The tidal evolution of anisotropic subhaloes: A new pathway to creating isotropic and cored satellites

TL;DR

This study uses idealised N-body simulations to demonstrate that the tidal evolution of anisotropic subhaloes can lead to isotropic, cored satellites, highlighting the impact of initial velocity anisotropy on mass loss and structural transformation.

Contribution

It reveals how initial velocity anisotropy influences tidal mass loss and core formation, providing a new pathway for satellite evolution without baryonic feedback.

Findings

Radially anisotropic subhaloes lose more mass and form cores.

Tangentially anisotropic subhaloes are more resistant to disruption.

Tidal stripping causes isotropisation of velocity distributions.

Abstract

It is common practice, both in dynamical modelling and in idealised numerical simulations, to assume that galaxies and/or dark matter haloes are spherical and have isotropic velocity distributions, such that their distribution functions are ergodic. However, there is no good reason to assume that this assumption is accurate. In this paper we use idealised $N$-body simulations to study the tidal evolution of subhaloes that are anisotropic at infall. We show that the detailed velocity anisotropy has a large impact on the subhalo's mass loss rate. In particular, subhaloes that are radially anisotropic experience much more mass loss than their tangentially anisotropic counterparts. In fact, in the former case, the stripping of highly radial orbits can cause a rapid cusp-to-core transformation, without having to resort to any baryonic feedback processes. Once the tidal radius becomes comparable to the radius of the core thus formed, the subhalo is tidally disrupted. Subhaloes that at infall are tangentially anisotropic are far more resilient to tidal stripping, and are never disrupted when simulated with sufficient resolution. We show that the preferential stripping of more radial orbits, combined with re-virialisation post stripping, causes an isotropisation of the subhalo's velocity distributions. This implies that subhaloes that have experienced significant mass loss are expected to be close to isotropic, which may alleviate the mass-anisotropy degeneracies that hamper the dynamical modelling of Milky Way satellites.