  We investigate effects of baryon dissipation on the dark matter virial
scaling relation between total mass and velocity dispersion and the velocity
bias of galaxies in groups and clusters using self-consistent cosmological
simulations. We show that the baryon dissipation increases the velocity
dispersion of dark matter within the virial radius by 5% - 10%. The effect is
mainly driven by the change in density and gravitational potential in inner
regions of cluster, and it is larger in lower mass systems where gas cooling
and star formation are more efficient. We also show that the galaxy velocity
bias depends on how galaxies are selected. Galaxies selected based on their
stellar mass exhibit no velocity bias, while galaxies selected based on their
total mass show positive bias of ~10%, consistent with previous results based
on collisionless dark matter- only simulations. We further find that
observational estimates of galaxy velocity dispersion are unbiased with respect
to the velocity dispersion of dark matter, provided galaxies are selected using
their stellar masses and and their velocity dispersions are computed with more
than twenty most massive galaxies. Velocity dispersions estimated with fewer
galaxies, on the other hand, can lead to significant underestimate of dynamical
masses. Results presented in this paper should be useful in interpretating
high-redshift groups and clusters as well as cosmological constraints derived
from upcoming optical cluster surveys.
