Galaxy seems to lack dark matter, stumping astronomers

Astronomers baffled by distant galaxy that doesn't have any dark matter

Astronomers baffled by distant galaxy that doesn't have any dark matter

So scientists were more than a bit surprised to find a galaxy that has little to no dark matter at all.

"Finding a galaxy without dark matter is unexpected because this invisible, mysterious substance is the most dominant aspect of any galaxy."
From the W.M. Keck Observatory, they measured the velocity of clusters of stars (called globular clusters) within DF2 and found they were moving slower than expected.

Van Dokkum and colleagues identified the galaxy, NGC 1052-DF2, using a low-budget setup called the Dragonfly Telephoto Array in New Mexico, which they designed from 48 commercial cameras and paparazzi-style lenses. "This thing is astonishing", said team member Pieter van Dokkum, "a very big blob that you can look through".

Van Dokkum said NGC1052-DF2 is so sparse that "it is literally a see-through galaxy".

Speaking with Space.com, Pieter van Dokkum of Yale University explained that for a galaxy with a star mass of 200 million solar masses, the "normal" would be 80 billion solar masses of dark matter. Instead, van Dokkum's team found those star clusters moving languidly around NGC 1052-DF2, a sign that there may well be very little or no dark matter within that galaxy at all.

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The galaxy was studied by many telescopes (Jemini and Kek Hawaii, Space Hubble, etc.) to cross-check the results.

Paradoxically, the discovery of a galaxy without dark matter may actually confirm that the stuff actually exists by contradicting hypotheses advanced by dark matter doubters. The result is typically the spiral arms or gaseous discs you can see in other galaxies. In addition to the dearth of dark matter, DF2 is unusual in another way: it is roughly the size of our Milky Way Galaxy, but contains only 1/200 the number of stars.

In other words, the question arises: if a galaxy did not start from dark matter, which then gravitationally attracted common matter, how did it form? It confounds astronomers' theories of the universe, which says galaxies need dark matter to form. "It challenges the standard ideas of how we think galaxies form". The formation of NGC 1052-DF2 may have been helped by powerful winds emanating from the young black hole that was growing in the center of NGC 1052. So how do astronomers know they exist? It moves just as fast as the regular old laws of astrophysics would suggest, when calculated based on the mass of the stars researchers can see.

The galaxy is about 65 million light-years away in the constellation of Cetus and is about the same size as the Milky Way. The galaxy is a complete mystery, as everything about it is odd.

"It is conventionally believed to be an integral part of all galaxies, the glue that holds them together and the underlying scaffolding on which they are built", said co-author Allison Merritt from the Max Planck Institute for Astronomy, in Germany. That stellar sparseness means it does not look much like a typical spiral galaxy, but rather a loosely connected, ghostly blob of star-pocked gas and dust. It may have been formed from gas interaction blowing out or into a larger galaxy. This is only expected if dark matter is bound to ordinary matter through nothing but gravity. Scientists have still not been able to decode the mystery surrounding dark matter. Three of them appear similar to NGC 1052-DF2. However, if that was the case, DF2 would exhibit the exact same abundance of dark matter that all other galaxies - even other Ultra-Diffuse Galaxies - appear to have. It's possible that this galaxy formed on top of dark matter just like any other, and the neighboring galaxy stripped the dark matter away, he says.

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