Journal Article

Measuring the escape velocity and mass profiles of galaxy clusters beyond their virial radius

Ana Laura Serra, Antonaldo Diaferio, Giuseppe Murante and Stefano Borgani

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 412, issue 2, pages 800-816
Published in print April 2011 | ISSN: 0035-8711
Published online March 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2010.17946.x
Measuring the escape velocity and mass profiles of galaxy clusters beyond their virial radius

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The caustic technique uses galaxy redshifts alone to measure the escape velocity and mass profiles of galaxy clusters to clustrocentric distances well beyond the virial radius, where dynamical equilibrium does not necessarily hold. We provide a detailed description of this technique and analyse its possible systematic errors. We apply the caustic technique to clusters with mass M200≥ 1014 h−1 M extracted from a cosmological hydrodynamic simulation of a ΛCDM universe. With a few tens of redshifts per squared comoving megaparsec within the cluster, the caustic technique, on average, recovers the profile of the escape velocity from the cluster with better than 10 per cent accuracy up to r∼ 4r200. The caustic technique also recovers the mass profile with better than 10 per cent accuracy in the range (0.6–4) r200, but it overestimates the mass up to 70 per cent at smaller radii. This overestimate is a consequence of neglecting the radial dependence of the filling function . The 1σ uncertainty on individual escape velocity profiles increases from ∼20 to ∼50 per cent when the radius increases from r∼ 0.1r200 to ∼4r200. Individual mass profiles have 1σ uncertainty between 40 and 80 per cent within the radial range (0.6–4) r200. When the correct virial mass is known, the 1σ uncertainty reduces to a constant 50 per cent on the same radial range. We show that the amplitude of these uncertainties is completely due to the assumption of spherical symmetry, which is difficult to drop. Other potential refinements of the technique are not crucial. We conclude that, when applied to individual clusters, the caustic technique generally provides accurate escape velocity and mass profiles, although, in some cases, the deviation from the real profile can be substantial. Alternatively, we can apply the technique to synthetic clusters obtained by stacking individual clusters: in this case, the 1σ uncertainty on the escape velocity profile is smaller than 20 per cent out to 4r200. The caustic technique thus provides reliable average profiles which extend to regions difficult or impossible to probe with other techniques.

Keywords: gravitation; techniques: miscellaneous; galaxies: clusters: general; cosmology: miscellaneous; dark matter; large-scale structure of Universe

Journal Article.  11302 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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