Paper day! I'm thrilled about the extensive new work by Salvatore Taibi, postdoc in my group (incl S. Khoperskov, @GalacticRAVE @satellitegalaxy). We study the observed Vast Polar Structure (VPOS) of the Milky Way and end up with a weirdly puzzling result.
arxiv.org/abs/2310.13521
There are many papers looking at the planes of satellite galaxies problem via cosmological simulations. This approach is of limited utility to learn about the origin of the observed sat. planes. We instead focus on empirically investigating the MW’s satellite plane, the VPOS.

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.

After three days of #MONDat40 in the UK, I'm ready for some MONDy Python: Modified Newtonian Dynamics according to Monty Python memes. Bringing up MOND at a cosmology conference.

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

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.

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).

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.

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.

https://twitter.com/DudeDarkmatter/status/1022803635862949888

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.

Finally read this nice preprint by Forero-Romero & Arias: arxiv.org/abs/1805.03188…
They use an interesting approach to measure the overall asphericity of the distribution of the top 11 to 15 satellites around the MW and M31, and use it to compare to cosmological simulations. One of the paper's main finding is that the MW system is highly exceptionally flattened. Similar satellite arrangements are rare in LCDM simulations. This agrees with what I've been saying, so for me this is reassuring, though not super exciting.

Paper time! I had the honor to contribute to one of the first Gaia DR2 papers, lead by Tobias Fritz, as part of the HSTPROMO collaboration including @tonysohn, @astronitya, and others. Here's a little thread of what we find. The title already kind of says it all: "Gaia DR2 Proper Motions of Dwarf Galaxies within 420 kpc: Orbits, Milky Way Mass, Tidal Influences, Planar Alignments, and Group Infall".
uci.voxcharta.org/2018/05/02/gai…

I've finally found some time to look at the Gaia proper motion measurements for the MW satellite galaxies, see: astro.rug.nl/~breddels/dggc/

So here's a short thread with some plots. Here's a plot of the orbital poles for the MW satellites compared to the normal direction of the satellite plane / Vast Polar Structure (VPOS). The first one is for the Gaia proper motions, the second for non-Gaia measurements (mostly HST). They don't look all that different!

In case any of you are in the Monterey, CA area: This Saturday, 14 April, I will give a free public lecture on the "Dance of Dwarf Galaxies" and possible implications for Dark Matter.

Time: 7:30pm
Place: Monterey Peninsula College, Lecture Forum #102

mira.org/events.htm This is organized by MIRA, the "Monterey Institute for Research in Astronomy", which has an interesting history: In 1972, a bunch of @CWRUastro grad students decided to rather start their own institute instead of going through the usual job market. mira.org/museum/story.h…

As promised last week, my review on the Planes of Satellite Galaxies Problem is now out on the arXiv: arxiv.org/abs/1802.02579 “The Planes of Satellite Galaxies Problem, Suggested Solutions, and Open Questions" Some time ago I was invited by MPLA to write a brief review on the Planes of Satellite Galaxies issue. Since there wasn't any review on the topic out there yet, and because people had been asking me frequently for a good introduction to the topic, I happily accepted.

Thread: Let me give you a summary of what @VoltarCH, @lellifede, Helmut Jerjen and I report in our new Science paper (science.sciencemag.org/content/359/63…) Satellite Galaxies Planes exist around the Milky Way and M31. The satellite galaxies show coherence in their velocities (red receding, blue approaching in these views). For the MW, proper motions even show that some co-orbit along the plane.