I'm being facetious. It's a very interesting paper, and though there are some parts I would quibble with, I'm surprised it hasn't been accepted despite being submitted back in November. The authors analyse a stellar stream around the Milky Way using data from the shiny new Gaia telescope, which gives super-awesome position, distance, and proper motion measurements (that's movement across the sky, as opposed to motion along the line of sight). This particular stream has a couple of odd gaps in it, which they claim are best explained by collisions with the long sought-after dark matter halos. So exactly what I'm interested in, just on a smaller scale.
They describe their preliminary investigations in paper I. The stream is clearly visible in a standard colour-magnitude plot and also in a plot of proper motions and positions. It also has a distinct "spur" extending north from one of the gaps, running roughly parallel to the main body of the stream. They identify all the stars in this stream and its spur by drawing polygons in the different plots, which as far as I can tell they do by eye. This clearly works, though it wouldn't be surprising if there were some stars included which aren't really in the stream. So the gaps may look like they're just lower-density regions with fewer stars present, but they could well be really empty gaps.
As far as I can tell they either don't use or don't have distance measurements. I don't know why this is. They are able to fit the orbit of the stream though, and identify its likely progenitor star cluster.
In the first paper they briefly speculate that a dark matter halo could have disrupted the stream to create the gaps. In this paper they go further, trying to constrain the nature of the perturber. They don't really consider other explanations much, though they do at least consider whether the gaps could have formed simply as part of the normal disruption of a cluster. They say no, the resulting stream in that case should be smooth.
Their model is a test particle model of a cluster ejecting stars as it orbits in a gravitational potential mimicking the Milky Way, with another massive particle used for the perturber (which is given different masses, sizes, encounter times, distances and velocities). Since the stars are test particles, which experience the effects of gravity but are themselves massless, they're ejected from the cluster artificially, so this isn't a self-consistent model of the stream formation. That's fine, because that's not the goal : they're more interested in the disruption of the stream, which test particles are perfectly adequate for. It means their models are computationally cheap, but of limited predictive power.
They run a bunch of simulations to explore the possible parameter space, though I'm not sure how many (at least 25) or how many test particles they use. I'm also not sure why a full n-body simulation would be so prohibitive : the progenitor cluster only has a mass of 70,000 Suns, so the particle number doesn't need to be crazily high. Anyway, their tests define the broad regions of parameter space of the perturber. They also allow them to predict the line of sight motions of the stars, which could be tested with future data. You can watch an example of one of their simulations below :
More images here, but there's not much information to explain what's going on. The movie shows the stream as it would appear on the sky from the centre of the Galaxy. I don't know if that strong, stream-wide warp is due to the encounter or simply its orbit around the Milky Way.
I'm a bit surprised that an encounter with a massive compact object can leave such neat gaps (or at least depleted regions). Based on similar simulations that I've run, I would have expected the stream to be much more distorted near the impact points, rather than stars being so neatly removed without bothering their neighbours much. It would have been nice to see what the simulation looks like from above; maybe things do look more distorted from other angles.
The authors seem pretty convinced by the dark matter halo explanation for the perturber. I definitely want to agree with them, and there's nothing obviously wrong with the analysis, but I think they're a rather too confident in their conclusions. They have quite a protracted discussion on whether it could be due to a black hole, which feels forced and unnecessarily exotic, and a lot of speculation about what they could do in the future. Sure, it might help constrain dark matter models, but don't count your chickens and all that.
Their constraints on the perturber mean that it is compatible - in terms of size and mass - with dark matter halos predicted in CDM simulations, but only just. It would have been nice to have more discussion on this point, and in particular to try and test what would happen to the stream if it encountered more typical dark halos. This should be easy since the simulations should be computationally dirt cheap. There's also not too much discussion of context either, i.e. how many similar streams are known, do any others have gaps, etc. But it's definitely an interesting result, and with lots of scope for further analysis and tests, it's also one that could potentially get a lot more interesting in the future.
The Spur and the Gap in GD-1: Dynamical evidence for a dark substructure in the Milky Way halo
We present a model for the interaction of the GD-1 stellar stream with a massive perturber that naturally explains many of the observed stream features, including a gap and an off-stream spur of stars. The model involves an impulse by a fast encounter, after which the stream grows a loop of stars at different orbital energies.
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