Ask Ethan: What’s The Real Story Behind This Dark Matter-Free Galaxy?
“I read a study that said the mystery of a galaxy with no dark matter has been solved. But I thought that this anomalous galaxy was previously touted as evidence FOR dark matter? What’s really going on here, Ethan?”
Imagine you looked at the Universe, and saw a galaxy unlike any other. Whereas every other galaxy we’ve ever looked at exhibited a large discrepancy between the amount of matter that’s present in stars and the total amount of gravitational mass we’d infer, this new galaxy appears to have no dark matter at all. What would you do? If you’re being a responsible scientist, you’d try to knock down this galaxy by any scrupulous means possible. You’d wonder if you had mis-estimated one of its properties. You’d try to re-confirm the measurements with different instruments and techniques. And you’d wonder if there weren’t an alternative explanation for what we were seeing.
Well, if you read that the galaxy has dark matter after all, and the mystery has been resolved, you should definitely read this instead. The story is far from over, and even if the new team’s results hold up, there’s still a mystery at play here.
The Galaxy That Challenged Dark Matter (And Failed)
“It’s fair to say that galaxies come in a great variety of shapes, sizes, densities and masses. Despite all that we know, there is so much we’re still learning as far as how they form, evolve, and grow up in the Universe. But whenever you have a surprising observation, the first thing you need to check is whether the conclusion it leads you to holds up when you make your observation using a different method.
These new observations don’t prove that dark matter exists, but they do remove a primary reason for doubting it. Instead of a single object that lacks a cosmic explanation, we now have an object that’s consistent with the observations of many similar objects in the same class. NGC 1052-DF2 is an interesting object that deserves further study, but it’s unlikely to have no dark matter at all. Although observations will always be our guide, this result highlights how important it is to independently verify your work before drawing grand, revolutionary conclusions.”
Earlier this year, a study came out identifying an ultra-diffuse galaxy 65 million light years away. While it was the size of the Milky Way, it had only 0.5% of the stars our galaxy has inside of it. When scientists measured the motions of the globular clusters around it, they found that they weren’t moving relative to one another at all, indicating an extremely low mass for the galaxy: a mass so low that it implied the galaxy had no dark matter. This shouldn’t be possible! Did this mean that there was a flaw with the measurements? That dark matter didn’t exist?
No; it meant we needed a better, independent method to draw a responsible conclusion. Well, that’s what science just delivered! Happy end-of-2018 to us all!
5 Ways To Make A Galaxy With No Dark Matter
“2.) Ejected from galactic mergers. When two galaxies smash together, they usually merge entirely, but sometimes there is ejected material. Sufficient amounts could create a baryons-only galaxy.”
Last week, astronomers announced the discovery of the ultra-diffuse galaxy NGC 1052-DF2 (or DF2), which appears to be completely free of dark matter. Other similar galaxies have been seen before, but all contain more, not less, dark matter than you’d have expected on average. This first galaxy ever seen without the gravitation-altering effects of dark matter was touted to defy theory, but it does no such thing. In fact, there are many explanations that lead directly to galaxies such as DF2 as an inevitable consequence, including one that was put forth in a predictive fashion as much as 20 years ago.
Come find out the five ways to make a galaxy without dark matter, and learn why this is such an important test of dark matter theory itself!