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. 
These satellite planes are a serious challenge for ΛCDM, because similarly extreme structures are extremely rare in cosmological simulations (of order 1:1000). Typically, dark matter sub-halos are much more randomly distributed (like in the shown Via Lactea simulation). 
Still, some people have argued that maybe both the Milky Way and M31 are exceptional, also because they form a group. So we decided to look at another galaxy for which a spatial satellite plane had been reported before.
That galaxy is Centaurus A, a massive and isolated elliptical galaxy about 4 Mpc away.
(Image credit: Christian Wolf and the SkyMapper team / Australian National University)
(Image credit: Christian Wolf and the SkyMapper team / Australian National University)
Looking at the spatial distribution of satellites, @satellitegalaxy and others found two parallel planes (adsabs.harvard.edu/abs/2015ApJ...…). We re-analysed the system with additional satellite candidates and found it more in line with one plane (adsabs.harvard.edu/abs/2016A%26A.…). 
But the truly remarkable property of the satellite galaxy planes around the Milky Way and the Andromeda galaxy is their kinematic coherence. They seem to rotate. Only if that is the case for Centaurus A we can claim to have found a similar structure.
Luckily, the Centaurus A satellite plane is seen approximately edge-on. That allows us to look for a kinematic coherence using line-of-sight velocities. We found 16 satellite galaxies for which such velocities are available. So, what does it look like?
(Ok, I realize that given that you are reading a summary of a paper published in Science, you can probably guess what comes next.)
Voila: of the 16 satellite galaxies with velocities, 14 show a coherent trend: the satellites in the North approach (blue), those in the South recede (red) relative to Centaurus A. 
That kinematic trend is furthermore aligned along the major axis of the satellite distribution, as expected for a rotating plane of satellites. Planetary nebulae (top right panel before, small dots here) show a similarly aligned velocity trend. 
We did a number of tests to determine the statistical significance of this alignment, and found it very unlikely to be a chance result (for example, only 0.4% of all cases in a simple coin-flip experiment result in the same coherence).
Here is a movie that gives a better impression of the 3D distribution of the system. You first see the full satellite distribution, then the best-fit plane is added in grey, and finally the velocities are blended in. The narrow line shows the direction to the Milky Way.
This kinematically coherent, apparently rotating satellite plane in itself is already a major finding. But we also wanted to know how this structure compares with the standard model of cosmology.
We selected anlog host galaxies from simulations, and mock-observed their satellite galaxy systems. Then we measured each satellite system's on-sky flattening (so we don't need to worry about distance uncertainties) and the kinematic coherence, just as done for the observed case.
This plot shows the flattening (horizontal) and the kinematic coherence (vertical axis). The data is from the Millennium-2 simulation. Satellite systems as flattened and as coherent as observed (green corner) are extremely rare, only 1 in 1000 simulated hosts shows what we found. 
What about baryonic physics? We tested that by comparing to the Illustris simulation. It does not look much better, as expected because the spatial distribution and the orbits of satellite galaxies aren't substantially affected by whatever internal baryonic physics is going on. 
To summarize, we found Centaurus A to hosts a planar distribution of satellite galaxies with strong signs of coherent rotation. It is as rare in cosmological simulations as the MW and M31 satellite planes, thus corroborating the Planes of Satellite Galaxies Problem of ΛCDM.
PS: In case you'd like to learn more, I should mention that I just finished a review on the Planes of Satellite Galaxies Problem. It will appear on the arXiv next week. Oh, and if you prefer listening to reading, you can of course invite me or the other authors to give a talk. 😉 
