Dotting the i's and crossing out the dark matter

This paper revisits one of our old friends, those galaxies without dark matter.

When last I checked in on this, it seemed to be settled that indeed they do lack dark matter, and they can be explained by conventional (though rare) interactions which can strip the dark matter but leave behind a surviving stellar core. This is probably still the case. The whole distance debacle appears to have been decisively settled such that the low velocity dispersions of the objects are indeed consistent with little or no dark matter. The authors note in the introduction, however, that a few diehards maintain that maybe we're just seeing them close to face-on, which would hide the signatures of rotation.

Dedicated readers will recall that I myself thought this quite plausible for some other, similar objects, until a recent paper finally convinced me that this just isn't tenable. This is why it's important to check every hypotheses as carefully as possible. The only thing that ever really settles the arguments is better data.

So what the present authors have done is gone and get measurements of how fast the stars are moving in the notorious NGC 1052-DF4. Already the globular cluster data is clear that it can't possibly have dark matter, with a velocity dispersion of just 4 (!) km/s. But direct measurements of the stars in the galaxy itself would be much more decisive, because nobody's ever really going to be happy with data from just seven globular clusters.

And the measurements, if sufficiently precise, can help beyond settling the major issue. As they say, different formation scenarios (such as tidal stripping versus collisions) are expected to result in different amounts of dark matter remaining, though always of course on the low side. Determining just how low this is requires extremely precise data, the kind you can only get from the stars themselves.

So that's what they go and do. The have 14+ hours on the Keck telescope and find the measured velocity dispersion, though larger than from the globular clusters at 9 km/s, is still entirely consistent with the galaxy having no dark matter whatsoever, and inclination angle effects can be neglected. Making this measurement is a much harder task than in regular galaxies, because here the dispersion is so low that even motions of individual stars need to be properly accounted for. After various corrections, their final estimate of the true velocity dispersion is just 6 km/s. The traditional NFW profile for the dark matter for an object like this would give a mass about three orders of magnitude greater than what they observe.

(I confess to being a little caught-out here. The globular clusters are found further away than the stars but their velocity dispersion is lower...? So used to declining rotation curves am I that this seemed really weird ! Then I remembered this is exactly what's supposed to happen according to Kepler)

They also give a much better explanation as to why the globular cluster population of these objects is interesting in itself, compared to other papers which I find never get the point across. For normal galaxies there's a nice simple linear relation between total dark matter mass and number of globular clusters. These objects, according to that relation, are consistent with much more massive dark matter halos. That is, they have far more globular clusters than one would expect based on their small/zero mass of dark matter. They don't fit the standard models of galaxy formation at all. 

In fact they're outliers in at least three different ways : they have less dark matter than expected given their stellar mass; more globular clusters than expected for their stellar mass; and more massive individual globular clusters than is typical. And their globular clusters are, they say, "remarkably consistent in colour".

They don't here speculate much as to what all this means for the formation scenario, and in this case, given the controversy that has engulfed these objects, I can't say I blame them. They do however note that there is some tension with cored dark matter profiles as well as the standard NFW ones, though it's a rather weak tension. More interestingly, they say that the velocity dispersion is "clearly inconsistent with MOND", which predicts > 12 km/s. This is fun because previously claims of inconsistency were shown to be premature because they hadn't accounted for the external field effect, a MONDian effect whereby nearby galaxies can change each other's dynamics in a way that doesn't happen in standard gravitational models.

We shall see where that one goes in due course, I'm sure. Still there are many questions about these objects. Since there are two in the same group, the presumption is they must have formed the same way. And somehow they have to have survived in the group without being destroyed by tidal forces, which is counter-intuitive for large, low-density objects. So if the major issue is settled, it hardly feels like we've heard the last of these yet.