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What happened to this galaxy’s dark matter?
Back in 2018, astronomers used Hubble to observe a relatively nearby galaxy and found it presented a baffling mystery: It appeared to have little or no dark matter.
This was so weird they decided to follow up to make sure their results made sense. So they took more Hubble observations — a lot more — and the mystery only deepened. It appears to have even less dark matter than they originally thought.
This is unexpected, and bizarre. Why doesn’t this galaxy have a feature pretty much every other galaxy has?
Okay, to start, the galaxy is called NGC 1052-DF2 (let’s call it DF2 for short) and was discovered in 2000. It lies near the much larger and brighter elliptical galaxy NGC 1052 in the sky, and was found to be physically close to it, too.
DF2 is faint, and very diffuse (so diffuse, in fact, that you can see background galaxies right through it). Given the initial distance measured to be around 65 million light years from us, that makes it a dwarf galaxy but a biggish one, about 20,000 light years across.
The weirdness started when some short Hubble observations showed it has several globular clusters around it, huge collections of hundreds of thousands of stars. These clusters orbit galaxies, and how fast they orbit depends on the mass of the galaxy; more mass means more gravity and faster orbits.
The astronomers measured the velocities of the clusters using the immense Keck telescope, and found a mass for DF2 of about 200 million times the mass of the Sun, and no more than about 300.
Here’s where it gets truly weird: They also looked at the visible light coming from the galaxy. Assuming it all comes from stars (a very good assumption) they also get a mass of… 200 million times the Sun.
BUT. We know that galaxies are surrounded by a halo of invisible stuff we call dark matter, and that this generally outmasses the visible stuff by a factor of five or so. That’s not the case for DF2. Assuming they got everything right, the total mass of the galaxy is about or not much more than the visible mass. That means it has very little or no dark matter.
And that’s very weird indeed.
As it happens, there’s an out: If they got the distance wrong, this situation can be fixed. If it’s a lot closer to us than 65 million light years, say around 42 million light years, then the galaxy isn’t as bright as we think, the visible mass drops but the total doesn’t, and that means it does have a dark matter halo. In fact, using the same Hubble data, some other astronomers did calculate a much closer distance from the galaxy. So who’s right?
To find out, they got more Hubble observations. The first time they observed the galaxy they got a 4,500 second exposure. This time, though, they went deep, getting a total of nearly 90,000 seconds, almost 25 hours, 20 times the first exposure.
The images showed much fainter stars, including a lot of stars called red giants. These are stars like the Sun but nearing the ends of their lives. When they run out of hydrogen fuel in their cores they swell up and cool off. Eventually they start to fuse helium, which makes them shrink a bit and heat up. The beauty of this is that the brightness of these stars are all the same when they switch to helium fusion, making them great distance indicators. We know how bright they really are when this happens, so by measuring their apparent brightness the distance to the stars (and the galaxy they’re in) can be found.
Here’s the really fun bit: They looked at 5,400 red giants in the galaxy to get the distance, and the new measurement puts NGC 1052-DF2 about 72 million light years from us, even more distant than before!
That makes things worse. It means the total brightness of the visible matter in the galaxy is higher (since it’s farther away than we thought) and so the dark matter content is even lower.
If we accept that the galaxy is truly at this distance it’s really hard to understand why it has no dark matter. There are some hypotheses on how a dark-matter-haloless dwarf galaxy can form, but they take some special conditions. One of them is that it forms near a much bigger galaxy that can strip the halo away from it. As it happens NGC 1052 is pretty close to DF2, so it’s possible this is what happened.
In the end these new Hubble images deepen the problem. It’s possible that the globular cluster velocities measured to get the galaxy’s total mass were off enough that the dark matter mass calculated is wrong, but that’s not covered by these new images. Perhaps more observations from ground-based telescopes can tighten that up.
Consider this whole thing an update, a chapter in the mystery novel that gives more information but doesn’t seem to help further the narrative other than dismissing a red herring. Fun, but there are still a lot of chapters to go.