sigma}$) may be made by combining observables completely internally to the SKA. We account for effects such as contamination by wide-angle terms, interferometer noise and beams in the intensity maps, nonlinear enhancements to the power spectrum, stacking multiple populations, sensitivity to the magnification slope, and the possibility that number counts and intensity maps probe the same tracers. We also derive a new expression for the covariance matrix of multitracer redshift-space correlation function estimators with arbitrary orientation weights, which may be useful for upcoming surveys aiming at measuring redshift-space clustering with multiple tracers.