With spectral data from Keck, van Dokkum's team found clusters of stars were moving much slower than expected, which indicates less mass in a system. It's coming in at around one two-hundredths the mass of our Milky Way. Normally, we infer that there's dark matter around because the galaxy appears to have a lot more matter than the amount provided by the stars we can see. "The stars in the galaxy can account for all of the mass, and there doesn't seem to be any room for dark matter".
Some have thought that what we think of as "dark matter" isn't actually another form of matter, but may be just another expression of how normal matter affects the universe. This galaxy without dark matter has never been observed before and it brings confusion to everything we've known about this invisible and odd thing that is so present in the cosmos.
Van Dokkum plans to use Keck to search for more galaxies like NGC 1052-DF2. Imaging them allowed the research team to determine their motions orbiting the galaxy, which could, in turn, provide a measure of the amount of mass that is involved in keeping the objects in place. And that normalcy is deeply weird.
Alternatively, in the absence of any direct dark-matter detections, some theorists have suggested its existence is illusory-and that something else drives galaxy evolution and gravitational lensing.
A galaxy far, far away is perplexing astronomers not because of what it contains, but because of what it doesn't: dark matter. Now researchers are pondering possible explanations for this missing dark matter in NGC 1052-DF2.
Dark matter is the glue which holds everything together in a galaxy.
It's a "game changer" galaxy, astronomers are saying, and it's like nothing we've ever seen before. Mysterious "dark matter" makes up the rest. Another idea is that it formed from matter spewed out by quasars.
"Basically, it was the seed for drawing in normal matter", Bauer told Live Science.
A galaxy entirely bereft of dark matter raises vexing questions that, so far, have stumped astronomers. "In those theories, every galaxy should show a dark matter signature, as it's not due to dark matter at all but due to the laws of physics", van Dokkum explained.
Most paths to explaining NGC 1052-DF2's formation implicate its galactic neighbors. They gravitationally attract ordinary, or baryonic, matter which eventually settles within the extant cloud of dark matter. "I indeed believe there is no dark matter that needs to be added to explain these observations".
"I don't find that [point of view] at all compelling", Bauer said. He also took note of the fact that the group had discovered a few other comparably astounding galaxies to investigate. "It looked like a diffuse blob sprinkled with very compact star clusters", said co-author Shany Danieli, a Yale graduate student.
If the current theory is wrong, that will also affect the strategies of the experiments trying to catch dark matter particles on Earth, says Bullock. Luckily, their studies might be given an early boost as Hubble images of 23 other ultra-diffuse galaxies seem to suggest that three of them are similar to NGC 1052-DF2. "So, this is at the hairy edge of credible".
And what about NGC 1052-DF2? The result of this would be that a signature usually attributed to dark matter should always be detected, and is an unavoidable outcome of the presence of ordinary matter.
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