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Astronomers Find a Galaxy Free of Dark Matter

March 31, 2018
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Astronomers Find a Galaxy Free of Dark Matter

This is the first time astronomers find an entire galaxy that’s devoid of dark matter. A poorly-understood building block of our universe.

Astronomers on Wednesday (March 28) announced the discovery of the first and only known galaxy without dark matter. Astronomers think that dark matter makes up about 27 percent of our universe.

“We thought that every galaxy had dark matter and that dark matter is how a galaxy begins,” said lead author Pieter van Dokkum, an astrophysicist at Yale University, in a press release. “This invisible, mysterious substance is the most dominant aspect of any galaxy. So finding a galaxy without it is weird. It challenges the standard ideas of how we think galaxies work, and it shows that dark matter is real: it has its own separate existence apart from other components of galaxies.”

The new discovery could entirely change the theories of how galaxies form.

NGC1052-DF2

The eerie galaxy in question, NGC1052-DF2 (or DF2 for short), is a small galaxy in a collection of galaxies dominated by the much larger elliptical NGC 1052.  It lies some 65 million light-years from Earth.

Researchers detected the see-thru galaxy using the Dragonfly Telephoto Array. The new Mexico-based telescope is built of camera parts that scientists designed to detect very faint galactic structures. Then, they further analyzed it by collecting more data using the Hubble Space Telescope as well as the Gemini North and Keck Observatories in Hawaii.

Unlike mirror-based devices, the mobile Dragonfly Telescope Array is composed entirely of nano-coated lenses, 48 in all.

The DF2 is about the same size as our galaxy, the Milky Way, but has 100 to 1,000 times fewer stars. The scopes detected 10 compact groups of these stars, also known as globular clusters, within the galaxy, according to a press release from Gemini.

However, those globular stars were moving much slower than the scientists’ models previously predicted. Thus, suggesting that there was less mass in the system than it would be expected if dark matter was present.

“If there is any dark matter at all, it’s very little,” says Pieter van Dokkum of Yale, leader of the research team.“The stars in the galaxy can account for all of the mass, and there doesn’t seem to be any room for dark matter.”

Researchers determined the speed of the rotating globular clusters around DF2 by analyzing the absorption lines of spectra. This allowed them to determine each cluster’s velocity, which they then used to calculate the overall mass of the galaxy. The Keck Observatory took the spectra. Image Credit: Gemini Observatory/NSF/AURA/W.M. Keck Observatory/Jen Miller/Joy Pollard

Formation of DF2

“There is no theory that predicted these types of galaxies. The galaxy is a complete mystery, as everything about it is strange,” said van Dokkum. “How you actually go about forming one of these things is completely unknown.”

But researchers have some theories about it.

One possible theory they talk about in the paper is that DF2 is actually a tidal dwarf galaxy. This type of galaxy can form during galactic mergers, which often fling out baryonic material – an ordinary matter that is made up of protons, neutrons, and electrons. But DF2 appears to have fewer metals than scientists would expect for a tidal dwarf galaxy.

The second theory suggests the galaxy formed when winds from a nearby quasar swept up large clouds of low-metallicity gas. But researchers point out that the galaxy may be too diffuse for this to be a likely scenario. Finally, the researchers suggest DF2 may have formed when portions of gas flowing toward NGC 1052 broke away due to jet-induced shocks from the larger galaxy’s black hole.

Currently, no one knows what holds together something as big as a galaxy without dark matter. Also, it will be very difficult to even understand how it formed.

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Thumbnail image: The “see-through” galaxy, NGC 1052-DF2 without a central region. Credit: NASA, ESA, and P. van Dokkum (Yale University)

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