Dark matter is the mysterious substance that's believed to permeate the universe, outweighing regular matter by a ratio of five to one. It's thought to play a central role in the formation of galaxies, which made the discovery last year of a galaxy with no dark matter quite surprising. But now, astronomers have examined the oddball galaxy a bit closer – and found that it's not so special after all.
Evidence abounds that there's more to the universe than meets the eye. For more than 80 years it's been established that galaxies are moving much faster than they should be if their mass was just what we could see as stars. That led astrophysicists to infer the existence of huge amounts of unseen material, dubbed "dark matter."
This strange stuff also helped plug up holes in other theories, such as how galaxies form in the first place. Since dark matter has such a strong gravitational influence, regular matter is drawn to it, until the density of all that material causes it to collapse in on itself, firing up as new stars.
But this neat little origin story was thrown into doubt last year, thanks to an unassuming little galaxy called NGC1052-DF2 (or DF2, for short). Calculations by astronomers at Yale University showed that the movement speed of DF2 was consistent with the mass of the matter we could see as stars and planets, meaning it had very little, if any, dark matter.
Although it might seem to disprove the theory of dark matter, that observation was actually taken as further evidence for the mysterious material. After all, if dark matter was just a mathematical mistake, that same "mistake" should be readily applied to every single galaxy observed. Its absence in this case was therefore seen as evidence that dark matter is a physical substance, which is usually (but not always) found blobbing around galaxies.
That said, it did also raise new questions about the birth of galaxies. If they didn't need dark matter to form, how else could they get started?
But it now seems that last year's study was itself affected by a different mathematical mistake. Astronomers from the Instituto de Astrofísica de Canarias (IAC) in Spain have examined the galaxy in more detail, and found that one wrong measurement might have thrown off the whole thing.
According to the team on the new study, red flags were raised when it was noticed that all the strange features about DF2 were dependent on its distance from Earth, while those properties that don't rely on the distance all seemed normal. The team used five independent methods to calculated the distance for themselves – and in all five cases, the galaxy appeared much closer than had previously been found.
Where the original study has calculated DF2's distance to be about 64 million light-years away from Earth, the IAC's revised figure is closer to 42 million. And when the other properties are calculated again based on that new distance, all the weirdness goes away.
With the new figures, the total mass of the galaxy is found to be about half of what was previously thought. The mass of the stars accounts for about a quarter of that, leaving the remaining three quarters to be made of dark matter. In short, DF2 is just like any other galaxy, the team says.
This new finding has plenty of important implications. For one, it removes that doubt about the origin story of galaxies that the possible existence of a dark matter-deficient galaxy raised. But it also now calls into question another study that later "confirmed" the anomalies of DF2, as well as a third that claimed to have found a second example of a galaxy light on the dark stuff.
While some people may see this new study as evidence of how fallible science can be, in our eyes it reinforces the scientific method. After all, it's important that bold claims like a galaxy with no dark matter are investigated by independent teams, and even when they're found to be wrong, science is all about revising our understanding based on new information.
The new research was published in the journal Monthly Notices of the Royal Astronomical Society.
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