Thread by @8minutesold

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…

The paper presents proper motions for 39 satellite galaxies and candidates, based on spectroscopically confirmed stars. This is similar to @joshuadsimon's recent paper, and our PMs show agreement with his. Our sample includes more distant dwarfs, though.

In addition to providing these proper motion measurements, we also put an emphasis on interpreting what we find. Much of this will be looked into in much more detail in follow-up papers, but let me give you some highlights.

Integrating the orbits, we find that the resulting orbital parameters (apocenters, velocities) looks more reasonable a potential with a heavy MW (1.6 x 10^12 M_sun) than in a light one (0.8 x 10^12 M_sun).

On average a satellite spends more time at apocenter (where it is slowest) than at pericenter. Interestingly, we find more MW satellites closer to pericenter. This has implications for the missing satellite problem: there must be distant dwarfs at their apocenter yet to be found

Crater II appears to have a rather eccentric orbit, reaching about 20-30 kpc. This can result in substantial tidal stripping and could help explain the very small observed velocity dispersion in this dwarf galaxy.

It had been suggested that the Crater-Leo objects were accreted as one group, due to their alignment on the sky and common distance. We don't find a clear association, their orbital poles seem to be all over the place, but they are also quite uncertain.

I also looked at which satellites of the 39 satellites orbit along the VPOS, and how close they come to the orbital planes I predicted for each of them.

In summary, 6 satellites can't orbit in the VPOS due to their position, 6 have well constrained orbital poles that don't align, 11 are consistent within their uncertainties with orbiting in the VPOS, 16 have median poles aligned with the VPOS. Of these, 5 counter-, 11 co-orbit.

I am of course working on a more in-depth analysis related to the VPOS than what was possible in just three days, so stay tuned.

That's it for now, good night!