Astronomical controversies are fascinating; they are usually exciting and, more often than not, tells us that we understand very little. Galaxy Dragonfly 2 (DF2) could be the perfect example of this. When it was discovered, it was announced that this might be a galaxy without dark matter. Exciting and puzzling!
More of such galaxies have since been discovered and follow-up observations by a different group found DF2 and others to be much closer than the original measurement. Based on this, they suggested that this meant they were not as big as they appeared and so they weren’t actually lacking dark matter. But now another twist has occurred. DF2 has been observed again by the original team and it turns out that it is not at the distance that was originally estimated. It is even further away from us! (queue dramatic music)
The new observations were possible thanks to the keen eye of the Hubble Space Telescope. The new findings are reported in The Astrophysical Journal Letters and the distance of DF2 is now estimated to be 72 million light-years. The team estimates that this galaxy contains 1/400th of the dark matter that it should have.
"We went out on a limb with our initial Hubble observations of this galaxy in 2018," senior author Professor Pieter van Dokkum, from Yale University, said in a statement. "I think people were right to question it because it's such an unusual result. It would be nice if there were a simple explanation, like a wrong distance. But I think it's more fun and more interesting if it actually is a weird galaxy."
The new distance was obtained by looking at aging red giant stars. These objects all reach the same peak brightness in their evolution, which allows researchers to use them to estimate the distance. They are a sort of luminous milestone.
"Studying the brightest red giants is a well-established distance indicator for nearby galaxies," lead author Zili Shen, a graduate researcher at Yale University, explained.
Dark matter remains a hypothetical substance. It was theorized to explain the peculiar rotation of galaxies, but its true nature is yet to be determined despite many different experiments. For this reason, some have proposed that there’s no such thing as dark matter but that our laws of physics have to be rewritten. The existence of DF2 and similar galaxies instead provide a clear indication that dark matter is a substance.
"In our 2018 paper, we suggested that if you have a galaxy without dark matter, and other similar galaxies seem to have it, that means that dark matter is actually real and it exists," van Dokkum said. "It's not a mirage."