How did a satellite galaxy of the Milky Way form? Physicists offer an explanation

Located about 380,000 light-years from Earth, Crater 2 is one of the Milky Way’s largest satellite galaxies. Crater 2 is extremely cold and has slow-moving stars and low surface brightness. How this galaxy formed remains unclear.

“Many attempts have been made to reproduce the unusual features of Crater 2 since its discovery in 2016, but it has proven challenging,” said Hai-Bo Yu, professor of physics and astronomy at the University of California, Riverside, whose team now offers an explanation for the origin of Crater 2 in an article titled “Self-interacting dark matter interpretation of Crater II” published in The Astrophysical diary letters.

A satellite galaxy is a smaller galaxy orbiting a larger host galaxy. Dark matter makes up 85% of the matter in the universe and can form a spherical structure called a dark matter halo under the influence of gravity. The invisible halo penetrates and surrounds a galaxy like Crater 2. The fact that Crater 2 is extremely cold indicates that the halo has a low density.

Yu explained that Crater 2 developed 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 moon’s gravity. In theory, the tidal interactions can reduce the density of the dark matter halo.

However, the latest measurements of Crater 2’s orbit around the Milky Way suggest that the strength of the tidal interactions is too weak to lower the density of dark matter in the satellite galaxy to be consistent with its measurements – if dark matter consists of cold, collisionless particles. , as expected based on the prevailing cold dark matter theory, or CDM.

“Another puzzle is how crater 2 could have a large size, because the tidal interactions would reduce its size as the satellite galaxy evolves in the Milky Way’s tidal field,” Yu said.

Yu and his team use a different theory to explain Crater 2’s properties and origins. It’s called self-interacting dark matter, or SIDM, and it can convincingly explain several dark matter distributions. It proposes that dark matter particles interact through a dark force, colliding strongly with each other near the center of a galaxy.

“Our work shows that SIDM can explain the unusual features of crater 2,” Yu said. “The main mechanism is that the interactions between dark matter and the halo of Crater 2 thermalize the halo of Crater 2 and produce a shallow-density core, that is, the density of dark matter is flattened at small radii. In a CDM halo, on the other hand, the density would increase sharply.” to the center of the Milky Way.”

According to Yu, in SIDM a relatively small strength of the tidal interactions, consistent with what would be expected from measurements of Crater 2’s orbit, is sufficient to reduce the dark matter density in Crater 2, consistent with observations.

“Importantly, the size of galaxies also expands in a SIDM halo, which explains the large size of Crater 2,” Yu said. “Dark matter particles are simply 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 2 formed.”

Yu was joined in the research by Daneng Yang of UCR, and Xingyu Zhang and Haipeng An of Tsinghua University in China.