To those of you interested in the controversy over the wide binary stars test of MOND, with some papers finding MOND behavior and another excluding MOND at extremely high sigma, let me summarize in a🧵what the discussion among the teams seems to have settled on here at #MONDat40:
Context: You can test the the law of gravity in the low acceleration regime via wide binary stars. This is relevant for Modified Newtonian Dynamics (MOND), which should act at accelerations around and below a0≈1.2×10−10 m/s2, where gravity is stronger than expected from Newton.
Here at #MONDat40, we heard three conflicting WB study results:
Kyu-Hyun Chae and independently Xavier Hernandez find agreement with MOND: a deviation from Newtonian expectations for wide-enough binaries.
Indanil Banik et al. find no MOND-like behavior & report 16 sigma tension.
Planes of satellite galaxies vs. LCDM. There’s a lot of literature on this debate. Since it would be a shame if relevant papers wouldn’t be cited and old arguments needlessly repeated, now is as good a time as any to go over some common methodological issues to look out for.
Planes of satellites are spatially flattened arrangements with coherent kinematics; potentially co-rotating. The latter seems the case for many of the 11 classical MW satellite galaxies. To measure tension with LCDM, one looks for similarly extreme structures in simulated systems
Matching only either the observed flattening or the kinematic coherence does not provide a good estimate of the frequency of analogs in simulations. The intriguing property is that the observe system is both flattened & correlated. ui.adsabs.harvard.edu/abs/2020MNRAS.…ui.adsabs.harvard.edu/abs/2014MNRAS.…
Paper day! "The Milky Way’s Disk of Classical Satellite Galaxies in Light of Gaia DR2", accepted in MNRAS, is out on the arXiv today: arxiv.org/abs/1911.05081
The paper is a whopping 20 pages long, so let me summarize the key takeaways here.
We know since Lynden-Bell (1976) and Kunkel & Demers (1976) that the distribution of known Milky Way satellite galaxies, especially the 11 brightest ones, is highly flattened. They form a disk/plane perpendicular to the MW.
Proper motion (PM) measurements have indicated that many of these satellites also orbit along this plane in the same direction (i.e. co-orbit).
Which is a problem for our standard model of cosmology LCDM, where similarly coherent satellite systems are very rare.
Some thoughts on the recent paper looking at the evolution of planes of satellites in the EAGLE simulation. Specifically, they look at the 11 classical satellites around the Milky Way, i.e. the brightest members of the Vast Polar Structure (VPOS). arxiv.org/abs/1904.02719…
I like that they nicely confirm what I've been finding with many other simulations (they just don't speak about that much). The 11 classical satellites (yellow) have a minor-to-major axis ratio of c/a = 0.18, and highly clustered orbital poles (directions of angular momentum).
In EAGLE, the authors find that only 1% of their simulated systems are as flattened as the observed VPOS, and <1% show as well aligned orbital poles.
I've zoomed in and highlighted those parts of the PDF here.
New preprint on the arXiv today to which I had the honor to contribute a little: "The Magellanic System: the puzzle of the leading gas stream", lead by Thor Tepper-García and @JossBlandHawtho. arxiv.org/abs/1901.05636
Let me try to summarize it in a short thread.
The Magellanic Stream is a gas structure emanating from the Magellanic Clouds that stretches over 150º across the south of the Milky Way, behind the Clouds. In front there's a gaseous feature called the Leading Arm - usually thought to be the Stream's counterpart running ahead.
Many properties of the Stream, as well as a Leading Arm-like feature, can be reproduced in simulations that model a mutual tidal interaction of the Magellanic Clouds during their first-time infall into the Milky Way halo.
Not everybody seems to realize it, but working on Dark Matter alternatives such as MOND requires a lot of knowledge: you have to understand not only that theory, but also it’s more popular alternative, and the successes and challenges faced by both (or more) approaches.
None of these approaches is without issues, but we all tend to weight different lines of evidence differently. Thing is, if you don’t know much about MOND, it is easily dismissed. But that’s a quite subjective reason.
If you work on MOND, though, you are in a constant debate about its feasibility and all the evidence in favor of Dark Matter. That’s not only with colleagues, you are debating yourself all the time.