Galaxies without dark matter are rare and were once thought impossible. Find out how a new hypothesis about their formation may explain how all galaxies form and what dark matter is.
When I’m at the cottage in the summer, the night sky is usually clear and dark. That gives me a perfect view of the Milky Way on moonless nights. It’s a cloudlike, glowing belt of light that’s unmistakable between April and October here in the northern hemisphere.
Technically, every star in the sky is part of the Milky Way Galaxy. However, the luminous band stargazers call the Milky Way is a dense accumulation of stars between us and the galactic plane. It’s hazy because the stars are so close together that our unaided eye can’t resolve them individually.
The eclectic scientist Lord Kelvin was studying the Milky Way in 1884 when he came to a realization. He’d been trying to estimate its mass by measuring the velocity of the stars around the Milky Way’s galactic centre.
Answer Very Different from Mass of Individual Stars
He used that data to estimate the Milky Way’s gravity and from there its total mass. Inexplicably, the answer he got was very different from the figure he got by adding up the mass of all its individual stars.
That’s when scientists started musing about something they began to call “dark matter” or “missing matter.” A range of prominent astronomers in the early twentieth century including Henri Poincare, Jacobus Kapteyn, Knut Lundmark, and Jan Oort all weighed in on the odd discrepancy.
The Swiss astrophysicist Fritz Zwicky was studying a group of galaxies called the Coma Cluster in 1933. He calculated that the cluster had 400 times more mass based on its gravitational effects than he could account for based on its visible stars. The missing matter clearly wasn’t any minor discrepancy.
We Can’t Account for 85% of the Universe’s Mass
We now know Zwicky’s estimate was on the high side. Even so, his basic finding was correct. We still can’t account for about 85% of the Universe’s mass. Scientists continue to call that missing 85% dark matter.
Last week, the journal Nature published a study that looked at the mystery from the opposite point of view. There are a few galaxies without dark matter, and they may shed some light on the nature of this unexplained phenomenon.
Astronomers recently discovered a 6 million light-year long row of dwarf galaxies they call “bullet galaxies.” the group seems to have formed from a collision between two larger dwarf galaxies about 8 billion years ago.
Two Galaxies That Don’t Show Usual Effects of Dark Matter
This string formation contains two galaxies called DF2 and DF4 whose motions don’t show the usual effects of dark matter. The researchers found them first and then located between three and seven similar star groupings that may also be galaxies without dark matter.
These oddball galaxies are hard to explain because scientists believe dark matter is the foundation of all galaxies. It’s the dark matter that seems to draw in all the gas from which stars form. Something must have separated out the dark matter in these unusual galaxies.
The hypothesis is that when two dwarf galaxies collide, the dark matter might keep moving in the same direction without being disturbed by the collision. It could be unaffected because dark matter doesn’t seem to interact with ordinary matter.
Gas Clouds Would Form Galaxies Without Dark Matter
Despite this, the gaseous ordinary matter would still collide. This would create several clumpy gas clouds that would then evolve into galaxies without dark matter.
One reason the researchers believe the two unusual galaxies came from a collision is that they’re moving away from each other in a way that suggests they used to be in the same place.
That could explain that row of up to 11 galaxies. Their alignment may result from a collision similar to an impact astronomers know took place causing a formation called the Bullet Cluster. They’re calling this new, hypothetical collision a “bullet dwarf collision.”
Calling Proposed Collision a “Bullet Dwarf Collision”
On each end of the row, there are two faint, peculiar galaxies. These could be the dark matter that moved on unperturbed by the collision with some stray stars that also survived.
The team’s leader is Pieter van Dokkum of Yale University. He described the attraction of the hypothesis saying, “This single explanation explains so many odd things about these galaxies.”
It was the combination of all these observatons that led to the team’s fascinating explanation. “It was staring you in the face once you knew what to look for,” Professor van Dokkum said.
“Staring You in the Face Once You Knew What to Look For”
Our Universe has a peculiar, self-organizing property that we don’t fully understand. It’s difficult to grasp how the matter disbursed throughout the Universe comes together to form stars, which then group together into highly structured galaxies.
Humanity is working to pull together a New Story explaining how the Universe formed and our place within it. To complete that narrative, we need to unveil the mystery of dark matter and its role in galaxy formation.
The discovery of these two dwarf galaxies without dark matter and the row of similar galaxies to which they appear to belong may help to explain the behaviour of dark matter and the formation of ordinary galaxies. It may even shed light on whether dark matter is self-interacting like ordinary matter.
Team Is First To Admit Their Idea Is Still a Hypothesis
The team is the first to admit that their idea is still a hypothesis. Other scientists point out there could be other explanations for how these galaxies without dark matter formed besides a collision.
It’s not altogether clear that this string of galaxies is close together. They could merely seem to form a row from our point of view on Earth while actually being vast distances apart. They could be unrelated to one another apart from conincidentally being in our line of sight.
We also don’t know the masses of the two galaxies at each end of the line. Until scientists measure that, it’s too soon to say they’re made of the dark matter from those original colliding dwarf galaxies.
If Measurements Are Off, Galaxies Would Be Unremarkable
Also, the team’s measurements of the distance to the two galaxies without dark matter might be off. If they turn out to be closer to Earth than the researchers thought, their motion would be consistent with the normal quantity of dark matter for dwarf galaxies, making them unremarkable.
All of these questions need answers before astronomy promotes this hypothesis to a scientific theory. The good news is that, in the process of resolving all these issues, our understanding of dark matter and galaxy formation is almost certain to improve, regardless of the final conclusion the team reaches.
We always have more to learn if we dare to know.
Galaxies without dark matter perplex astronomers
Giant collision created galaxies devoid of dark matter
A trail of dark-matter-free galaxies from a bullet-dwarf collision
The 5 Big Questions We Need Cosmology to Answer
Gravitomagnetism: Does It Explain Away Dark Matter?
Mass Discrepancy: Is Dark Matter Hiding in Plain Sight?