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.

This movie shows satellite galaxies around the Milky Way (upper left) and Andromeda (lower right). The dwarf galaxies around our own galaxy are mostly aligned along a plane; a subset of Andromeda's satellites are also along a plane, but their distribution isn't quite so compact.
Pawlowski M., Kroupa P. & Jerjen H. / Monthly Notices of the Royal Astronomical Society 2013

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

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

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

Large and Small Magellanic Clouds
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.
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, 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.

Comments


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Rod

February 15, 2021 at 3:45 pm

'Astronomers are starting to understand why the dwarf galaxies around the Milky Way are aligned along a plane.', very interesting report. My observation. The arXiv paper link shows much, the structures last perhaps 0.5 Gyr to max 3.0 Gyr in the simulations. There is also indications that the structures are destroyed over timescales compared to globular clusters in the dwarf galaxies (see page 2 of the paper), "...Pawlowski et al. 2017 noted that integrating present-day satellite orbits either forward or backward in time typically leads to the disintegration of the plane." The arXiv paper, 'Planes of satellites around Milky Way/M31-mass galaxies in the FIRE simulations and comparisons with the Local Group', https://ui.adsabs.harvard.edu/abs/2020arXiv201008571S/abstract, October 2020, "These planes are generally short-lived, surviving for < 500 Myr. However, if we select hosts with an LMC-like satellite near first pericenter, the fraction of snapshots with MW-like planes increases dramatically to 7-16 per cent, with lifetimes of 0.7-3 Gyr, likely because of group accretion of satellites."

My observation. Ongoing efforts to show the plane does not conflict (too much perhaps) with LCDM cosmology and then their short-life times is also intriguing 🙂

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Andrew James

February 16, 2021 at 1:36 am

Sorry. None of this comment makes any sense to me. I read the paper, and none what you say relates this. Understanding coplanetary orbits is the key to this.

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KELLYMCCOY

February 16, 2021 at 4:47 am

I believe that black holes are simply a
Visual effect of many things that would in a rotation be illusions I have seen mimicking light change optical images.
Its completely a optical illusion.

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Rod

February 16, 2021 at 8:50 am

Okay Andrew James. The arXiv paper has 23 references to "dark matter", is looking at issues of forming structures like the planes using LCDM cosmology. There are 14 dwarf galaxies in the study and these have specific ages (they are not 500 million years old). As the arXiv paper abstract says "...We use 14 Milky Way/Andromeda-(MW/M31) mass host galaxies from the FIRE-2 simulations. We select the 14 most massive satellites by stellar mass within host <= 300 kpc of each host and correct for incompleteness from the foreground galactic disk when comparing to the MW. We find that MW-like planes as spatially thin and/or kinematically coherent as observed are uncommon, but they do exist in our simulations. Spatially thin planes occur in 1–2 per cent of snapshots during = 0 - 0.2, and kinematically coherent planes occur in 5 per cent of snapshots. These planes are generally short-lived, surviving for < 500 Myr..."

So as I connect dots, there are *old* dwarf satellite galaxies in the study, very young plane structures relative to the age of the dwarf satellite galaxies (including H-R ages of stars in them), and 23 references to dark matter too 🙂

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Andrew James

February 16, 2021 at 5:37 pm

You say; "...forming structures like the planes using LCDM cosmology." but the abstract and discussion says "We conclude that planes of satellites are not a strong challenge to LCDM cosmology." LCDM cosmology does not predict these planes, but it might influence them.

To quote the article: "The lack of strong correlations between planarity and other properties of the host-satellite systems leaves us with few physical explanations for the MW's highly coherent satellite plane. Our most promising result points to the presence of the LMC near first pericenter as a likely primary driver of planarity."

The interactions of these satellite dwarf galaxies are controlled by the biggest masses, driving the rest in into a plane. As the LMC orbits the Milky Way, the planarity is broken or disrupted, but may reformed again later in the LMC orbit when it returns to pericentre. e.g. resonance Multiple orbits likely make this planarity formation less apparent. The problem is similar to how component behave in multiple stars, where orbits of principal components align in preferential directions. Such a hypothesis is testable and supported by observing other galaxies showing planarity.

As gravitation is the main driver of planarity, the additional influence of dark matter seems from observation and simulation in these studies to be "...we do not interpret this as a tension with
LCDM cosmology.' e.g. This planarity is little influenced by LCDM effects.

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Rod

February 17, 2021 at 9:32 am

I like to ask good questions using Who, What, Where, When, How, and Why. Why the surprise reported here? My observation. Here is what the S&T report said "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." This report from July 2015 could explain the surprise encountered. 'Satellite Dwarf Galaxies in a Hierarchical Universe: Infall Histories, Group Preprocessing, and Reionization', https://ui.adsabs.harvard.edu/abs/2015ApJ...807...49W/abstract, July 2015. From the arXiv paper attached. On page 10 "6.2.2. Implications for observed associations in the Local Group The significant group-infall fractions (30 - 60 %) that we found may help to explain the many observed associations between satellite galaxies (and stellar streams) within the halos of the MW and M31. Li & Helmi (2008) showed that infalling groups can remain coherent and share similar orbital planes for up to ~ 8 Gyr (see also, Klimentowski et al. 2010; Sales et al. 2011; Slater & Bell 2013), which we find is also the typical time that satellites have been within the MW/M31 halo. See also Deason et al. (2015), for detailed phase-space distributions at z = 0 of infalling LMC-mass groups."

The Sky & Telescope report based upon the new model indicates the plane structures last only 0.5 Gyr to ~ 3.0 Gyr so I do understand the surprise, thus why the surprise :). CDM has other surprises too like the cosmological constant used in GR and expanding universe. Wrong value for the CC destroys the universe, no form or structure will form in the BB model as space expands.

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Rod

February 17, 2021 at 9:34 am

https://ui.adsabs.harvard.edu/abs/2015ApJ...807...49W/abstract

This NASA ADS link should work.

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Yaron Sheffer

February 17, 2021 at 12:09 pm

I think the commenting app is breaking up the link structure. So instead of clicking, I find that copying the entire line and pasting it as a url address in a new window does bring up the expected page.

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