Measuring cosmic velocities with 21cm intensity mapping and galaxy
  redshift survey cross-correlation dipoles
  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 21cm intensity mapping, demonstrating
that the dipole may be measured at modest significance ($\lesssim 2\sigma$) by
combining the upcoming radio survey CHIME with the future redshift surveys of
DESI and Euclid. More significant measurements ($\lesssim~10\sigma$) will be
possible by combining intensity maps from the SKA with these of DESI or Euclid,
and an even higher significance measurement ($\lesssim 100\sigma$) 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, non-linear 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 multi-tracer
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.
