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The Cluster-EAGLE project : velocity bias and the velocity dispersion–mass relation of cluster galaxies.

Armitage, Thomas J. and Barnes, David J. and Kay, Scott T. and Bahé, Yannick M. and Dalla Vecchia, Claudio and Crain, Robert A. and Theuns, Tom (2018) 'The Cluster-EAGLE project : velocity bias and the velocity dispersion–mass relation of cluster galaxies.', Monthly notices of the Royal Astronomical Society., 474 (3). pp. 3746-3759.


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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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2017 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:16 November 2017
Date deposited:17 January 2018
Date of first online publication:23 November 2017
Date first made open access:17 January 2018

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