Satellite galaxy has left gravitational ‘wake’ in Milky Way’s outer reaches

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A 2019 image of the Large Magellanic Cloud, the Milky Way Galaxy's largest satellite galaxy, by the European Southern Observatory's VISTA telescope. (ESO/VMC Survey)

New observations have revealed that, as it orbits, the Large Magellanic Cloud is disturbing stars on the outskirts of the Milky Way galaxy, an interaction that can be used to improve galactic models and test theories of gravity and dark matter.

Publishing their findings April 21 in Nature, Harvard University astronomers led the creation of one of the first all-sky maps of the galactic halo that extends far from the Milky Way's center. Among other results, they found observational evidence of unusually high stellar density in the halo they characterized as a "wake," which was left behind in the path of the Large Magellanic Cloud.

Of the dozens of galaxies orbiting the Milky Way, the Large Magellanic Cloud is the largest and one of the closest, at roughly 1% of the Milky Way's mass and 163,000 light-years away. The galaxy is moving through the Milky Way's galactic halo, a mostly spherical and relatively sparse formation that contains many old stars, plasma and dark matter. It and the Small Magellanic Cloud are the only galaxies visible to the naked eye in the Southern Hemisphere.

In 2019, a collaboration led by astronomers at the University of Arizona investigated how the Large Magellanic Cloud would behave when it moved through the halo's dark matter, which appears to exert large gravitational effects on visible matter but has not yet been directly observed or identified. Their computer simulations predicted a high-density "wake" behind the satellite galaxy, as well as disturbances of halo stars in the Milky Way's northern sky.

The new study was intended to test these predictions with new observations of the galactic halo, lead author Charlie Conroy said.

"They predicted this very striking feature that we should see in the data, and so that's partly what motivated us to go out and try to find it," said Conroy, a professor of astronomy at Harvard.

Conroy said he and his co-authors faced a "needle-in-a-haystack problem" when trying to map the stars in the halo: Halo stars are less numerous and farther away than those nearer to the Earth, so it was a challenge to pick them out of a crowded night sky.

Their solution was to use observations from the European Space Agency's Gaia satellite and NASA's Wide-field Infrared Survey Explorer satellite to gather data on the visible and infrared light emitted by stars, which allowed the researchers to calculate the stars' distance and determine which are in the halo. 

The final map included 1,301 stars between 195,000 and 326,000 light-years away from the galactic center. For comparison, the sun is about 26,000 light-years away from the galactic center.

The all-sky map revealed a high-density wake behind the Large Magellanic Cloud as well as the northern-sky disturbances, largely aligning with the Arizona simulations. Conroy said the wake observed in the stars likely corresponds with a similar disturbance in the invisible dark-matter halo around them, like tracking leaves in a lake to measure the waves.

"It's not the exact same physics, but it's the same kind of picture, that the boat is traveling and it creates this wake as it moves through the water," Conroy said. "In a similar way, as the Large Magellanic Cloud moves through our halo, it's creating a wake behind it."

The wake provides a new avenue for testing different theories for dark matter — including those that replace dark matter with a modified form of gravity — by seeing whether they accurately predict the wake, Conroy said.

"We think that this observation and the nature of this wake might hold kind of unique clues to the nature of dark matter," he said. "We don't know for sure — people have to run additional tests and simulations — but there's some promise that this is going to be a new way to study that old problem."

The wake also acts as a new line of evidence that the Large Magellanic Cloud is making its first passage around the Milky Way, according to the researchers. They said they would expect the wake to be much weaker or nonexistent if the satellite had completed multiple orbits. Whether the galaxy has completed an orbit has spurred some debate between experts, in part because of uncertain estimates of the galaxies' masses.

Furthermore, the disturbances in the Milky Way's northern-sky halo contradict assumptions that the stars are in equilibrium and simply orbit the galactic center, and they should be taken into account for future modeling of the galaxy, according to the researchers.

Although the all-sky halo map largely recreated the simulations' results, there were some differences between the two. The density of the wake was about 40% higher, according to the observations, and the disequilibrium across the Milky Way's northern sky was more lopsided.

The Arizona team is running new simulations to try to close the gap by tweaking assumptions about the Milky Way's mass and the Large Magellanic Cloud's mass and orbit, according to Conroy. He and his lab, meanwhile, plan to use more observational data to learn more about the velocities of the halo stars, which can be used to test more predictions of the simulations.

The study, "All-sky dynamical response of the Galactic halo to the Large Magellanic Cloud," published April 21 in Nature, was authored by Charlie Conroy, Rohan Naidu, Ana Bonaca and Benjamin Johnson, Center for Astrophysics | Harvard & Smithsonian; and Nicolás Garavito-Camargo, Gurtina Besla and Dennis Zaritsky, University of Arizona.

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