How did Andromeda's dwarf galaxies form? Hubble Telescope finds more questions than answers

[Admin]

A family portrait of the Andromeda galaxy and its orbiting dwarf satellites raises questions about how galaxies evolve.

How did Andromeda's dwarf galaxies form? Hubble Telescope finds more questions than answers : Read more

 

[Torbjorn Larsson]

The survey also revealed that half of Andromeda's dwarf galaxies orbit in a unique, flat plane, all moving in the same direction — a configuration not observed around other galaxies, including our own.

Their series of references dips back to the original plane discovery paper to point out a recent upset as a potential culprit, specifically a collision with the nearby and seemingly collision stripped M32 galaxy:

In the original discovery paper, McConnachie & Irwin (2006b) provided a
detailed discussion of possible dynamical and cosmological explanations for the M31 lopsided satellite distribution; we refer the reader to this paper for more details. Among several of these explanations, one is that much of the satellite population was accreted recently and has not had time to fully phase mix. This scenario is broadly consistent with various pictures of M31ʼs recent accretion history (e.g., Ferguson & Mackey 2016; D’Souza & Bell 2018; McConnachie et al. 2018; Mackey et al. 2019) and could reconcile the extreme properties of M31 with respect to the broader galaxy population.

From the article of Cooper:

the phenomenon is actually a prediction of an alternative theory of gravity, Modified Newtonian Dynamics or MOND,

And it points back to an earlier Cooper article with a discussion that also mentioned MOND, but did not have the recent upset hypothesis as explanation for the "why now" question. This time neither the paper nor any other references mentions MOND, which seems to be a posed as a candidate rival hypothesis, but really isn't. Mostly astronomers may have the luxury to forego that MOND does not pass relativistic tests and is cherry picked to pass a very limited set of other dynamical tests, and very few remains to do so.

Since Milgrom's original proposal, MOND has seen some successes. It is capable of explaining several observations in galaxy dynamics,[7][8] a number of which can be difficult for Lambda-CDM to explain.[9][10] However, MOND struggles to explain a range of other observations, such as the acoustic peaks of the cosmic microwave background and the matter power spectrum of the large scale structure of the universe. Furthermore, because MOND is not a relativistic theory, it struggles to explain relativistic effects such as gravitational lensing and gravitational waves. Finally, a major weakness of MOND is that all galaxy clusters, including the famous Bullet cluster, show a residual mass discrepancy even when analyzed using MOND.[7][11][12]

Since MOND is dying, it is likely by now doing general readers a disservice to mention it. Of course, people will differ in their judgment on this point, but it bears valid criticism: MOND is a small fringe and accumulates papers rejecting it (by supporting the successful cosmological LCDM) all the time.