The velocity distribution of galaxies in clusters is not universal ; rather , galaxies are segregated according to their spectral type and relative luminosity .
We examine the velocity distributions of different populations of galaxies within 89 Sunyaev Zel ’ dovich ( SZ ) selected galaxy clusters spanning 0.28 < z < 1.08 .
Our sample is primarily draw from the SPT-GMOS spectroscopic survey , supplemented by additional published spectroscopy , resulting in a final spectroscopic sample of 4148 galaxy spectra—2868 cluster members .
The velocity dispersion of star-forming cluster galaxies is 17 \pm 4 % greater than that of passive cluster galaxies , and the velocity dispersion of bright ( m < m ^ { * } -0.5 ) cluster galaxies is 11 \pm 4 % lower than the velocity dispersion of our total member population .
We find good agreement with simulations regarding the shape of the relationship between the measured velocity dispersion and the fraction of passive vs. star-forming galaxies used to measure it , but we find a small offset between this relationship as measured in data and simulations in which suggests that our dispersions are systematically low by as much as 3 % relative to simulations .
We argue that this offset could be interpreted as a measurement of the effective velocity bias that describes the ratio of our observed velocity dispersions and the intrinsic velocity dispersion of dark matter particles in a published simulation result .
Measuring velocity bias in this way suggests that large spectroscopic surveys can improve dispersion-based mass-observable scaling relations for cosmology even in the face of velocity biases , by quantifying and ultimately calibrating them out .