Armitage, T. J.; Barnes, David J.; Kay, Scott T.; Bahé, Yannick M.; Dalla Vecchia, C.; Crain, Robert A.; Theuns, Tom
Bibliographical reference
Monthly Notices of the Royal Astronomical Society, Volume 474, Issue 3, p.3746-3759
Description
We use the Cluster-EAGLE simulations to explore the velocity bias
introduced when using galaxies, rather than dark matter particles, to
estimate the velocity dispersion of a galaxy cluster, a property known
to be tightly correlated with cluster mass. The simulations consist of
30 clusters spanning a mass range 14.0 ≤ log10(M200
c/M⊙) ≤ 15.4, with their sophisticated subgrid
physics modelling and high numerical resolution (subkpc gravitational
softening), making them ideal for this purpose. We find that selecting
galaxies by their total mass results in a velocity dispersion that is
5-10 per cent higher than the dark matter particles. However, selecting
galaxies by their stellar mass results in an almost unbiased (<5 per
cent) estimator of the velocity dispersion. This result holds out to z =
1.5 and is relatively insensitive to the choice of cluster aperture,
varying by less than 5 per cent between r500 c and r200
m. We show that the velocity bias is a function of the time spent
by a galaxy inside the cluster environment. Selecting galaxies by their
total mass results in a larger bias because a larger fraction of objects
have only recently entered the cluster and these have a velocity bias
above unity. Galaxies that entered more than 4 Gyr ago become
progressively colder with time, as expected from dynamical friction. We
conclude that velocity bias should not be a major issue when estimating
cluster masses from kinematic methods.
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