Satellite Galaxy Crater II The Milky Way's Crater 2 (or Crater 2) is located in the constellation Crater, about 380,000 light-years from Earth. This galaxy is very cold, very diffuse, and has a low surface brightness. According to a new study, Crater 2 exists thanks to self-interacting dark matter.
Location of Crater II and other Milky Way moons at distances between 100,000 and 400,000 parsecs from the Sun. Image courtesy of Torrealba others., doi: 10.1093/mnras/stw733.
” discovery “Since 2016, there have been numerous attempts to recreate the anomalous properties of Crater II, but these have proven extremely difficult,” said Haibo Yu, a professor at the University of California, Riverside.
Dark matter makes up 85% of the matter in the universe, and under the influence of gravity it can form spherical structures called dark matter halos.
Invisible halos permeate and surround galaxies like Crater II, and the fact that Crater II is so cold indicates that its halo is low density.
“Crater II developed within the Milky Way's tidal field and experienced tidal interactions with its host galaxy, similar to how Earth's oceans experience tidal forces due to the Moon's gravity,” Professor Yu said.
“In theory, tidal interactions can reduce the density of dark matter haloes.”
However, recent measurements of Crater II's orbit around the Milky Way suggest that if dark matter is made of cold, collisionless particles, as predicted by the prevailing cold dark matter theory (CDM), the strength of the tidal interactions is too weak to reduce the dark matter density in the satellite galaxy enough to match the measurements.
“Another mystery is why Crater II is so large when, as the satellite galaxy evolves in the Milky Way's tidal field, tidal interactions should reduce its size,” said Professor Yu.
Professor Yu and his colleagues put forward a different theory to explain the properties and origin of Crater II.
This is called self-interacting dark matter (SIDM) and can plausibly explain the diverse distribution of dark matter.
The theory proposes that dark matter particles self-interact through the dark force and collide with each other forcefully near the center of the galaxy.
“Our study shows that SIDM can explain the anomalous properties of Crater II,” said Professor Yu.
“The key mechanism is that dark matter self-interaction thermalizes Crater II's halo and creates a shallow dense core, i.e. the dark matter density flattens out at a small radius.”
“In contrast, in a CDM halo, the density would increase rapidly towards the center of the galaxy.”
“In SIDM, the strength of the relatively weak tidal interaction, consistent with what is expected from measurements of Crater II's orbit, is sufficient to reduce the dark matter density in Crater II, consistent with observations.”
“Importantly, the size of galaxies is also increasing within the SIDM halo, which could explain the large size of Crater II.”
“Dark matter particles are only more loosely bound in the cored SIDM halo than in the pointed CDM halo.”
“Our study shows that SIDM is a better option than CDM for explaining the origin of Crater II.”
of study Published in Astrophysical Journal Letters.
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Zhang Xingyu others2024. Interpreting self-interacting dark matter in Crater II. Apu JL 968, L13; doi: 10.3847/2041-8213/ad50cd
Source: www.sci.news
