And as the article explains, none of the explanations the mainstream has come up with solve the problem.Satellite Galaxies All in a Row—How So?
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Large galaxies like our Milky Way are surrounded by swarms of smaller satellite galaxies. In simulations of galaxy formation made using the standard model of cosmology, the Lambda Cold Dark Matter model (ΛCDM), these satellite galaxies end up orbiting mostly randomly. That is to say, you generally wouldn’t expect to find multiple satellite galaxies having orbits similar to each other and orbiting in the same direction. However, observations of the Milky Way, the Andromeda Galaxy (M31), and the relatively nearby galaxy Centaurus A have revealed the presence of large, coherent planes of satellite galaxies orbiting around all three. And not only are many of the satellites orbiting in a plane, but in each case the majority of the galaxies in the plane are also orbiting in the same direction. (See Figure 2 for an overview of the various planar structures around the three galaxies.)
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The Problem: Comparison to Simulations
The interesting part of all this is that in the best simulations we have right now such tightly correlated satellite galaxy planes are extremely rare. To measure just how rare Pawlowski searched two large simulations, the Exploring the Local Volume in Simulations (ELVIS) suite and Millennium-II, for galaxies comparable to the Milky Way, M31, and Centaurus A. He then looked at their satellite galaxies to see how they compared to what we observe in nature.
The result? Planes of satellite galaxies turned out to be extremely unlikely, showing up in a mere 0.1% (Millennium-II) to 0.5% (ELVIS) of galaxies. This might not be a problem if we only knew of one such structure, but we now know of three in the local universe which is significantly more than we’d expect to find based on these simulations. This discrepancy is known as the Planes of Satellite Galaxies Problem, and has been suspected for a number of years now (see for instance this earlier Astrobite).
Wonder if the authors are aware of EU and the work of Arp?So there we have it: dwarf satellite galaxies are too orderly. Pawlowski concludes that at this point it remains an open question how these planes of satellite galaxies are formed.
Apparently not yet …
https://astrobites.org/2021/08/31/satel ... n-a-row-2/
Satellite Galaxies — Still All in a Row!