Imprint of baryons and massive neutrinos on velocity statistics
  We explore the impact of baryonic effects (namely stellar and AGN feedback)
on the moments of pairwise velocity using the Illustris-TNG, EAGLE, cosmo-OWLS,
and BAHAMAS suites of cosmological hydrodynamical simulations. The assumption
that the mean pairwise velocity of the gas component follows that of the dark
matter is studied here at small separations, and we find that even at pair
separations of 10-20 $h^{-1}\mathrm{Mpc}$ there is a 4-5% velocity bias. At
smaller separations, it gets larger with strength varying depending on the
subgrid prescription. By isolating different physical processes, our findings
suggest that the large scale velocity bias is mainly driven by stellar rather
than AGN feedback. If unaccounted for, this velocity offset could possibly bias
cosmological constraints from the kinetic Sunyaev-Zel'dovich effect in future
cosmic microwave background (CMB) surveys. Furthermore, we examine how the
first and the second moment of the pairwise velocity are affected by both the
baryonic and the neutrino free-streaming effects for both the matter and gas
components. For both moments, we were able to disentangle the effects of
baryonic processes from those of massive neutrinos; and below pair separations
of 20 $h^{-1}\mathrm{Mpc}$, we find that these moments of the pairwise velocity
decrease with increasing neutrino mass. Our work thus paves a way in which the
pairwise velocity statistics can be utilised to constrain the summed mass of
neutrinos from future CMB surveys and peculiar velocity surveys.
