Satellite planes still not a thing

A while back I was raving on about claims that satellite galaxies lie in narrow planes around their host galaxies. This is potentially really interesting, since the standard models of galaxy formation, flawed as they are, predict that satellites should be found in roughly-spherical clouds rather than narrow planes. You can find a full write up here :
(which is uber-long, but you can read section 2 in isolation - I'll try and make a shorter, more bloggy version soon)

In short, claims for such planes use dodgy statistical analysis and in one case border on the nonsensical. There are three main claims for these systems :
- The Milky Way. This one is solid and probably unimpeachable. And it's genuinely interesting and deserves an explanation.
- Andromeda. This one is marginal - it was found by deliberately selecting the thinnest structure, which is guaranteed to find a thin structure even in a spherical cloud ! There's a hint of a structure when you consider the motions of the galaxies, but I wouldn't call it any more than a hint.
- Centaurus A. The claim was that this elliptical galaxy had not one but two planes of satellites, which to me looked to be simply absurd. The paper didn't even explain how the planes were selected, they just arbitrarily decided there were two "distinct" planes because why not.

This latest paper, which has passed peer review ( now says that the previous claims for two planes were wrong and that there's only one plane. But there isn't, as the gif below shows. There just isn't. At best, there's a marginal hint that the cloud is elongated, but take away a mere two galaxies and even that vanishes (equally, if the surveys have missed any galaxies, that apparent elongation could easily disappear).

You can watch the author's own video here :
But I don't like it. Halfway through, they select the structure they want you to see - I defy anyone to claim they would have spotted this by themselves in the gif below !

What is a bit better is that here they have measurements of the line of sight motions of the galaxies, and that is consistent with rotation. But they don't know the true 3D motion of the sources across the sky, and in any case it's still a very thick structure and not at all plane-like. Furthermore, other models (which I know via private communication that the authors are aware of but don't cite) have shown that interactions between galaxies can perturb their satellite clouds into planes and preferentially destroy satellites moving in certain directions. So even the apparent rotation of this system isn't particularly impressive.

The paper suffers from all the same problems as the previous satellite plane papers. They make a very strong claim from this spheroidalish cloud of points, claiming that it's in "serious tension with the expectations from the standard model" and that finding "three such systems is extremely unlikely." But it isn't and they haven't. Various mechanisms have been proposed to explain the planes, which they largely ignore. Their estimate of how rare the planes in the standard models smells extremely fishy - they don't comment on the leading paper which does find such planes, they demand a very precise match between the simulations and observations (down to the number of satellites, rather than, say, the fraction in a plane; also the isolation of the system seems - maybe - excessive). And then they make no comment on the systems which do match the observations in this model. That is a serious error : if there is a physical mechanism for the plane formation, it makes no sense to keep insisting they only form by chance - firstly because if Cen A is in a similar situation to the galaxies in the simulations, then you should absolutely expect it to have a plane, and secondly because you can't multiply probabilities if they're not independent. And of course, they insist that the Andromeda plane is definitely real, despite being marginal.

Even Snakes on a Plane was more believable than this.
Animated Photo
Shared publiclyView activity