Dark matter and the galaxy's history
The way the Milky Way rotates reveals that most of its mass is invisible. Its structure also records a long history of growth by mergers.
The rotation curve
A rotation curve plots the orbital speed of stars and gas against distance from a galaxy's centre. If most mass were in the visible bright centre, speeds should fall with distance, as planets slow farther from the Sun. Instead, the Milky Way's rotation curve stays roughly flat far beyond the visible disc.
Vera Rubin and dark matter
In the 1970s, Vera Rubin and Kent Ford measured rotation curves of many spiral galaxies and found them flat too. Together with earlier work by Fritz Zwicky on galaxy clusters in the 1930s, this provided strong evidence for dark matter: matter that has gravity but emits no light.
How much dark matter
The Milky Way's dark matter halo extends far beyond the visible disc and may contain about 1-1.5 trillion solar masses in total, several times more than all its stars and gas. Across the universe, dark matter makes up about 85% of all matter.
What dark matter might be
Dark matter is not ordinary matter hidden in dim stars or gas, as shown by many observations, including measurements of the early universe. Leading candidates are new kinds of particles, such as weakly interacting massive particles (WIMPs) or axions. Despite decades of searching with underground detectors, it has not been directly detected.
Building the galaxy
Galaxies grow by merging with smaller ones. Gaia data revealed that about 8-11 billion years ago, the Milky Way merged with a galaxy now called Gaia-Enceladus, whose stars still move in distinctive orbits. Streams of stars, such as the Sagittarius stream, trace dwarf galaxies being torn apart today.
Satellite galaxies
Dozens of small satellite galaxies orbit the Milky Way, including the Large and Small Magellanic Clouds, visible from the Southern Hemisphere. The Large Magellanic Cloud is about 160,000 light-years away, and the Small Magellanic Cloud about 200,000 light-years.
The future: Andromeda
The Andromeda Galaxy, about 2.5 million light-years away, is approaching the Milky Way at about 110 km/s. The two galaxies may collide and merge in several billion years. Stars are so widely spaced that few, if any, would collide directly, though the galaxies' shapes would be transformed; recent studies suggest the merger is likely but not certain.
Worked example
At the Sun's distance, orbiting at 230 km/s, the mass inside the Sun's orbit can be estimated with Newton's laws: about 100 billion solar masses. Measurements of stars and gas much farther out, still moving fast, imply far more mass at greater distances than visible matter can account for, which is the dark matter halo.
Key takeaways
The Milky Way's rotation curve stays flat far from the centre, unlike what visible matter predicts. Vera Rubin's measurements, building on Zwicky, provided strong evidence for dark matter. The galaxy's dark halo far outweighs its stars; dark matter is about 85% of all matter. Its nature is unknown; candidates include WIMPs and axions. The Milky Way grew by mergers, such as Gaia-Enceladus. Satellite galaxies include the Magellanic Clouds. Andromeda may merge with the Milky Way in billions of years.
Flashcard
Orbital speed versus distance from centre
Check your understanding
What does a flat galaxy rotation curve suggest?