Made-to-measure dark matter haloes, elliptical galaxies and dwarf galaxies in action coordinates

Using realistic cosmological simulations of Milky Way sized haloes, we study their dynamical state and the accuracy of inferring their mass profiles with steady-state models of dynamical tracers. We use a new method that describes the phase-space distribution of a steady-state tracer population in a spherical potential without any assumption regarding the distribution of their orbits. Applying the method to five haloes from the Aquarius ΛCDM N-body simulation, we find that dark matter particles are an accurate tracer that enables the halo mass and concentration parameters to be recovered with an accuracy of 5%. Assuming a potential profile of the NFW form does not significantly affect the fits in most cases, except for halo A whose density profile differs significantly from the NFW form, leading to a 30% bias in the dynamically fitted parameters. The existence of substructures in the dark matter tracers only affects the fits by ∼ 1%. Applying the method to mock stellar haloes generated by a particle-tagging technique, we find the stars are farther from equilibrium than dark matter particles, yielding a systematic bias of ∼ 20% in the inferred mass and concentration parameter. The level of systematic biases obtained from a conventional distribution function fit to stars is comparable to ours, while similar fits to DM tracers are significantly biased in contrast to our fits. In line with previous studies, the mass bias is much reduced near the tracer half-mass radius.

Section: Fitting the Halo Potential With Dm As Tracersmentioning

confidence: 96%

“…There are more flexible DFs that can improve over the one assumed in Wang et al (2015), for example, by allowing for varying anisotropies (e.g. Wojtak et al 2008;Williams & Evans 2015b). More generally, there may exist a true model that describes well the DF of the tracers.…”

mentioning

confidence: 99%

The orbital PDF: the dynamical state of Milky Way sized haloes and the intrinsic uncertainty in the determination of their masses

Han¹,

Wang²,

Cole³

et al. 2015

“…Williams & Evans (2015) then showed that a model with two power-law regimes is generated by a DF of the form…”

Section: Distribution Function and Gravitational Potentialmentioning

confidence: 99%

Haloes light and dark: dynamical models of the stellar halo and constraints on the mass of the Galaxy

Williams

Evans

2015

Self Cite

We develop a flexible set of action-based distribution functions (DFs) for stellar haloes. The DFs have five free parameters, controlling the inner and outer density slope, break radius, flattening and anisotropy respectively. The DFs generate flattened stellar haloes with a rapidly varying logarithmic slope in density, as well as a spherically aligned velocity ellipsoid with a long axis that points towards the Galactic centre -all attributes possessed by the stellar halo of the Milky Way. We use our action-based distribution function to model the blue horizontal branch stars extracted from the Sloan Digital Sky Survey as stellar halo tracers in a spherical Galactic potential. As the selection function is hard to model, we fix the density law from earlier studies and solve for the anisotropy and gravitational potential parameters. Our best fit model has a velocity anisotropy that becomes more radially anisotropic on moving outwards. It changes from β ≈ 0.4 at Galactocentric radius of 15 kpc to ≈ 0.7 at 60 kpc. This is a gentler increase than is typically found in simulations of stellar haloes built from the mutiple accretion of smaller systems. We find the potential corresponds to an almost flat rotation curve with amplitude of ≈ 200 kms −1 at these distances. This implies an enclosed mass of ≈ 4.5 × 10 11 M within a spherical shell of radius 50 kpc.

“…When proposing the DFs, one is free to choose the integrals of motion, to be either classical integrals such as energy and angular momentum or the more theoretically appealing actions (see e.g. Posti et al 2015;Williams & Evans 2015a, for such recent models on halo stars).…”

Section: Introductionmentioning

confidence: 99%

The orbital PDF: general inference of the gravitational potential from steady-state tracers

Han

Wang

Cole

et al. 2015

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details.