Kepler's Krazy Kurves

I'm going to hate myself for the title but meh.

This paper is about the rotation curve of the Milky Way. Of course we all know galaxy rotation curves are flat, and that's one of the biggest reasons we think the Universe is dominated by dark matter*. But even so, if we go far enough away (so that the great bulk of the mass is interior to a given position), rotation curves should eventually start to decline in the good old-fashioned Keplerian way.

* Well not really, but see below.

Now I'd thought that this was something that hadn't been previously seen in any galaxies, or if it was it would be one or two exceptional and/or highly uncertain cases with dodgy measurements. So given that this paper claims to have detected the Keplerian decline for the Milky Way, I was expecting them to make a big song and dance about this, possibly also cheerleading for either vindicating the Standard Model or for finding new MOND-compatible evidence. I'd have been happy to read it either way, but they don't do this at all.

Instead, the press releases* pick up on something I would tend to regard as fairly trivial : the exact mass of the Milky Way. Which is normally the astrophysical equivalent of a dick-measuring contest, pointless, petty, and greatly exaggerated.

* I do want to say that Brian Koberlein is one of the best science communicators out there, lest this should be interpreted in any way disparaging – as I usually am toward press releases.

And to some extent that's true. The poor authors go through an enormous amount of very careful work to measure the rotation curve to as great a distance as they possibly can, but it isn't at all clear what they get out of this that's new. It looks, from both their figures and text, like this result is all but confirming existing findings, with any differences seeming to be marginal at best. Sure, it's an important confirmation (we shouldn't underestimate the importance of replicability !), but it would have been a lot clearer if they'd just stated this. As it is, they extrapolate by a factor of four in distance to find a difference from previous findings of the total mass by a factor two. Ain't nobody caring if the Milky Way has one or two trillion solar masses of stuff. 

They make another couple of annoying, though much more minor, claims which could also easily have been clarified. They say they estimate a radius for the Milky Way which is almost twice as large as the standard 13.4 kpc, but don't say under what conditions : presumably they mean the distance at which they directly detect material in orbit. If so this is uninteresting, because nobody thought the galaxy had a sharp edge*. And they say that their measurements can help resolve the important argument about whether the Milky Way's dark matter halo has a central density spike ("cusp") or a flat "core", but don't actually investigate this at all.

* I may well be misreading what they're intending to claim here, but this could have been reworded very easily.

But they do implicitly make it very clear that yes, the Milky Way does have a dark halo. The rotation curves that they derive from all the individual components (stars, gas, dust, and separate components of the bulge and disc) do not come anywhere near close enough to explaining the orbital motions without dark matter. Sure, the curve is Keplerian, but it's still way faster than expected, even at the outermost edges, than from the baryons alone. This is an important reminder that really it's the speed of rotation, not the shape of the rotation curve, which determines whether there's dark matter present or not.

And I have a lot of praise for other aspects of the paper too. They note that one of the earliest flat rotation curves comes from 1925 (!), much earlier than I thought – though the figure itself shows a very high scatter, and nobody would hold this as in any way conclusive. Still, a good bit of archaeological research there. 

More importantly, while their Milky Way mass measurement may be consistent with other values found from similar methods, it's substantially lower than those from other techniques. Estimates made using extragalactic sources, like the motions of the Magellanic Clouds, have found masses up to five times greater than the present estimate. What they say this likely means is that these other methods are just not robust enough : these other components aren't in nice stable orbits, so of course their motions don't really reflect the true mass. And kudos to the authors for emphasising that our knowledge of our own Galaxy is still changing. That's what research is for, after all.

While it's disappointing that they don't venture any guesses as to what the Keplerian decline might mean for cosmology, this is probably a wise move. They do, however, give a nice overview of the (as I suspected) extremely limited and suspicious claims for other such detections in other galaxies, making it clear that the Milky Way is quite unique in this regard. And this points to a pet topic of mine, that those claiming Standard Model adherents have to point to the special nature of the Milky Way to avoid conflicts with theoretical predictions are absolutely right to do so. Sure, we should assume we're a typical spiral galaxy by default, and it's therefore right to be surprised when we find it does unusual things that typical spirals shouldn't do (like having satellite galaxies in a plane). But it's wholly wrong to insist that we must be typical even when measurements clearly show that we're not.

In this specific case they point to two possible interpretations. First, it might only be the unique nature of the methodology that gives us this Keplerian decline, and if we had comparable data for other galaxies we might well see the same thing. Second, perhaps more interestingly, the Milky Way's other unusual attributes (being a four-armed isolated spiral which hasn't experienced any recent major mergers) might indicate other types of galaxies we should examine to see if they too show the same thing. Maybe this declining rotation curve is a consequence of that, in which case, other similar galaxies should show the same feature. This makes things testable.

Finally, they note that the classic NFW profile often used to approximate the dark matter halo doesn't work in this case, as it can't predict a declining rotation curve (as you go further and further away, says NFW, you just keep enclosing more and more mass). The Einasto profile has no such problems so we should all use that instead.

What began for me as a mildly baffling disappointment turned out to be a really very nice piece of work. It doesn't have any direct bearing on the Standard Model, but it does suggest further evidence that our Milky Way is weird – and that the measurement technique, especially within our own Galaxy, really matters. It's that comparison with different techniques I think should have been emphasised much more in both the paper and the press releases. Without that, the whole main point of the paper is easily lost.