To β or not to β: can higher order Jeans analysis break the mass–anisotropy degeneracy in simulated dwarfs?

Abstract

We test a non-parametric higher-order Jeans analysis method, GravSphere, on 32 simulated dwarf galaxies comparable to classical Local Group dwarfs like Fornax. The galaxies are selected from the APOSTLE suite of cosmological hydrodynamics simulations with Cold Dark Matter (CDM) and Self-Interacting Dark Matter (SIDM) models, allowing us to investigate cusps and cores in density distributions. We find that, for CDM dwarfs, the recovered enclosed mass profiles have a bias of no more than 10 per cent, with a 50 per cent scatter in the inner regions and a 20 per cent scatter near the half-light radius, consistent with standard mass estimators. The density profiles are also recovered with a bias of no more than 10 per cent and a scatter of 30 per cent in the inner regions. For SIDM dwarfs, the mass and density profiles are recovered within our 95 per cent confidence intervals, but are biased towards cuspy dark matter distributions. This is mainly due to a lack of sufficient constraints from the data. We explore the sources of scatter in the accuracy of the recovered profiles and suggest a χ2 statistic to separate successful models from biased ones. Finally, we show that the uncertainties on the mass profiles obtained with GravSphere are smaller than those for comparable Jeans methods, and that they can be further improved if stronger priors, motivated by cosmological simulations, are placed on the velocity anisotropy. We conclude that GravSphere is a promising Jeans-based approach for modelling dark matter distributions in dwarf galaxies.

Dark matter density profiles in dwarf galaxies: linking Jeans modelling systematics and observation

“…However, for data sets with ࣠1000 stars from both parameter sets -on par with the existing dwarf galaxy measurements -we cannot determine whether the underlying halo has an inner slope of γ = 0 or γ = 1 in a statistically significant manner, consistent with previous Jeans modelling-based results in the literature (e.g. Walker et al 2009;Read, Walker & Steger 2018;Genina et al 2020). We further note that, for the smaller sample sizes, the fact that the posterior γ distributions are unconstrained implies that the results are highly sensitive to the choice of priors on γ , and we therefore choose to present the full posterior distributions rather than to quote recovered median values or quantiles.…”

“…The degeneracy between the velocity anisotropy, β(r), and the enclosed mass profile, M(<r), is a known complication in Jeans modelling (e.g. Merrifield & Kent 1990;Wilkinson et al 2002;Lokas & Mamon 2003;De Lorenzi et al 2009;Read & Steger 2017;Genina et al 2020). It can be seen from equation (12) that β(r) and σ 2 r (r) are degenerate with each other, which, combined with equation ( 8), implies that β(r) is degenerate with M(<r).…”

“…While this was chosen for demonstrative purposes, for CDM haloes, β has been found to be consistent with 0 for dwarf galaxies in the APOSTLE simulations (Genina et al 2020) and in the central regions of isolated dwarfs from the FIRE simulations (González-Samaniego et al 2017), further motivating our choice.…”

“…Bonnivard, Maurin & Walker 2016b;Ichikawa et al 2017Ichikawa et al , 2018Horigome et al 2020), the effect of assuming equilibrium for systems which are not in equilibrium (El-Badry et al 2017), the effect of nonspherical models (Bonnivard et al 2015a;Hayashi et al 2016;Klop et al 2017), the degeneracy between the enclosed mass and velocity anisotropy (e.g. Merrifield & Kent 1990;Wilkinson et al 2002;Lokas & Mamon 2003;De Lorenzi et al 2009;Read & Steger 2017;Genina et al 2020), and the presence of potentially large fractions of binary stars in the dwarf galaxies (e.g. McConnachie & Côté 2010;Minor et al 2010;Geha et al 2013;Spencer et al 2017Spencer et al , 2018Minor et al 2019).…”

The surprising accuracy of isothermal Jeans modelling of self-interacting dark matter density profiles

“…In general, it is non-trivial to infer the DM distribution from observations (e.g. Kowalczyk et al 2013;Harvey et al 2018;Oman et al 2019;Genina et al 2020;He et al 2020) and it is important to test how these difficulties impact the inferred SIDM cross-section when isothermal Jeans modelling is applied to observations. This was recently done in the context of galaxy cluster observations by Sagunski et al (2020), who found that the inferred cross-section from isothermal Jeans modelling using mock observations of simulated clusters was in agreement with the crosssection used in the simulations.…”