The nature of core formation in dark matter haloes: Adiabatic or impulsive?

Abstract

It is well established that the central deficit of dark matter (DM) observed in many dwarf galaxies disagrees with the cuspy DM haloes predicted in the collisionless and cold DM (CDM) model. Plausible solutions to this problem are based on an effective energy deposition into the central halo with an origin that is either based on baryonic physics (e.g. supernova-driven gas blowouts; SNF) or on new DM physics (e.g. self-interacting DM, SIDM). We argue that the fundamental difference between the two is whether the process is impulsive or adiabatic, and explore novel ways to distinguishing them by looking at the response of stellar orbits. We perform idealized simulations of tracers embedded in a 1.48× 1010M⊙ spherical halo, and model the creation of a ∼1kpc DM core in SIDM with σ T/mχ = 2 cm2g-1, and in SNF injecting energy into the halo via a sudden mass removal equivalent to O(10%) of the halo's potential energy. Choosing idealized initial orbital configurations for the tracers, we find that radial actions are conserved (changed) in the SIDM (impulsive) case. The adiabaticity of the SIDM case prevents tracers from changing their orbital family during core formation, whereas SNF separates tracers of initially the same family to a variety of orbits. We show that these key features remain in a cosmological halo, albeit for a few dynamical times. The number density and velocity dispersion profile of a Plummer sphere with r1/2 = 500 pc change only marginally under adiabatic core formation, whereas SNF causes a substantial expansion of the sphere driving it out of Jeans equilibrium. Our results point towards promising ways of differentiating adiabatic from impulsive core formation.