Scientists Uncover a Galaxy with Nearly No Stars, Held Together Almost Entirely by Dark Matter
TL;DR
Astronomers have discovered Dragonfly 44, a galaxy composed of 99.99% dark matter, with far fewer stars than the Milky Way. Despite being nearly invisible, dark matter is essential in holding the galaxy together, defying expectations of galaxy formation. The discovery raises new questions about dark matter’s role in galaxy formation and how this massive galaxy remains intact with so little visible matter. It could change our understanding of dark matter and how galaxies evolve.
Astronomers have discovered a galaxy nearby with a similar mass to the Milky Way, yet it contains less than 1 percent of its stars.
The galaxy is so faint that it has remained undetected for decades. Now, the team behind its discovery has determined how it has avoided being torn apart by its lack of stars — it’s composed of 99.99 percent dark matter.
Dark matter is thought to account for about 27 percent of the mass and energy in the observable Universe. While its gravitational force is detectable, it does not emit any form of light or radiation that we can observe.
Despite extensive searching, we still don’t know what dark matter is. However, this unseen matter is vital for the stability of the Universe. Galaxies spin at such high speeds that they would disintegrate if only their own gravity were keeping them intact. Something else must be holding them, and physicists suspect it’s dark matter.
In fact, cosmology’s standard model suggests that for every gram of atoms in existence, there’s at least five times more dark matter in the Universe. Now, scientists have identified a galaxy that’s nearly entirely made of dark matter.
Named Dragonfly 44, the galaxy was first spotted in 2014 when a team using the WM Keck Observatory and the Gemini North Telescope in Mauna Kea, Hawaii, discovered several ‘fluffy galaxies’ in the Coma Cluster, located about 320 million light-years away.
“If the Milky Way is a sea of stars, then these newly discovered galaxies are like wisps of clouds,” said researcher Pieter van Dokkum from Yale University at the time. “We are beginning to form some ideas about how they were born, and it’s remarkable they have survived at all,” he added.
“They are found in a dense, violent region of space filled with dark matter and galaxies whizzing around, so we think they must be cloaked in their own invisible dark matter ‘shields’ that are protecting them from this intergalactic assault.”
Now, van Dokkum and his team have been able to test their theory, and by calculating the mass of Dragonfly 44, they say they have enough proof that dark matter is indeed what’s keeping this galaxy intact. The researchers observed the velocities of stars in Dragonfly 44 over 33.5 hours across six nights and used that data to estimate the galaxy’s total mass.
An object’s mass increases with velocity because faster speeds boost its kinetic energy. This means that the faster the stars move, the more massive the galaxy must be. By measuring the stars’ speeds at around 47 kilometers per second, the team estimated that the galaxy has a mass approximately 1 trillion times that of the Sun — far too heavy for the stars alone to hold together.
“Motions of the stars tell you how much matter there is,” van Dokkum explained to Avery Thompson at Popular Mechanics. “They don’t care what form the matter is, they just tell you that it’s there. Using the Keck Observatory, we found many times more mass indicated by the motions of the stars than there is mass in the stars themselves.”
After estimating that Dragonfly 44 needs to be composed of 99.99 percent dark matter to stay together, the team has officially identified the darkest known galaxy in the Universe. A similarly dark galaxy was found earlier this year in the Virgo cluster, but with 99.96 percent dark matter, it’s now been surpassed.
While this discovery is fascinating, it has raised more questions than answers. Currently, no candidate for dark matter has produced enough evidence to explain its makeup, and until recently, the dark matter galaxies known to us have been relatively small.
Dragonfly 44 is massive, and no one can yet explain how it became so large — and remained so — with such little visible matter. At least, now we have a galaxy brimming with dark matter to examine further, right?
“It’s hard to argue with the observations, yet the conclusion from this paper runs counter to my understanding of how galaxies are formed,” said astronomer Marla Geha from Yale University, who was not part of the research, in an interview with New Scientist.
“I’m hoping these objects are rather rare and/or only form in special environments such as a dense galaxy cluster. Otherwise we may need to rewrite galaxy formation.”
The research has been published in Astrophysical Journal Letters.
