Astronomers have long considered dark matter the invisible framework holding galaxies together. Without its substantial gravitational pull, galaxies would likely disintegrate due to their rotational speed. However, scientists are increasingly finding galaxies that appear to be entirely missing this crucial component.

Third Galaxy Lacking Dark Matter Discovered

The latest discovery, detailed in a new paper available on arXiv, focuses on a galaxy named NGC 1052-DF9. This finding, led by Michael Keim, Pieter van Dokkum, and their team from Yale, provides further evidence for a controversial galaxy formation theory known as the “Bullet Dwarf” collision scenario.

The Initial Discovery: NGC 1052-DF2

In 2018, Dr. van Dokkum’s team published research on another ultra-diffuse galaxy, NGC 1052-DF2. This galaxy is comparable in size to the Milky Way but contains up to 500 times fewer stars. Its diffuse nature allows background galaxies to be visible through it.

DF2 was the first indication that galaxies could exist without dark matter. The discovery demonstrated that dark matter is a distinct physical substance capable of being separated from ordinary matter. This presented a challenge to Modified Newtonian Dynamics (MOND), a theory attempting to explain galactic rotation curves without invoking dark matter.

MOND vs. Dark Matter

MOND proposes that gravity behaves slightly stronger than expected at extremely low accelerations, such as those experienced by stars at the edges of galaxies. If MOND were correct, DF2’s stars would be moving faster than predicted by its visible mass. However, observations showed the stars moving at a slower pace, consistent with standard Newtonian dynamics.

This presented a paradox for MOND: if it’s a fundamental law of physics, it should apply universally. The “normal” gravity observed in DF2 suggested that the “extra gravity” seen in other galaxies wasn’t a universal rule.

Confirmation and Further Discoveries

Initial skepticism regarding the distance calculations of DF2 was addressed when the Hubble Space Telescope confirmed the initial measurements. Subsequently, the team discovered another galaxy, DF4, forming a linear tail with DF2 and exhibiting similar properties.

NGC 1052-DF9 and the 'Bullet Dwarf' Collision

The latest paper introduces NGC 1052-DF9, which aligns with the pattern established by DF2 and DF4. This strengthens the evidence for a series of ultra-diffuse galaxies lacking dark matter. The research team believes the most likely explanation is the “Bullet Dwarf” Collision theory.

How the Collision Works

The “Bullet Dwarf” collision occurs when two gas-rich dwarf galaxies collide at high speeds. Dark matter halos pass through each other due to their gravitational interaction, while the normal matter (gas clouds) collide.

This collision separates the gas from its dark matter, triggering star formation and leaving behind galaxies devoid of dark matter. DF9’s lack of dark matter supports this theory. The team plans to study additional galaxies along the trail, though their increasing distance makes observation more challenging.

NGC 1052-DF9 serves as evidence for both the existence of dark matter and the violent processes involved in galaxy formation.