Key Takeaways:

  • Astronomers found 19 more galaxies lacking dark matter
  • This challenges our understanding of galaxy formation
  • It suggests alternative ways for galaxies to form without significant dark matter
  • Studying these galaxies can help us understand their formation process
  • More work is needed to confirm the findings and learn more about these unusual galaxies

Researchers have found 19 additional galaxies that lack dark matter. These unusual galaxies are mostly composed of ordinary matter, such as protons, neutrons, and electrons, which make up everything we are familiar with, rather than dark matter.

The controversial recent discovery of two other galaxies without dark matter is supported by the new discovery, which was published in Nature Astronomy. The mysterious material, which makes up the majority of the universe’s matter, is believed to be the fundamental building block of all galaxies and the primary force behind the initial formation of galaxies. The fact that so many galaxies lack the exotic matter implies that astronomers are fundamentally misunderstanding the formation and evolution of galaxies.

In a press release, lead author Qi Guo of the Chinese Academy of Science stated, “This result is very hard to explain using the standard galaxy formation model and thus encourages people to revisit the nature of dark matter.”

Dark matter and galaxy rotation

Astronomers Vera Rubin and Kent Ford of the Carnegie Institution made the most famous observation of our nearest neighbor, the Andromeda galaxy, in the late 1970s. When they did, they found that the galaxy was not rotating in the way they had anticipated. In our solar system, the planets rotate around the Sun at different rates. Mercury is moving much faster up close than Neptune is from a distance. In Andromeda, on the other hand, objects visible on the outer rim of the galaxy move at the same speed as those in close orbit around the galaxy’s center.

Ford and Rubin were confused. It implied that enormous quantities of invisible matter must be present throughout the Andromeda galaxy, extending far from its center. In the end, their finding proved to be the initial tangible evidence of dark matter.

Astronomers infer the existence of dark matter largely based on the fact that the rotation curves of galaxy’s are not what you would expect without some form of hidden mass spread throughout the entire galaxy. In this simulation, the galaxy on the left shows what rotation would look like without the effects of dark matter, while the right shows rotation with dark matter. Note how the stars and gas on the outside of the right galaxy are spiraling much faster than those in the left galaxy. Ingo Berg/Wikimedia Commons

In the decades that followed, astronomers discovered that dark matter—a material that interacts with matter and light only through the force of gravity—seemed to be abundant in every galaxy. Then, in 2018, a strange, ghostly galaxy known as NGC 1052-DF2 was discovered by scientists led by Pieter van Dokkum of Yale University. It appeared to contain very little, if any, dark matter.

“We thought that every galaxy had dark matter and that dark matter is how a galaxy begins,” van Dokkum said in a press release after the discovery. This material is mysterious and invisible, and it dominates every galaxy. It is therefore surprising to find a galaxy without it. It contradicts accepted theories about how galaxies function and demonstrates the existence of dark matter, which exists independently of other components of galaxies.

A few months later, NGC 1052-DF4 was discovered by van Dokkum and colleagues as a second galaxy without of significant dark matter. Similar to NGC 1052-DF2, this ultra-diffuse galaxy caused some controversy among astronomers.

Ignacio Trujillo, an astronomer at the Instituto de Astrofisica de Canarias in Spain, was among the critics.

“Something that caught my attention very early on was the fact that the galaxy [DF2] was not only anomalous for not having dark matter, but also for having an extraordinarily bright population of globular clusters,” Trujillo told Astronomy. “I remember thinking: ‘Two anomalies at the same time really looks odd.’”

galaxydf2darkmatter
NGC1052-DF2, imaged here by the Hubble Space Telescope, is a large, but very diffuse galaxy thought to contain a negligible amount of dark matter. NASA/ESA/P. van Dokkum (Yale University)

But after an academic back-in-forth, where Trujillo and van Dokkum traded salvos in the form of research papers, the answer to whether these galaxies are really missing their dark matter still remains uncertain.

A growing list of galaxies without dark matter

However, the finding of 19 additional galaxies lack of dark matter now decreases the strangeness of DF2 and DF4. Furthermore, astronomers will need to give careful consideration to the implications of this expanding population of dark matter-free galaxies if the most recent results hold true.

Guo and her colleagues used data from the Arecibo Observatory in Puerto Rico and the Sloan Digital Sky Survey to investigate the nature of 324 dwarf galaxies, and they found the most recent batch of galaxies lacking dark matter. They investigated the rotational speed of hydrogen gas around each galaxy, following in the footsteps of Rubin and Ford, using this data. They also calculated the amount of normal matter they contained, which included both stars and gas.

After doing some math, Guo and her colleagues found that 19 out of the 324 dwarf galaxies they looked into had enough visible matter to account for the hydrogen motions in the galaxies alone. Stated differently, these galaxies appear to be lacking a significant amount of dark matter.

According to the study, “Our results suggest that a population of dwarf galaxies could form in a particular way such that much less dark matter is required than for those in the Local Group [our cosmic neighborhood] and those found in simulations.”

What relevance does the discovery of galaxies without of dark matter have? Van Dokkum asserts that this would imply that our understanding of the formation of galaxies is incomplete. Currently, scientists believe that galaxies form only when the gas and dust required to initiate star formation are drawn to the galaxy by the gravitational pull of massive amounts of dark matter.

“The thing is, we have no idea how star formation would proceed in the absence of dark matter,” van Dokkum explained. “All we can say is that there must have been very dense gas early on in their history.” The galaxies couldn’t produce any new stars if they couldn’t.

Guo and her colleagues indicate that in the future, additional work should be done by astronomers to map the motions of hydrogen gas inside these galaxies. And with that, they hope to discover more about the initial processes leading to the formation of these galaxies devoid of dark matter.

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