Get Those Mother**ing Satellites Off My Mother**ing Plane

It's been a good long while since I looked at any planes of satellites papers, so let's see what's new in this controversial arena.

For those not in the know, the idea is that there are significant numbers of galaxies whose small satellite companions orbit around them in thin planes. This is not a natural prediction of cosmological models, which show that they should orbit them in spheroidal clouds instead. So this has been seized on as a major challenge to the standard model and even the dark matter paradigm itself.

Now the plane of satellites around our own Milky Way is really very clear and unarguable. But as I go into at great length here, claims for other such planes are not in the least bit convincing, and I tend to view the whole field as awash with some bloody daft statistical biases from people who really should know better. Honestly I think it's just (ahem) plain silly. A very short recent summary that uses a lot of the same arguments that I do can be found here.

But today I turn my attention to this more substantial offering. This paper concentrates only on the plane around the Milky Way. As I've said, this feature is very interesting. I don't think it necessarily means that all of science is wrong, but I'd like to know how it formed all the same.

In this paper the authors use one of the big new all-singing all-dancing cosmological simulations, within which they try and look for Milky Way analogues that have planes. They impose limits on the mass of the host galaxy that are fairly generous, but they have a rather strict policy for considering the 14 brightest satellites of the galaxy. Together with the mass of the satellites, this ensures a direct like-for-like comparison with the observations. At this mass range the observations should be complete (meaning they haven't missed any) and the observations have sufficient resolution to simulate them accurately.

Actually, while their mass range for the host galaxy is quite large, I suspect their constraints on the satellites are if anything too strict. 14 is a pretty arbitrary number, really, and they only report on one single Milky Way analogue that has a plane in agreement with the observations. This bit is my only gripe with the paper : they state their selection criteria for the host galaxy very clearly, but they don't actually state their criteria for a match regarding the plane; my guess is that if they relaxed these parameters a bit, they might find considerably more planes. There is no need, after all, to insist on a perfect match of all parameters.

Anyway, of the 548 halos of the correct mass in their simulations, 404 are as isolated as the Milky Way, and of those 231 have the correct number of satellites. Only 1 of these has a compatible plane (the above caveats notwithstanding), but 1 in 200 is already quite a bit higher than some other estimates. Remarkably, this galaxy also has a very similar large-scale environment to the Milky Way, with its nearest massive neighbours matching the properties of Andromeda and Centaurus A pretty well. These environmental conditions weren't used as a selection criteria at all.

The plane itself is also very similar to that observed. Not only does it have a similar geometry but it's also rotating, and at a very high inclination angle with respect to the stellar disc of the host galaxy. This is really an excellent match; even if the mass of the Milky Way analogue is a bit low, it's still within observational constraints on the Milky Way. To match this closely on so many parameters at a rate of 1 in 200 is very impressive result indeed.

Now one of the frustrating aspects of many anti-standard-model papers is that they search simulations for analogues of the observed planes, find some, and then only compute the frequency at which they're found and nothing else. Observational planes, they say, are very common, whereas simulated planes are very rare. They conclude that this means there's a conflict between the standard model and observations. The problem here -  well, one of them - is that they seldom if ever attempt to examine how those few planes that they do find actually form in their simulations... and this can make a world of difference. 

What I mean by this is that saying that they're rare overall in the simulations might be correct but potentially irrelevant. For example, giant redwoods are very rare among trees, but if you walk through Giant Sequoia National Park you shouldn't be amazed that if you've found far more than random chance would suggest. The approach of those claiming the planes challenge the standard model is in essence entirely statistical, neglecting the physical processes at work, treating galaxies are random when in fact they're anything but.

Here the authors find that the plane results from two mechanisms. First, the satellites infall along large-scale filaments, preferentially leading to the formation of elongated structures. Second, while most of the satellites here are indeed orbiting around the galaxy, three are just coincidences : their true 3D velocities will take them in quite different directions, so it just so happens that at one particular moment, the plane appears to have more members than it really does. It's partly "real", but partly transient.

At this point I want to raise another issue. Some years back, having a protracted email discussion with a rather strongly pro-plane group, one response about the infall of filaments was rather brief :

"I had already talked to Libeskind, and he never even wanted to suggest that the filaments are related to the VPOS. They are far too thick anyway."

Well, Libeskind is one of the three authors of this paper, and not only do they here explicitly state that the filaments are part of the formation mechanism, but they several times cite his earlier papers as claiming that as well. The above quote is thus demonstrably not correct. You can see why these wholly erroneous claims tend to annoy me quite a lot.

Is this the last word on the matter ? No, though it probably should be. If planes really were as common as some suggest, with practically every nearby massive galaxy having a planar system, we'd likely have a big problem. The authors here are careful to state that their findings don't say anything about these other systems, but in my view, none of these other systems are even remotely comparable to the Milky Way system and really aren't worth bothering with. 

In short, models do predict planes. Not many, but more than some claim, and those which form seem to be in very good agreement with the observations both on large and small scales. By showing that there are physical processes leading to the formation of planes, this is a strong rebuttal of claims that planes pose a serious challenge to the standard model. I remain convinced that this, like many claims against the standard model, is a non-issue.