A dynamical attractor in the evolution of dwarf spheroidal galaxies

TL;DR

This study uses N-body simulations to reveal a dynamical attractor in dwarf spheroidal galaxies, linking their structural properties to dark matter halo characteristics and evolutionary processes like internal heating and tides.

Contribution

It identifies a universal dynamical attractor state in dSphs influenced by dark matter subhaloes, explaining their structural diversity and evolutionary paths.

Findings

Dwarf spheroidals reach a dynamical attractor characterized by specific size and velocity dispersion ratios.

Tidal stripping accelerates the evolution toward this attractor state.

Mass-luminosity relations are consistent with abundance matching but with systematically lower halo masses.

Abstract

We use controlled $N$-body experiments to study the dynamical evolution of dwarf spheroidal galaxies (dSphs) embedded in dark-matter (DM) haloes containing a large population of dark subhaloes. We show that stellar orbits subject to stochastic force fluctuations irreversibly gain energy and expand toward a dynamical attractor characterized by a stellar half-light radius $r_{\rm half} \approx r_{\rm max}$ and a velocity dispersion $\sigma \approx 0.5\,v_{\rm max}$, where $v_{\rm max}$ is the peak circular velocity of the host halo at radius $r_{\rm max}$. This state is reached both in isolation and under tidal stripping, although tidal mass loss significantly accelerates the evolution. Assuming that the Milky Way (MW) dSphs have reached this state, we find that the inferred halo masses collapse onto narrow sequences as a function of $r_{\rm half}$. Under this assumption, MW satellites with $r_{\rm half} \lesssim 1\,\mathrm{kpc}$ follow the tidal tracks of cuspy haloes, while larger systems deviate in a manner consistent with cored DM profiles. Moreover, the mass--luminosity relation follows the slope expected from abundance matching, but with halo masses systematically lowered from their peak values at fixed luminosity. These results suggest that the structural diversity of dSphs is largely an evolutionary outcome driven by internal heating and tides, rather than by the conditions of star formation. This framework predicts that isolated, early-quenched dSphs should have systematically larger sizes than satellites, a prediction testable with upcoming surveys.