The Large Magellanic Cloud may be tearing its smaller neighbor apart

A small galaxy on the Milky Way’s doorstep may be slowly coming apart under the influence of its larger neighbor.

Astronomers tracking thousands of massive stars across the Small Magellanic Cloud, or SMC, found that stars on opposite sides of the galaxy are moving away from each other rather than following the orderly rotational pattern expected for a disk. The eastern side generally moves toward the Large Magellanic Cloud, or LMC, while the western side moves in the opposite direction.

The findings, published in The Astrophysical Journal Supplement Series, come from a Nagoya University team led by Satoya Nakano and Kengo Tachihara. Using data from the European Space Agency’s Gaia mission, the researchers assembled a catalog of 7,426 candidate stars with masses at least eight times that of the Sun and mapped their motions across the SMC.

“When we first got this result, we suspected that there might be an error in our method of analysis,” Tachihara said. “However, upon closer examination, the results are indisputable, and we were surprised.”

The Small Magellanic Cloud lies roughly 200,000 light-years from Earth and is one of the Milky Way’s nearest dwarf-galaxy neighbors.

Its proximity makes it unusually useful for studying galactic interactions. Astronomers can resolve individual stars across much of the galaxy rather than seeing only their combined light.

Nakano and colleagues concentrated on massive stars because their lives are short. Stars more than eight times the Sun’s mass generally survive for only tens of millions of years before ending as supernovae.

That youth is valuable because such stars have had relatively little time to move independently of the gas clouds that formed them. Their motions can therefore provide clues about how the SMC’s interstellar gas is moving.

The researchers selected their candidates from Gaia Data Release 3 using stellar brightness, color and parallax. They identified 7,426 massive-star candidates distributed across the galaxy’s main body, bar, wing and parts of the Magellanic Bridge connecting the SMC toward the LMC.

Once the team removed the SMC’s overall motion through space, a striking internal pattern appeared.

Many stars were moving toward the galaxy’s outer regions. Some reached inferred transverse velocities near 100 kilometers per second, potentially fast enough to escape the SMC altogether.

More importantly, large groups of stars on opposite sides of the galaxy moved in different directions.

The eastern regions generally traveled southeast, in the direction of the LMC. Western regions moved northwest, away from it.

“The stars in the SMC were moving in opposite directions on either side of the galaxy, as though they are being pulled apart,” Tachihara said. “Some of these stars are approaching the LMC, while others are moving away from it, suggesting the gravitational influence of the larger galaxy.”

The researchers divided concentrations of massive stars into nine large structures, each roughly hundreds of light-years across. These groups also showed distinct motions and line-of-sight velocities, strengthening the case that the pattern was not simply produced by a few runaway stars.

Galaxies often show organized rotation, with stars and gas orbiting a common center.

The Large Magellanic Cloud displays that kind of motion clearly. The SMC has long been harder to understand because its irregular shape and large depth along our line of sight complicate attempts to reconstruct its structure.

Previous observations of neutral hydrogen gas have sometimes been interpreted as evidence of SMC rotation.

The new stellar measurements do not fit that picture well.

Instead of forming a rotational velocity field, the massive stars show an east-west separation pattern. The researchers argue that this is more consistent with the SMC being stretched by its interaction with the LMC.

They also found that the stellar radial velocities do not align well with the rotation inferred from some hydrogen maps.

“If the SMC is indeed not rotating, previous estimates of its mass and its interaction history with the Milky Way and LMC might need to be revised,” Nakano said.

Because young massive stars should broadly follow the gas from which they formed, the result also raises the possibility that some gas motions previously interpreted as rotation actually reflect a more complicated three-dimensional tidal structure.

The LMC is substantially more massive than the SMC, making tidal forces a natural explanation for the smaller galaxy’s distorted motions.

As the two galaxies pass near each other, gravity can pull more strongly on the near side of the SMC than the far side. Over time, that difference stretches stars and gas, producing tidal tails and other structures.

The team also considers ram pressure as a possible contributor.

If the SMC moves through gas associated with the LMC or its surrounding halo, that material can push against the SMC’s interstellar gas. Such pressure could strip gas and alter star-forming regions even though it acts differently from gravity.

The present study does not establish exactly how much of the observed motion comes from tides and how much might involve ram pressure. More precise stellar distances will be needed to reconstruct the three-dimensional geometry.

The researchers plan to use stellar evolutionary modeling to improve those distance estimates.

Rotation is commonly used to estimate the mass of galaxies.

The faster stars or gas orbit a galaxy, the more gravitational mass must be present to keep them bound. If a measured velocity gradient is caused by tidal stretching rather than rotation, applying a rotational model could produce misleading mass estimates.

That matters not only for the SMC’s visible matter but potentially for estimates of its dark matter.

The new interpretation could also alter simulations of how the SMC, LMC and Milky Way have interacted over billions of years. Those models are used to explain structures including the Magellanic Bridge and the enormous stream of hydrogen trailing around the Milky Way.

A different internal velocity field could require parts of that history to be recalculated.

The SMC is also useful because it contains relatively few heavy elements compared with the Milky Way.

That low metallicity makes some of its star-forming conditions resemble those thought to have existed more commonly in the early Universe.

Interactions between young galaxies were also much more frequent billions of years ago. Watching the Magellanic Clouds disturb one another can therefore provide a nearby laboratory for understanding how encounters reshape galaxies, redistribute gas and trigger star formation.

“We are unable to get a ‘bird’s-eye view’ of the galaxy in which we live,” Tachihara said. “As a result, the SMC and the LMC are the only galaxies in which we can observe the details of stellar motion.”

The new measurements suggest that those motions are recording a violent history in real time.

Rather than behaving like a small, smoothly spinning galaxy, the SMC may be a system under stress, with its stars and gas being stretched into new structures as its larger companion steadily reshapes it.

These resources explore the structure, tidal evolution and star-formation history of the Small and Large Magellanic Clouds.

Surviving the waves: evidence for a dark matter cusp in the tidally disrupting Small Magellanic Cloud: Uses thousands of red giant stars and Gaia proper motions to model the SMC while explicitly accounting for its ongoing tidal disruption. (Monthly Notices of the Royal Astronomical Society, 2024)

UVIT Study of the MAgellanic Clouds (U-SMAC) – I. Recent star formation history and kinematics of the Shell region in the north-eastern Small Magellanic Cloud: Maps recent star formation and stellar motions in the northeastern SMC and connects one major episode to interaction with the LMC. (Monthly Notices of the Royal Astronomical Society, 2024)

Congratulations, it’s twins!: Summarizes evidence that the SMC may contain two line-of-sight components separated by several kiloparsecs, emphasizing how complex its three-dimensional structure may be. (Nature Astronomy, 2024)

Research findings are available online in The Astrophysical Journal.

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