Something 'fishy' is happening with the Milky Way's dark matter halo

a dark glowing stretch of gases and light that is the milky way galaxy.
The Milky Way, seen from our location inside it by the Gaia spacecraft. (Image credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO.)

Stars have been caught crawling around the outskirts of the Milky Way more sluggishly than expected, a slow motion that scientists say can only be explained if our dark matter galaxy map is wrong.

Specific Velocities of stars around the edges of galaxies have historically been dead giveaways for the presence of dark matter in those galaxies. This is because astronomers can measure a galaxy's "rotation curve," which charts the orbital velocities of stars against their distances from the center of a galaxy.

If no dark matter were present (and hence the gravitational influence it offers) were absent, stars would begin to slow down the farther they orbit from the center of a galaxy. Instead, however, in the 1960s and early 1970s, astronomers Vera Rubin and Kent Ford noticed that the rotation curves of galaxies were flat. In other words, the orbital motion of stars did not drop off with distance. They maintained pace. The explanation for this, scientists believe, is that galaxies are ensconced within haloes of dark matter. These haloes are thought to be densest at the center of the galaxy; it is the gravity from this dark matter that keeps stars moving.

Related: How the songs of stars can help perfect Gaia's sweeping map of our galaxy

But here's the thing — because we sit inside our galaxy, and lack a bird's eye view of it, measuring the rotation curve of our Milky Way has proven more difficult. 

What is needed is accurate distance information so that we know how far from the galactic center various outlying stars are. In 2019, Anna-Christina Eilers of the Massachusetts Institute of Technology (MIT) led a research team that used the European Space Agency's star-measuring Gaia mission to chart the orbital velocities of stars out to 80,000 light-years from the galactic center.As expected, the researchers found a flat rotation curve with only the merest hint of a decline in velocity for only the outermost stars in that sample.

However, new results that combine Gaia measurements with those from APOGEE (Apache Point Observatory Galactic Evolution Experiment), performed on a ground-based telescope in New Mexico, USA, and which measures the physical properties of stars to better judge their distance, have indeed measured the Milky Way's rotation curve for stars out farther than ever before, to about 100,000 light years.

"What we were really surprised to see was that this curve remained flat, flat, flat out to a certain distance, and then it started tanking," sLina Necib, who is an assistant professor of physics at MIT, said in a statement. "This means the outer stars are rotating a little slower than expected, which is a very surprising result."

"At these distances, we're right at the edge of the galaxy where stars start to peter out," added MIT's Anna Frebel in the same statement. "No one had explored how matter moves around in this outer galaxy, where we’re really in the nothingness."

The decline in orbital velocity at these distances implies that there is less dark matter in the center of our galaxy than expected. The research team describe the galaxy's halo of dark matter as having been "cored," somewhat like an apple. The crew also says there's not enough gravity from what dark matter there seems to exist there, to reach all the way out to 100,000 light years and keep stars moving at the same velocity.

"This puts this result in tension with other measurements," said Necib. "There is something fishy going on somewhere, and it’s really exciting to figure out where that is, to really have a coherent picture of the Milky Way."

The next step, says Necib, is to employ high-resolution computer simulations to model different distributions of dark matter within our galaxy to see which best replicates the falling rotation curve. Models of galaxy formation could then try to explain how the Milky Way galaxy arrived at its specific, cored-out distribution of dark matter — and why other galaxies didn't that.

The findings were published on Jan. 8 in the journal Monthly Notices of the Royal Astronomical Society

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Keith Cooper
Contributing writer

Keith Cooper is a freelance science journalist and editor in the United Kingdom, and has a degree in physics and astrophysics from the University of Manchester. He's the author of "The Contact Paradox: Challenging Our Assumptions in the Search for Extraterrestrial Intelligence" (Bloomsbury Sigma, 2020) and has written articles on astronomy, space, physics and astrobiology for a multitude of magazines and websites.

  • Questioner
    A wildly alternative proposal:

    I posit that what can be referred to as the 'dark matter' effect is a function of the 'interiors' of black holes.

    Gravity relates to the distribution of the speed of time passage, namely time dilation (slowing).
    At the event horizon of a black hole time essentially stops.

    So what happens in the interior of a black hole?

    Continuing the progression one might guess time reverses in a black hole's interior.

    Further one might speculate that would potentially create gravitation like effects far more powerful than known gravity.

    Time in the Universe generally operates within the guidelines of the laws of physics, and the limited degrees of freedom that allows for.

    Inverting time likely has the same (limited) degrees of freedom as the forward passage of time.
    Extending from the interior of a black hole inverting time Initially randomly might trace nearly directly back through space-time, but as the time inversion sequencing continues in all likelihood it drifts further & further from (our) space-time's specific sequence.
    I suggest that might be why the 'dark matter' effect is somewhat inconsistent. Somewhat bounded random variability.

    Galactic 'dark matter' effects generally center on a galaxy's massive central black hole.
    This proposal fits quite well with the tight correlation between a galaxy's central black hole size and the hypothesized DM for that galaxy.

    I will continue with the idea that the effect happens on (connects with) the interior of stars rather than as an effect of their surrounding space-time.
    It would have no direct effect on space-time/gravity.
    The way to test this would be to observe planets orbiting those most effected stars and see if their planets are or are not influenced by the hypothesized DM gradient of space-time/gravity.

    Not sure how soon our technologies will have that observational capacity.