We tested for the first time whether such an arrangement is common for analogs in cosmological simulations (TNG & EAGLE). Turns out that no, the observed system is highly unexpected. At a level of 0.3%! Even considering the look-elsewhere effect (that analogs might be weird in some other sense). - ThreadSky

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8minutesold.bsky.social • 48 days ago

We tested for the first time whether such an arrangement is common for analogs in cosmological simulations (TNG & EAGLE). Turns out that no, the observed system is highly unexpected. At a level of 0.3%! Even considering the look-elsewhere effect (that analogs might be weird in some other sense).

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8minutesold.bsky.social•48 days ago

Interestingly the observed M31 system (open black circle) also shows the most population-wide asymmetry. Analogs in the simulations that are similarly extreme (black crosses) are cases where fewer satellites (smaller cone population, vertical) are more clustered (smaller opening angles, horizontal).

8minutesold.bsky.social•48 days ago

So, either M31 is an extreme outlier to cosmological expectations, or an outright challenge to the cold dark matter model of cosmology. What speaks for the latter is that the unexpected lopsidedness is on top of the existence of a plane of satellites around M31, which already challenges the model.

8minutesold.bsky.social•48 days ago

I am super thrilled that I can finally share Jamie's fantastic work publicly, after we've been working on it for two years. Jamie will present this at a few upcoming conferences, too, and we are working on follow-up projects already … though he also needs to focus on wrapping up his PhD soon.

8minutesold.bsky.social•48 days ago

If you want to know more, read the article or our related Nature Astronomy research briefing written for a broader audience: https://www.nature.com/articles/s41550-025-02481-2

Of course we have also prepared a press release: https://www.aip.de/news/andromedas-asymmetrical-challenge-cosmology

And feel free to contact me if you have any questions!

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