Astrophysicists Offer Explanation for Origin of One of Milky Way’s Largest Satellites

The satellite galaxy Crater II (or Crater 2) of the Milky Way is located approximately 380,000 light-years away from Earth in the constellation of Crater. This galaxy is extremely cold and exceptionally diffuse, and has low surface brightness. According to new research, Crater II exists thanks to a self-interacting dark matter.

“Since its discovery in 2016, there have been many attempts to reproduce Crater II’s unusual properties, but it has proved very challenging,” said University of California, Riverside’s Professor Hai-Bo Yu.

Dark matter makes up 85% of the Universe’s matter, and it can form a spherical structure under the influence of gravity called a dark matter halo.

Invisible, the halo permeates and surrounds a galaxy like Crater II. The fact that Crater II is extremely cold indicates its halo has a low density.

“Crater II evolved in the tidal field of the Milky Way and experienced tidal interactions with the host galaxy, similar to how Earth’s oceans experience tidal forces due to the gravity of the Moon,” Professor Yu said.

“In theory, the tidal interactions can reduce the density of the dark matter halo.”

However, the latest measurements of the orbit of Crater II around the Milky Way suggest the strength of the tidal interactions is too weak to lower the satellite galaxy’s dark matter density to be consistent with its measurements — if dark matter is made of cold, collisionless particles, as expected from the prevailing cold dark matter theory (CDM).

“Another puzzle is how Crater II could have a large size, as the tidal interactions would reduce the size when the satellite galaxy evolves in the tidal field of the Milky Way,” Professor Yu said.

Professor Yu and his colleagues invoke a different theory to explain Crater II’s properties and origin.

Called self-interacting dark matter (SIDM), it can compellingly explain diverse dark matter distributions.

It proposes that dark matter particles self-interact through a dark force, strongly colliding with one another close to the center of a galaxy.

“Our work shows that SIDM can explain the unusual properties of Crater II,” Professor Yu said.

“The key mechanism is that dark matter self-interactions thermalize the halo of Crater II and produce a shallow density core, that is, the dark matter density is flattened at small radii.”

“In contrast, in a CDM halo, the density would increase sharply toward the center of the galaxy.”

“In SIDM, a relatively small strength of tidal interactions, consistent with what can be expected from measurements of Crater II’s orbit, is sufficient to lower Crater II’s dark matter density, consistent with observations.”

“Importantly, the galaxy size also expands in a SIDM halo, which explains Crater II’s large size.”

“Dark matter particles are just more loosely bound in a cored SIDM halo than in a ‘cuspy’ CDM halo.”

“Our work shows that SIDM is better than CDM at explaining how Crater II originated.”

The study was published in the Astrophysical Journal Letters.

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Xingyu Zhang et al. 2024. Self-interacting Dark Matter Interpretation of Crater II. ApJL 968, L13; doi: 10.3847/2041-8213/ad50cd