From Sky & Telescope: “The Alignment of the Milky Way’s Entourage Explained”

From Sky & Telescope

February 15, 2021
Monica Young

Astronomers are starting to understand why the dwarf galaxies around the Milky Way are aligned along a plane.

Over the past couple decades, astronomers have discovered dozens of new dwarf galaxies orbiting our own. But as the numbers increased, it became apparent that something unexpected was happening. Rather than being distributed all the way around our galaxy, the satellites mostly align along a thin sheet, or plane, with most of them orbiting the Milky Way in the same direction. Picture them as pepperoni pre-emptively stuck on a thrown disc of pizza dough.

The alignment surprised astronomers. Some thought the structure might not even be real, but new observations [Astronomy & Astrophysics] confirmed it’s really there. Meanwhile, others argued that the plane’s existence could challenge our current understanding of how dark matter helps galaxy systems form.

Now, a more detailed look at the formation of galaxies and their dwarf entourages, to appear in the MNRAS, suggests that while the alignment is rare, it’s not completely unexpected. The key seems to be the presence of a large satellite that dominates the small-galaxy attendants like the proverbial big fish in a small pond.

Blazing a Trail with FIRE

This Milky Way-mass galaxy comes from the universe constructed by the FIRE simulations.
Credit: Latte Project.

Graduate student Jenna Samuel (University of California, Davis) and colleagues approached the problem using the Feedback in Realistic Environments (FIRE) computer simulation [Northwestern University*]. While early cosmological simulations included only dark matter, which interacts primarily via gravity and is thus easier to model, FIRE includes interactions with baryons, aka the “normal” matter that makes stars and galaxies visible.

Normal matter produces feedback that can counteract a galaxy’s gravity, such as supernovae and black hole jets, and including that feedback makes the artificial universe a little more realistic. As a result, the FIRE simulations has already helped solve some other controversies within the dark matter paradigm.

Whether the Milky Way’s thin sheet of satellites fits in that paradigm remains debated, though. Similar structures are exceedingly rare in the universes simulated with only dark matter — so much so that the very existence of the Milky Way’s satellite sheet might challenge the current notion of dark matter.

Samuel set out to see if that rarity persisted in the more realistic FIRE simulations. Selecting Milky Way-like galaxies and measuring the distribution of their satellites, she found that between 1 and 2% of these systems had satellites that fell along thin planes like our own galaxy’s. In other words, the phenomenon is rare but not outside the realm of possibility.

“The fact that we’re finding any at all is still pretty surprising,” Samuel added at January’s American Astronomical Society meeting. If the simulated universe can make thin sheets of satellites, then maybe there’s no problem with dark matter after all.

Most of those simulated structures were short-lived, though — they typically lasted less than 500 million years. Some think the Milky Way’s satellite sheet, on the other hand, could last up to a billion years or so.

Big Fish in a Small Pond

The Large Magellanic Cloud (left) and the Small Magellanic Cloud (right) are small satellite systems of our Milky Way visible from the Southern Hemisphere. The LMC’s mass is 10% that of the Milky Way, making it a giant among dwarfs. Credit: Akira Fujii.

But our galaxy’s satellite group is also a bit unusual: It’s dominated by the Large Magellanic Cloud (LMC), a dwarf galaxy with 10 billion solar masses, about 10% of the Milky Way’s heft. So Samuel rinsed and repeated, this time looking only at simulated galaxies with a giant dwarf among their satellites. When such a big fish influences the small pond, Samuel found that thin satellite planes were more common, occurring 7 to 16% of the time, and they lasted much longer, up to 3 billion years.

The LMC might be bringing in some of its own satellites, Samuel speculates. In an earlier study [The Astrophysical Journal], Ekta Patel (University of California, Berkeley) and colleagues found the same when they reconstructed the orbital histories of 18 of Milky Way’s satellites using data from the European Space Agency’s Gaia mission.

ESA (EU)/GAIA satellite .

But Samuel thinks that the LMC also had an impact on the orbits of dwarf galaxies already around or coming in toward the Milky Way.

“I agree with the Samuel study that the LMC plays a major role in the origin of the Milky Way’s plane of satellites and that the tension with cold dark matter will be resolved,” says Gurtina Besla (University of Arizona), who was not involved in the study. But she adds that there’s still work to be done to iron out the details and understand how the big-fish effect works. Her team is working on that problem, too, with more results coming soon.

Incidentally, another prediction came out of the recent analysis of the FIRE simulations. Over the past decade, sweeping sky surveys have enabled the discovery of dwarf galaxies around our own. Samuel’s analysis shows that this survey is nearly complete — but not quite. She predicts that five additional satellites with more than 100,000 solar masses could still be discovered out to a million light-years from the Milky Way.

*The FIRE project seeks to develop and explore cosmological simulations of galaxy formation that directly resolve the interstellar medium of individual galaxies while capturing their cosmological environment. FIRE aims to improve the predictive power of galaxy formation simulations by directly informing the implementation of sub-resolution processes with explicit small-scale models, thus reducing the reliance on adjustable model parameters.

See the full article here .


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Sky & Telescope, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

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“Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”