Thursday, 18 September 2025

Weaponising dark matter

Stephen Baxter's Xeelee sequence revolves around a war between baryonic and non-baryonic life forms. One memorable sequence features a pulsar being hurled at the Great Attractor (because reasons). Today's paper feels like it could easily fit within such a realm of the gloriously far-fetched, albeit it's not without some reasonable evidence too.

This is just a four-page letter so I'll keep this one very short indeed. Like the last paper, they claim to have found the signature of a very small dark matter halo, but this one's even smaller. The last one was about a billion solar masses, extremely small by galaxian standards but not outrageously so... this one, by contrast, is probably no more than a few tens of millions of solar masses, with a lower limit of just a few thousand.

Such features are certainly predicted in cosmological simulations. Basically, the higher the resolution, the more small dark halos result. But below a certain limit, nobody ever expected to have much chance of ever detecting them, since they'd have so little gravity they'd never attract enough gas to form a single star. And once you add in all the baryonic matter to the simulations (the boring normal matter of stars and gas), presumably most of the smallest ones would be disrupted.

The claim here is they've found a bullet wound in the Milky Way resulting from a collision with one of these minihalos. Actually, again like the last paper, this is not a discovery announcement so much as an independent confirmation by a different method. The original discovery came back in 2017 in the form of a molecular gas cloud with an unexpectedly high line width. 30 km/s is small by the standards of galaxies (the Milky Way would be more like 400 km/s), but with no stars to drive the motion, dynamics have an obvious appeal. Without any other visible material (at least nowhere near enough), dark matter is at least heavily implied.

Here they use Gaia data to look at the velocity of the stars in the vicinity and discover they have a vertical velocity anomaly : in a small region of the disc, the average velocity of the stars perpendicular to the disc drops, even while their dispersion increases. The original CO blob is slap-bang in the middle of this VVA, with the VVA being very much larger than the CO blob. Which would be an awfully suspicious coincidence.

A lump of dark matter colliding with the disc could certainly cause this. Its small size is certainly consistent with a very small halo. But I have no familiarity with stellar dynamics on these scales at all, so I can't tell you how unusual such features are and their figures don't really give much of an indication. They also don't even consider other explanations, which I suppose is fair given the limited available space, but it would have been nice to have mentioned something (besides the obvious impossibility of stellar winds and the like, there apparently being no stars here). And how much such features would we expect to find, if these minihalos exist in the numbers predicted by simulations ? Finally, their tie-in to Ultra Compact Galaxies more generally – which are largely thought to be the stripped cores of more massive galaxies – is just too speculative even for a letter.

In short, it's definitely a very interesting feature to report, but it's going to take a lot more work to say anything definitive about what it actually is

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