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The Latest Mystery of Galaxies Devoid of Dark Matter

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Dark matter, an invisible mass that makes up approximately 96% of the Universe, is a non luminous material that does not emit, absorb, or interact with light in any way. Dark matter exists in space and can take many different forms such as “cold dark matter” which are weakly interacting particles or “hot dark matter,” high energy, randomly moving particles that were created shortly after the Big Bang event nearly 13.8 billion years agoDark matter is distinguished by its gravitational attraction, being able to affect the speed of  clustered galaxies

The existence of Dark matter was first hypothesized in 1933 by Swiss astronomer, Fritz Zwicky, who was puzzled by the fact that the mass of the stars in the Coma cluster of galaxies, located in the constellation Coma Berenice, only accounted for 1% of the mass that was needed to keep the galaxies from escaping the cluster’s gravitational pull. This “missing mass” stumped scientists for decades until American astronomers, Vera Rubin, and W. Kent Ford established its existence through the examination of a similar phenomenon that states that “the mass of the stars visible within a typical galaxy is only about 10% of that required to keep those stars orbiting the galaxy’s center.”

Newton and Einstein both stated that mass and gravitational pull are proportionally related: the more mass an object has, the more gravitational pull it contains.Similarly, the further an object is from the center, the slower it should travel in orbit since the gravitational pull is weaker, which is why, according to Newtonian gravity, all the galaxies are pulling on each other. For example, the mass of the Sun pulls Mercury faster than Pluto since Mercury is much closer to the Sun. 

However, Fritz Zwicky and Vera Rubin, did not observe this phenomenon. While Rubin was observing the rotational curves of galaxies, like the Milky Way, similar to Zwicky, her observations were odd: she measured that as you continued to get further away, the velocity of the orbiting gas and dust remained constant. 

As a result, the phenomenon of dark matter was born which states that there is invisible matter surrounding the galaxy that keeps it whole and in–tact. It is this dark matter that exists in galaxies and has enough mass to keep the rotation speed of planets constant. 

However, 40 years after establishing that all galaxies have dark matter, recent research at the University of California, Irvine, and Pomona College have demonstrated  a surprising revelation that there may be galaxies that can exist without dark matter.. 

The team of collaborating astrophysicists, including James Bullock from UCI, reported that smaller galaxies collide with bigger galaxies that strip the smaller galaxies of their dark matter, reported in a new Nature Astronomy study. The possible explanation of galaxies being able to exist without dark matter began in 2018 when astrophysicists, Shany Danieli and Pieter van Dokuum of Princeton and Yale University discovered that there were two galaxies that seemed to exist without much of their dark matter. 

When the research was published, this  novel revelation threw the scientific community into confusion because of the previously held notion that galaxies not just contain but  need dark matter. This signaled to scientists that the modern  understanding of dark matter had to be revised. 

As a result, Moreno, an astronomy professor at Pomona College, and his team ran a computer-generated model that simulated the evolution of a portion of the universe and discovered seven galaxies without dark matter, due tocollisions with neighboring galaxies that were 1,000 times bigger , consequently being  stripped of most of their material. Left behind were only stars and residual dark matter.

A Professor at the University of Zurich, Robert Feldmann, who designed the new simulation stated that, “this theoretical work shows that dark matter-deficient galaxies should be very common, especially in the vicinity of massive galaxies.” 

James Bullock, an astrophysicist at UC Irvine and an expert on low mass galaxies explained how he and his team did not just construct their model so that they could construct galaxies without dark matter since the program was not “designed” in a way to create the collisions that the team actually found. 

Regarding dark matter research, James Bullock stated that, “We have a successful model, developed over decades of hard work, where most of the matter in the cosmos is dark. There is always the possibility that nature has been fooling us.” 

However Jorge Moreno stated that, “you don’t have to get rid of the standard dark matter paradigm.” 

Now astronomers understand how a galaxy can strip away its dark matter. Nevertheless, Bullock states, “It still doesn’t mean this model is right. A real test will be to see if these things exist with the frequency and general characteristics that match our predictions.”

Professor Moreno has indigineous roots and received permission from Cherokee leaders to name the 7 galaxies without dark matter after the 7 Cherokee clans: “Bird, Blue, Deer, Long Hair, Paint, Wild Potato and Wolf.” Jorge Moreno stated that he “feels a personal connection to these galaxies…More massive galaxies robbed the smaller galaxies of their dark matter…Many people of indigenous ancestry were stripped of our culture. But our core remains, and we are still thriving.”

Finding galaxies that lack dark matter challenges the long held belief that galaxies need dark matter to exist. Since these galaxies that are devoid of dark matter are rare, it will take more research of galaxies without dark matter to settle this puzzle. 

Sera Selin Guven is a STEM staff writer for the winter 2022 quarter. She can be reached at guvens@uci.edu.