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How to break the density-anisotropy degeneracy in spherical stellar systems

Read, Justin and Steger, P. (2017) How to break the density-anisotropy degeneracy in spherical stellar systems Monthly Notices of the Royal Astronomical Society.

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Abstract

We present a new non-parametric Jeans code, GravSphere, that recovers the density ρ(r) and velocity anisotropy β(r) of spherical stellar systems, assuming only that they are in a steady-state. Using a large suite of mock data, we confirm that with only line-of-sight velocity data, GravSphere provides a good estimate of the density at the projected stellar half mass radius, ρ(R1/2), but is not able to measure ρ(r) or β(r), even with 10,000 tracer stars. We then test three popular methods for breaking this ρ − β degeneracy: using multiple populations with different R1/2; using higher order ‘Virial Shape Parameters’ (VSPs); and including proper motion data. We find that two populations provide an excellent recovery of ρ(r) in-between their respective R1/2. However, even with a total of ∼7, 000 tracers, we are not able to well-constrain β(r) for either population. By contrast, using 1000 tracers with higher order VSPs we are able to measure ρ(r) over the range 0.5 < r/R1/2 < 2 and broadly constrain β(r). Including proper motion data for all stars gives an even better performance, with ρ and β well-measured over the range 0.25 < r/R1/2 < 4. Finally, we test GravSphere on a triaxial mock galaxy that has axis ratios typical of a merger remnant, [1: 0.8: 0.6]. In this case, GravSphere can become slightly biased. However, we find that when this occurs the data are poorly fit, allowing us to detect when such departures from spherical symmetry become problematic.

Item Type: Article
Divisions : Faculty of Engineering and Physical Sciences > Physics
Authors :
NameEmailORCID
Read, Justinj.read@surrey.ac.ukUNSPECIFIED
Steger, P.UNSPECIFIEDUNSPECIFIED
Date : 19 July 2017
Identification Number : 10.1093/mnras/stx1798
Copyright Disclaimer : © 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
Uncontrolled Keywords : Galaxies: haloes; Cosmology: dark matter; Galaxy: globular clusters: general; Galaxies: clusters: general, proper motions; Methods: miscellaneous
Depositing User : Clive Harris
Date Deposited : 15 Aug 2017 13:38
Last Modified : 15 Aug 2017 13:38
URI: http://epubs.surrey.ac.uk/id/eprint/841924

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