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).
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 !
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.
EDIT : That's "not impressive" in the sense that it contradicts CDM predictions. I don't dispute that the system is consistent with rotation or is even rotation dominated, which is, admittedly, definitely interesting. But a plane ? No, I can't accept that. I just don't see it.
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.
https://arxiv.org/abs/1802.00081
I looked at the gif yesterday and it felt like there is a parabolic pattern. Almost similar to the way the arms of a spiral galaxy look but spread around(over time) as if it was balancing towards the center point sucking parts of the matter while leaving drops. It feels like raindrops on a glass: the surface tension allows for drops to combine being sucked towards each other while small drops remain behind in the periphery both allowing for water to bridge the glass surface but still remaining as sort of a local minima.
ReplyDeleteThere's one point in the animation (well, two) where I can cover all of the points with one finger, whereas 90 degrees later it's more like three fingers wide. Does a 3:1 ratio constitute a plane? Seems pretty similar to what I recall of the Milky Way's version, but I didn't do the same test for that one.
ReplyDeleteIf you include the top two galaxies, them it does show a hint of an extension. Take them away and it really looks - at best - like a slightly squashed cloud. It's a heck of a leap to say this small squashed cloud poses any kind of challenge to CDM.
ReplyDeleteI think there's a gif of the Milky Way in the first link somewhere (I'm on mobile so it's hard to copy and paste), but that one is definitely much clearer.
I think you're talking about this one: 2.bp.blogspot.com
ReplyDelete...which is a little hard to evaluate since there are clearly some background ones that show up only in the non-minimal cross section (albeit just before it). To me it looks like they kill the plane idea, though admittedly the closer-in ones are reasonably compressed.
Do you have a handy pointer to the 3D positions of these guys, by any chance? I'm pretty sure my own model has no more than two thirds as many MW satellites, possibly only half of them, so it would be nice to fill it in a bit more and also to validate the existing data I've got.
For the Milky Way I used the catalogue of McConnachie :
ReplyDeletehttp://www.astro.uvic.ca/~alan/Nearby_Dwarf_Database.html
I've put the simpler version I used up here : https://drive.google.com/open?id=1ZwyS92lBuxT2IqwfkOTjEoEbvRMom5gh
I don't remember how I selected the satellites for the gif but it was probably just a simple radial distance cut, so there may well be a few background galaxies included.
Using the complete set of known Milky Way satellites, one gets a thick plane but still definitely a plane. Apparently if the selection is restricted to the brightest objects it gets much thinner, which doesn't happen for the other claimed planes, but I didn't try this myself.
For the Centaurus A plane in the above gif I used the table given in the paper. But then I read it more carefully : that table only includes the 16 galaxies with both distance and velocities. They say there's a total of 31 satellites with accurate distances, but don't provide a catalogue or direct reference. From the references they provide I could only find an additional 7, from this paper : http://adsabs.harvard.edu/abs/2016ApJ...823...19C
I've put the original Muller table online here :
https://drive.google.com/open?id=17rEk9ct8LZD8sE1TGGnwm0sa2i-xi5tC
And the additional table here :
https://drive.google.com/open?id=10cbTf2DZX-lk7qpXDUXLYAyHLaDRiYv3
And the latest gif, including the additional galaxies, is below.
I'm a little bit suspicious that I can reach the claimed total of 31 by including the other objects in the Crnojević paper, which includes streams and shells and individual parts of disrupted galaxies.
I've been reading a bunch more papers on plane finding and the perils of small number statistics. My main concern is that originally it was claimed there are two planes around Cen A, which is a fantastically unlikely configuration, but now apparently there's supposed to be one big one and that's unlikely too. Really I think you can see whatever you want in data sets this small. And if you slice the data even smaller, you get even more options. To my mind it's crazy to present this is a significant challenge to CDM, let alone with the confidence that Muller et al. seem to think.
Blog post in the works.
https://lh3.googleusercontent.com/C9wcO0w4ZlyfRsaFGyFVHm5Rhlii5Q1itHkrKmq70lvNvW3WH9qHjX8x6B4WhdSfay8HK4TcDdvKbb89elhymcNVJ8xnOut1Ngad=s0
Many thanks! It's 2am here, and I've got an oncall issue, but I'll take a look sometime tomorrow. I agree with your small-number concerns...but I'm also suspicious of "chi by eye," even when it looks pretty convincing (as here). The 26 Myr mass extinction periodicity didn't survive Fourier analysis despite what eyeball analysis suggested.
ReplyDelete