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 collision-less 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 idealised simulations of tracers embedded in a $1.48\times 10^{10}{\rm M}_{\odot}$ spherical halo, and model the creation of a $\sim1$kpc DM matter core in SIDM with $\sigma_T/m_{\chi} = 2\,{\rm cm}^2{\rm g}^{-1}$ and through SNF by impulsively removing a mass equivalent to $\mathcal{O}(10\%)$ of the halo's potential energy. Choosing idealised 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 timescales. The number density and velocity dispersion profile of a Plummer sphere with $r_{1/2} = 500$ 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.