NGC 1052-DF2, or DF2 for short, This Ghostly Galaxy Has Almost No Dark Matter

In addition to providing insight into how galaxies form, the unusual galaxy is helping strengthen the argument for the existence of dark matter, researchers said.

According to the new paper’s lead author, Pieter van Dokkum, a researcher from Yale University, the galactic find challenges the standard idea of how galaxies are born. While interactions between normal and dark matter have long been considered a key element in galaxy formation, the dearth of dark matter in NGC 1052-DF2 galaxy challenges that assumption.

“Dark matter is apparently not a requirement for forming a galaxy,”

– van Dokkum (Space.com)

NGC-1052-DF2
A Hubble Space Telescope image of the galaxy NGC 1052-DF2. Distant galaxies are visible through DF2 due to its lack of stars. Credit: P. van Dokkum/R. Abraham/STScI, Space Telescope Science Institute

What is Dark Matter? and What is Dark Matter Deficiency?

Dark Matter is a form of matter whose presence is discernable only through its gravitational interactions with baryonic, or “normal,” matter. The unseen matter seems to make up roughly 80 percent of the mass of the universe and is thought to play a key role in galactic evolution.

The mysterious material is considered the scaffolding of the universe. Small bits of dark matter clump together to create the bones of the scaffold, growing larger over time. This growth is accompanied by the formation of stars from the gas and dust of the galaxy. According to NASA, the interaction of stars and galaxies within the dark matter is thought to have produced the galaxies astronomers observe today.

But DF2 defies this idea. Van Dokkum and his colleagues were first drawn to the strange galaxy by a collection of 10 unusually bright compact objects orbiting around it. Using a combination of the light of the galaxy’s stars and their color, the scientists measured how much normal mass could be found within the galaxy. They found that DF2 is about the size of the Milky Way but has about 200 times fewer stars.

The mass of the combined stars is about 200 million times the mass of Earth’s sun, a unit known as a solar mass. The researchers then used the motion of the bright objects, classified as globular clusters, to calculate the total mass in the galaxy.

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dragonfly_telephoto_array
Roberto Abraham (far left) from the University of Toronto, Pieter van Dokkum (far right) from Yale University, and their team of Toronto and Yale graduate students pose with half of the 48-lens Dragonfly Telephoto Array in New Mexico.
Credit: University of Toronto

Unimaginary Dark! A see-through galaxy

DF2 lies about 6.5 billion light-years from the Milky Way, one of several galaxies found within a galactic group. This galaxy is not a recent discovery, but its swarm of unusually bright globular clusters caught the eye of the researchers, who had been studying a class of galaxies known as ultra-diffuse galaxies. These faint galaxies, which include DF2, can be as large as the Milky Way but shine only 1 percent as brightly.

DF2 Making the case for dark matter

Studies of galaxies in the 1950s first indicated that the universe contained more matter than could be measured with the naked eye. Measurements of the motions of galaxies and globular clusters have long suggested that these objects are spinning more rapidly than can be accounted for based on measurements of visible matter.

Ironically, although it is deficient in dark matter, DF2 may help argue the case for the existence of the unseen material. While most researchers seem to agree that dark matter dominates the universe, alternative explanations do exist. Modified Newtonian dynamics (MOND) proposes a modification of Newton’s laws to account for the observed discrepancies. Emergent gravity is another idea that suggests that space-time is made up of small elements whose collective motion produces the force of gravity. The pair are the most dominant of several scientific proposals arguing against dark matter.

“In those theories, dark matter is not real but an illusion, caused by our lack of knowledge of gravity on large scales,” van Dokkum said. “If that’s the case, every galaxy should show a dark matter signature  it’s not something you can turn on or off in those models.”

Until the discovery of DF2, that has been the case. But the very existence of a galaxy with virtually no dark matter makes the case that these non-dark-matter models are incorrect, he said.

“Paradoxically, the absence of dark matter in this galaxy is evidence for [dark matter’s] existence,”

– van Dokkum said.

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