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arXiv · 1708.00508

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

Thomas Joshua Armitage·David. J. Barnes·Scott. T. Kay·Yannick M. Bahé·Claudio Dalla Vecchia·Robert A. Crain·Tom Theuns
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Abstract

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 $14.0 \le \log_{10}(M_{\rm 200c}/\mathrm{M_\odot}) \le 15.4$, with their sophisticated sub-grid physics modelling and high numerical resolution (sub-kpc 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 $r_{\rm 500c}$ and $r_{\rm 200m}$. 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 $4 \, \mathrm{Gyr}$ 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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Thomas Joshua Armitage, David. J. Barnes, Scott. T. Kay, Yannick M. Bahé, Claudio Dalla Vecchia, Robert A. Crain, Tom Theuns. 2017-11-21. The Cluster-EAGLE project: velocity bias and the velocity dispersion - mass relation of cluster galaxies. https://doi.org/10.1093/mnras%2Fstx3020

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