Last year, Yale-led researchers announced the discovery of a galaxy with little to no dark matter, a finding that was received with both interest and skepticism. Now, follow-up research continues to show the peculiar lack of dark matter in this object and has discovered a second one with similar characteristics.
NGC 1052-DF2, or just DF2, was spotted using the Dragonfly Telephoto Array, an instrument dedicated to finding particularly faint objects. And that’s necessary! DF2 is an ultra-diffuse galaxy, as large as the Milky Way but with 100 to 1,000 times fewer stars.
To measure the mass of a galaxy such as this, the team track how globular clusters of stars move inside the galaxy. Based on how they move, they can work out how much matter there is. The latest measurements, as reported in The Astrophysical Journal Letters, were performed using the Keck Cosmic Web Imager on the W. M. Keck Observatory. With that instrument, they obtained a very precise speed for the globular clusters in DF2. A speed consistent with a galaxy having just the matter we can see, and no dark matter. This confirmed what they found last year.
“The fact that we’re seeing something that’s just completely new is what’s so fascinating,” lead author Shany Danieli, a graduate researcher who first spotted the galaxy about two years ago, said in a statement. “No one knew that such galaxies existed, and the best thing in the world for an astronomy student is to discover an object, whether it’s a planet, a star, or a galaxy, that no one knew about or even thought about.”
The second study, also published in The Astrophysical Journal Letters, focused on the discovery of NGC 1052-DF4, or DF4 for short. DF4 belongs to the same group of galaxies as DF2 and is an ultra-diffuse galaxy. For the team, this shows that DF2 is not an isolated case.
“Discovering a second galaxy with very little to no dark matter is just as exciting as the initial discovery of DF2,” said Professor Pieter van Dokkum, who is the lead author of the DF4 paper. “This means the chances of finding more of these galaxies are now higher than we previously thought. Since we have no good ideas for how these galaxies were formed, I hope these discoveries will encourage more scientists to work on this puzzle.”
The discovery of these galaxies has important consequences for cosmological theories. It strongly suggests that dark matter is indeed a substance, although we are none the wiser about its exact properties. The theories need to allow for galaxies to form outside dark matter halos or to somehow lose dark matter, and that is something that has not been considered before.
The team will now look for more candidates using the Dragonfly Telephoto Array before following up their observations using the Keck telescope.