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

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

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 ≤ 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.

Citation

Armitage, T. J., Barnes, D. J., Kay, S. T., Bahé, Y. M., Dalla Vecchia, C., Crain, R. A., & Theuns, T. (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), 3746-3759. https://doi.org/10.1093/mnras/stx3020

• Journal Article Type: Article
• Acceptance Date: Nov 16, 2017
• Online Publication Date: Nov 23, 2017
• Publication Date: Mar 1, 2018
• Deposit Date: Jan 16, 2018
• Publicly Available Date: Jan 17, 2018
• Journal: Monthly Notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Royal Astronomical Society
• Peer Reviewed: Peer Reviewed
• Volume: 474
• Issue: 3
• Pages: 3746-3759
• DOI: https://doi.org/10.1093/mnras/stx3020
• Public URL: https://durham-repository.worktribe.com/output/1341334
• Related Public URLs: https://arxiv.org/abs/1708.00508