The slow evolution of dark matter halos from cusp to core naturally produces extended stellar core-like distributions

¹ Instituto de Astrofísica de Canarias, La Laguna, Tenerife, E-38200, Spain
² Departamento de Astrofísica, Universidad de La Laguna, Spain
³ CeBio y Departamento de Ciencias Básicas, Universidad Nacional del Noroeste de la Prov. de Buenos Aires, UNNOBA, CONICET, Roque Saenz Peña 456, Junin, Argentina

^⋆ Corresponding authors: jos@iac.es, itc@iac.es, arplastino@unnoba.edu.ar

Received: 27 May 2025
Accepted: 14 August 2025

Abstract

Motivated by the observation of extended stellar cores in dark matter (DM)-dominated dwarf galaxies, this study investigates a simple mechanism by which stellar cores can form as a result of DM halo expansion. Several non-CDM models predict that the DM distribution thermalizes over time, transforming initially cuspy halos into cores. This transformation weakens the gravitational potential, allowing the stellar component to expand and form diffuse, core-like structures. Using analytical models and adiabatic invariants, we examine stellar systems with purely tangential, purely radial, and isotropic orbits evolving under a slowly changing potential. Across a wide range of initial and final conditions, we find that stellar cores form relatively easily, though their properties depend sensitively on these conditions. Orbit types preserve their nature during the DM halo expansion: tangential and radial orbits remain so, while isotropic orbits remain nearly isotropic in the central regions. Systems with circular orbits develop stellar cores when the initial stellar density logarithmic slope lies between −0.5 and −1.2, whereas radial systems do not form cores. Isotropic systems behave similarly to tangential ones, producing cores that are isotropic in the center but develop increasing radial anisotropy outward; the anisotropy parameter β grows from 0.07 at the core radius to ∼0.5 at three core radii. The theoretical and observational literature suggests initial DM profiles with steep slopes and stellar distributions that are shallower and isotropic at the center. Given these conditions, the mechanism predicts stellar cores with radii at least 40% that of the DM core and inner logarithmic slopes shallower than 0.6.