🔭 Dwarf galaxies are doomed... to look alike

Spheroidal dwarf galaxies are small galaxies that orbit giants like the Milky Way. Though they display very diverse appearances, a recent study reveals they share the same cosmic fate. As if guided by an invisible magnet, they all converge toward a stable and predictable shape.

This solves a long-standing puzzle. Dark matter models predict that its density should increase sharply at the center of these galaxies, forming a cusp. But observations often show a flatter distribution, like a core. This discrepancy between prediction and observation has puzzled astronomers for years.


Jorge Peñarrubia and Ethan Nadler from the University of Edinburgh and the University of California, San Diego propose an explanation. According to them, these galaxies always evolve toward the same equilibrium state. Regardless of their initial conditions, they end up adopting a similar configuration.

How does a galaxy reach this state? The stars inside are constantly jostled by small force fluctuations, like in a pinball machine. These disturbances come from dark matter subhalos, invisible clumps that give energy to the stars and widen their orbits.

This process speeds up when the small galaxy passes near a large one, like the Milky Way. Tidal forces pull on its outer layers, further expanding its stellar envelope. Even in isolation, a dwarf eventually reaches a predictable shape.

The researchers tested their model with computer simulations. They tracked billions of particles of stars and dark matter subhalos over billions of years. Result: a dwarf galaxy must lose more than 99% of its dark matter before it loses a significant number of its stars, making it a very robust system. Real data from galaxies around the Milky Way match these predictions.

This discovery changes our perspective. The diversity of spheroidal dwarfs is not due to different origins, but to a snapshot at different stages of their evolution toward the same equilibrium point.

Dark matter

Dark matter is an invisible substance that makes up about 85% of the matter in the Universe.

It cannot be seen directly because it neither emits nor absorbs light. Its existence is inferred from its gravitational effects on stars and galaxies. For example, galaxies spin so fast that they would fly apart without this invisible mass.

Dark matter forms halos around galaxies, and in spheroidal dwarfs it completely dominates the mass. Understanding its nature is one of the great challenges of modern cosmology.