Astronomers have long argued that dark matter is the invisible scaffolding that holds galaxies together. Without its immense gravitational pull, the galaxies’ rotating necks would force them to fly sideways. But now, scientists have found a series of galaxies that do not seem to have their dark matter completely. The latest in this string, known as NGC 1052-DF9, is described in a new paper, available in preprint on arXiv, by Michael Keim, Pieter van Dokkum and their Yale team. It gives credence to the strong theory of the formation of the galaxy known as the “Bullet Dwarf” collision, which has been a controversial topic for the past decade.
Back in 2018, Dr. van Dokkum and his team published another research paper describing the galaxy NGC 1052-DF2 (DF2). This galaxy was the size of the Milky Way, but had 500 times fewer stars. It was so scattered that you could actually see other, older stars shining through it. And it was the first clue that galaxies could exist without dark matter binding them together.
The discovery of DF2 proved that dark matter is a unique, physical substance that can be separated from ordinary matter. This was a severe blow to Modified Newtonian Dynamics (MOND), which was designed to explain why stars in the outer edges of galaxies move at great speeds. MOND says that at very low velocities, like the stars at the edges of the galaxy, the gravitational force works a little more strongly than expected.
Fraser discusses a dark matter with Dr. Surjeet Rajendran
MOND itself tries to explain the law of nature – while dark matter solves the same problem, but with physical matter. DF2 was a great playground to test those two concepts against each other. According to MOND, a massive galaxy like DF2 would develop the gravity it predicts for low-velocity internal conditions. So if MOND were true, the DF2 stars would be moving faster than their apparent mass is measured.
But that’s not what the researchers saw. They found that they move at a negative speed, which is perfectly described by the classical Newtonian unmodified forces. Thus, the discovery of DF2 presented a fatal paradox for MOND. If MOND is a fundamental law of physics, it should apply to everything – you can’t have a galaxy “out of” the laws of gravity. But the “normal” gravity observed in DF2 proved that the “normal” gravity observed in other galaxies was not a universal law.
As is often the case when new information is introduced in scientific discussions, there was considerable disagreement. A number of papers were published that questioned DF2’s spatial calculations, which could explain why its stars move the way they do. But Hubble looked at it, and confirmed the distance. In addition, Dr. van Dokkum and his team discovered another galaxy. Known as DF4, it forms a solid, green tail with DF2, and has many similarities.
Sabine Hossenfelder discusses the debate between MOND and dark matter. Credit – Sabine Hossenfelder YouTube Channel
This latest paper, which presents NGC 1052-DF9 (DF9), which fits well in the “tail” between DF2 and DF4, proves that approach. At this point the data strongly point to a series of very wide galaxies that appear to lack dark matter. So the question becomes – why do they lose dark matter?
According to the research team, the most likely explanation is the “Bullet Dwarf” Collision theory. Basically, that’s what happens when you crash two galaxies together at blinding speeds. The path between DF2, DF4, and DF9 strongly suggests that they were all created in the same catastrophic event. And that event may collide with “Bullet Dwarf”.
Imagine two young, gas-rich galaxies colliding. Since dark matter interacts only through gravity, the dark halos that hold these galaxies pass each other like ghosts. But ordinary objects, which, in this case, are giant clouds of gas, collide physically in massive collisions. The collision separates the gas from its dark matter, causing massive starbursts and leaving a series of galaxies completely devoid of dark matter.
DF9 missing dark matter, as the Bullet Dwarf Collision theory predicted, is a major achievement for the theory. But it may not be the last. After that, the team hopes to measure the kinematics for the fourth or fifth constellation on the way, although it is far away where little changes. For now, DF9 stands as proof of the existence of dark matter and the extreme and violent ways in which the universe can build galaxies.
Learn more:
MA Keim et al. – The Dark Third Lost Galaxy along the Trail of Galaxies in the field of NGC 1052
UT – Ultra-Diffuse Galaxy Found Without Dark Matter
UT – Galaxies Found with no Dark Matter at all
UT – A Large Group with a Dark Home
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