The phase-space structure of tidally stripped haloes

Abstract

We propose a new method for generating equilibrium models of spherical systems of collisionless particles that are finite in extent, but whose central regions resemble dark matter halos from cosmological simulations. This method involves iteratively removing unbound particles from a Navarro-Frenk-White profile truncated sharply at some radius. The resulting models are extremely stable, and thus provide a good starting point for N -body simulations of isolated halos. We provide a code to generate such models for NFW and a variety of other common density profiles. We then develop an analytic approximation to this truncated distribution function. Our method proceeds by analogy with the King model, truncating and shifting the original distribution function of an infinitely extended Navarro-Frenk-White profile in energy space. We show that the density profiles of our models closely resemble the tidally truncated density profiles seen previously in studies of satellite evolution. Pursuing this analogy further with a series of simulations of tidal mass loss, we find that our models provide a good approximation to the full distribution function of tidally stripped systems, thus allowing theoretically motivated phase-space calculations for such systems.

Power-law Pseudo-phase-space Density Profiles of Dark Matter Halos: A Fluke of Physics?

“…The quantity ρ/σ 3 turned out to be a power law in radius over about 3 decades, despite the fact that neither density nor the velocity dispersion are power laws, but in fact significantly deviate from a scale-free form. Since then, it has been confirmed by a number of studies that dark matter halos formed in cosmological N-body simulations (Drakos et al 2017;Butsky et al 1 The equation of state referred to here and other similar contexts in the literature is not the same as the relativistic equation of state, w = 0, for cold dark matter, used in the cosmological context. Here, the equation of state refers to the relation between mass density ρ, and dynamically generated "pressure" P of dark matter, where P = ρσ 2 , and σ is the velocity dispersion.…”

Mass-loss in tidally stripped systems: the energy-based truncation method

“…The satellite halo was modeled as an NFW halo with concentration c = 10. Initial conditions were generated using the publicly available code icicle (Drakos et al 2017), which assigns positions and velocities to each particle by sampling the mass profile and DF of a specified profile. Since the mass of NFW profiles diverges with increasing radius, particles were generated within a specified radius, rcut = crs, and unbound particles were iteratively removed.…”

“…Tidal stripping in minor mergers, for instance, is frequently studied using N -body simulations of a satellite halo evolving within a static host halo (e.g. Hayashi et al 2003;Kazantzidis et al 2004;Boylan-Kolchin & Ma 2007;Kampakoglou & Benson 2007;Peñarrubia et al 2008aPeñarrubia et al ,b, 2009Choi et al 2009;Peñarrubia et al 2010;Drakos et al 2017;Ogiya et al 2019;Delos 2019a). These idealized simulations provide a way to test the physical processes that govern subhalo evolution and determine the long-term fate of subhaloes as they lose mass, but they normally require interpretation with analytic models in order to extrapolate the results to the full range of situations relevant in calculations of annihilation rates, substructure lensing and semi-analytic models of galaxy evolution.…”

“…Overall, given a clear definition or way of distinguishing bound and unbound particles (which itself is problematic), an analytic model of mass loss needs to explain and predict three things: (1) which particles are lost, or in what order particles are lost from a system (what criterion and which particle properties are relevant) (2) how the structure responds to mass loss and how the properties of individual particles change as mass is lost, and (3) the overall mass-loss rate. In Drakos et al (2017), hereafter Paper I, we addressed the first two points, proposing that particles are lost based on their binding energy, but did not estimate the mass-loss rate. The main goal of this paper is to provide an estimate for the mass-loss rate, and show that it matches simulations without adjustment of free parameters.…”

Impact of substructure on local dark matter searches

“…In field halos (those not bound within larger halos), the NFW profile would be truncated at the virial radius [80][81][82][83], defined such that the mean density of the halo inside the virial radius is a factor of 200 larger than the critical density (see e.g. [25]).…”