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.
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! 
Main changes are:
- Leo I & II are more uncertain in Gaia, no surprise given their distance. For Leo II the Gaia uncertainties allow a radial orbit, so any orbital pole direction is allowed. For now I'd trust the HST data of the Leos more, and there Leo II aligns well.
- Leo I & II are more uncertain in Gaia, no surprise given their distance. For Leo II the Gaia uncertainties allow a radial orbit, so any orbital pole direction is allowed. For now I'd trust the HST data of the Leos more, and there Leo II aligns well.
- One nice aspect of the Gaia data it that it provides the first good measurement of Carina's proper motion. That one bugged me for years because its previous PM was consistent with co-orbiting in the VPOS, but only barely. The new data confirms it is part of the satellite plane!
- But you can't always win, so Bootes turns out to not co-orbit in the satellite plane. One might try to argue that it counter-orbits like Sculptor, but the two are quite offset. On the other hand, Bootes is relatively nearby, so might be worth checking the orbit in more detail.
Compared to these new proper motions, my predictions fare quite well (prediction is red line, most satellites cluster along it or have high uncertainties).
The face-on view of the VPOS continues to give the impression of a preferred sense of rotation.
The face-on view of the VPOS continues to give the impression of a preferred sense of rotation.
Here are exact comparisons to my proper motion predictions from adsabs.harvard.edu/abs/2013MNRAS.…. The magenta lines are the prediction for perfect alignment with a VPOS plane (thick for co-orbiting). 9 of 12 Gaia measurements (blue error bars) match quite well within their uncertainties.
Thinking back 5 years to when I first made these predictions, I am sure I had not expected such a good agreement. So I'm quite happy with what Gaia finds today.
And maybe more importantly, this independent measurement shows nice consistency to the the HST proper motions, which confirms what we've been doing and saying in the past years: many of the classical MW satellites co-orbit in the VPOS.
