Unveiling the Cosmic Enigma: Galaxies Without Dark Matter
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Chapter 1: The Revelation of NGC 1052-DF2
The realm of astrophysics has long been dominated by the concept of dark matter, with astronomers generally believing that every galaxy harbors a portion of its mass in this elusive substance. However, groundbreaking research indicates that certain galaxies may exist without any dark matter at all.
A notable example is NGC 1052-DF2, identified in 2015 as the first galaxy observed to have minimal to no dark matter. This ultra-diffuse galaxy has a tenuous grip on its stars and was discovered by a team led by Yale University graduate student Shany Danieli. Pieter van Dokkum from Yale expressed the excitement of uncovering such an unprecedented find, stating, "The fact that we’re seeing something that’s just completely new is what’s so fascinating. No one knew that such galaxies existed, and the best thing in the world for an astronomy student is to discover an object, whether it’s a planet, a star, or a galaxy, that no one knew about or even thought about."
The vast enigma of dark matter continues to perplex scientists. This invisible entity, which emits no radiation and interacts with ordinary matter solely through gravity, plays a crucial role in holding galaxies together. In fact, it significantly outweighs the visible matter in the cosmos.
The Search for Dark Matter
The first signs of dark matter emerged in 1933, when astronomer Fritz Zwicky examined galaxies within the Coma Cluster. He concluded that the cluster would disintegrate if it contained only the observable mass. This led to the hypothesis of an unseen force maintaining the cluster's integrity. Over time, it became clear that dark matter was not limited to intergalactic spaces.
In the late 1970s, Vera Rubin and Kent Ford conducted studies on the rotational dynamics of the Andromeda Galaxy. Their findings were astonishing; objects on the periphery of galaxies were rotating at similar speeds as those closer to the center, contrary to the expectations set by gravitational laws. Rubin, recalling Zwicky’s earlier work, proposed that the presence of dark matter within galaxies could account for these unexpected velocities. Today, it is estimated that dark matter constitutes about 84 percent of the universe's total mass, vastly surpassing the visible stars and galaxies.
Where Has the Dark Matter Disappeared?
In their investigations of DF2, astronomers observed that galaxies within clusters exhibited motion consistent with the presence of only ordinary matter. Van Dokkum remarked, "For decades, we thought that galaxies start their lives as blobs of dark matter. After that everything else happens: gas falls into the dark matter halos, the gas turns into stars, they slowly build up, then you end up with galaxies like the Milky Way. NGC 1052-DF2 challenges the standard ideas of how we think galaxies form."
This initial discovery of a galaxy defying the norm is rare in science. Often, such findings are later deemed erroneous; however, when they prove accurate, they can lead to revolutionary insights. Van Dokkum shared the uncertainty that accompanies such discoveries, stating, "If there’s one object, you always have a little voice in the back of your mind saying, ‘but what if you’re wrong?’ Even though we did all the checks we could think of, we were worried that nature had thrown us for a loop and had conspired to make something look really special whereas it was really something more mundane."
Chapter 2: The Confirmation of NGC 1052-DF4
In a follow-up study using the Keck Observatory, researchers identified a second ultra-diffuse galaxy, NGC 1052-DF4, which also showed little to no signs of dark matter. These galaxies, characterized by having less than one percent of the stars found in the Milky Way, occupy a similar spatial volume as our galaxy.
The existence of galaxies devoid of significant dark matter fortifies the earlier theories proposed by Zwicky, Rubin, and Ford, suggesting that normal and dark matter can coexist independently as well as together. This observation presents challenges for alternative theories that propose varying gravitational forces across different regions of the universe.
Danieli explained, "We want to find more evidence that will help us understand how the properties of these galaxies work with our current theories. Our hope is that this will take us one step further in understanding one of the biggest mysteries in our universe — the nature of dark matter."