Astronomers find 19 more galaxies missing their dark matter

Authored by astronomy.com and submitted by clayt6
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Recap: dark matter and galaxy rotation

In the late 1970s, astronomers Vera Rubin and Kent Ford of the Carnegie Institution observed our most well-known galactic neighbor: the Andromeda galaxy. And when they did, they discovered that the galaxy wasn't rotating the way they expected. In our solar system, the planets rotate around the Sun at different rates. Close-in Mercury moves much quicker than distant Neptune. However, in Andromeda, the visible stuff on the galaxy's outer rim moves just as fast as the stuff orbiting near the galaxy's core.

Rubin and Ford were perplexed. It meant the Andromeda galaxy must be saturated with huge amounts of invisible matter, stretching far from the galaxy's center. Ultimately, their discovery turned out to be the first direct evidence for dark matter.

Astronomers infer the existence of dark matter largely based on the fact that the rotation curves of galaxy's are not what you would expect without some form of hidden mass spread throughout the entire galaxy. In this simulation, the galaxy on the left shows what rotation would look like without the effects of dark matter, while the right shows rotation with dark matter. Note how the stars and gas on the outside of the right galaxy are spiraling much faster than those in the left galaxy.

Over the ensuing decades, astronomers realized that every galaxy seemed to be chock-full of dark matter, a substance that doesn't interact with regular matter or light, except through the force of gravity. Then, in 2018, researchers led by Pieter van Dokkum of Yale University found a bizarre, ghostly galaxy named NGC 1052-DF2 that seemed to contain very little, if any, dark matter.

"We thought that every galaxy had dark matter and that dark matter is how a galaxy begins," van Dokkum said in a press release after the discovery. "This invisible, mysterious substance is the most dominant aspect of any galaxy. So finding a galaxy without it is unexpected. 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."

Just a few months later, van Dokkum and his team uncovered a second galaxy without any appreciable dark matter: NGC 1052-DF4. And like NGC 1052-DF2, this ultra-diffuse galaxy raised a lot of eyebrows in the astronomical community.

One critic was astronomer Ignacio Trujillo of the Instituto de Astrofisica de Canarias in Spain.

"Something that caught my attention very early on was the fact that the galaxy [DF2] was not only anomalous for not having dark matter, but also for having an extraordinarily bright population of globular clusters," Trujillo told Astronomy. "I remember thinking: 'Two anomalies at the same time really looks odd.'"

buckwurth on December 12nd, 2019 at 22:43 UTC »

How could they tell they’re missing their dark matter?

clayt6 on December 12nd, 2019 at 22:43 UTC »

Stealing my comment from a good question on r/science concerning the previously "debunked" discovery of 2 other galaxies missing dark matter, DF2 and DF4.

Basically, the question of whether galaxies can really lack dark matter is still an ongoing debate. The most recent news I know of is that there were new observations taken with Hubble late this summer that the original researchers claim verifies that at least one of the galaxies (DF4) is located some 60 million light-years away, meaning it IS missing dark matter. But some other researchers think the galaxies are closer, some 42 million light-years, which would mean they are NOT missing dark matter like initially thought (which is where the dubunked part came in).

From this comment's linked article:

"I think this is definitive," co-author Pieter van Dokkum [the researcher saying the two galaxies don't have dark matter] of Yale University told Astronomy via email. "The TRGB [tip of the red giant branch] cannot be argued with: it is caused by well-understood stellar physics, and [is] as direct as distance indicators get."

But astronomer Ignacio Trujillo [one of the main researchers saying the two galaxies aren't missing dark matter] of the Instituto de Astrofisica de Canarias is skeptical of van Dokkum's conclusion. "They need to show that their analysis is not biased to produce a large distance first," he said. "I think there are a number of choices the authors have used that have not been justified. All of these choices seem to be selected to favor a larger distance than what the data suggests."

So all in all, it's still really unclear whether galaxies without dark matter exist, but the discovery of 19 more galaxies seemingly missing their dark matter (by an unaffiliated group of researchers) does make it seem more likely that they exist.

ObsceneGesture4u on December 12nd, 2019 at 22:24 UTC »

I get a feeling there’s a whole “something” missing about our understanding of the universe. Like when we discovered the electro-magnetic spectrum and saw how “active” the universe is within a whole spectrum that we, as humans, cannot see. Once we discover this “something” science will open up again into a whole new era of discovery