Dwarf spheroidal galaxies around the Milky Way display a puzzling range of sizes and velocity dispersions. The standard explanation: different star formation histories produced different structures. Compact dwarfs formed their stars in concentrated bursts. Extended dwarfs formed them more gradually. The diversity reflects the diversity of origins.
Peñarrubia and Nadler (arXiv:2603.00257) propose the opposite. The diversity reflects different positions along a single evolutionary trajectory. All dwarf spheroidals converge toward the same structural attractor — a characteristic state where the stellar half-light radius matches the dark matter halo's scale radius and the velocity dispersion reaches half the peak circular velocity. The diversity isn't in where they started. It's in how far they've traveled.
The mechanism is stochastic heating by dark matter substructure. Dark subhalos — small clumps of dark matter orbiting within the larger halo — randomly perturb stellar orbits as they pass through the dwarf galaxy. Each perturbation adds energy to the stellar system, gradually inflating it. The process is irreversible: stars gain energy stochastically but can't lose it the same way. Over billions of years, the stellar system expands toward the attractor state regardless of its initial configuration.
Tidal interactions with the Milky Way accelerate the process without changing the destination. A dwarf that has orbited closer to the galaxy, experiencing stronger tides, reaches the attractor faster. One in a wider orbit gets there more slowly. The observed range of sizes and dispersions maps onto different elapsed times along the same convergent path.
The structural insight: origin and trajectory are confounded observables. Two dwarfs that look different today could have formed identically and evolved differently, or formed differently and converged to different points along the same track. The attractor breaks the degeneracy — it predicts that isolated, early-quenched dwarfs (which formed their stars and stopped) should be systematically larger than satellite dwarfs at the same luminosity, because they've had more undisturbed time to inflate. The prediction is testable.
What looks like diversity of formation is diversity of evolution. The structure you observe doesn't tell you where the galaxy came from. It tells you where the galaxy is going.
Peñarrubia & Nadler, "A dynamical attractor in the evolution of dwarf spheroidal galaxies," arXiv:2603.00257 (2026).