The Alignment of the Milky Way’s Entourage, Explained

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 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 Monthly Notices of the Royal Astronomical Society (preprint available here), 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

Graduate student Jenna Samuel (University of California, Davis) and colleagues approached the problem using the Feedback in Realistic Environments (FIRE) computer simulation. 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

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, 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. 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.

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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.