Cosmic Heist: How the Milky Way Stole Several Small Galaxies

Not only is our galaxy involved in an ongoing merger with a neighbouring galaxy — the Large Magellanic Cloud — but it was responsible for the ‘theft’ of several small galaxies.

Visualization of the simulations used in the study. Top left shows dark matter in white. Bottom right shows a simulated Large Magellanic Cloud-like galaxy with stars and gas, and several smaller companion galaxies. (Ethan Jahn, UC Riverside.)

The Milky Way stands accused of the theft of several small satellite galaxies. It’s accusers, a team of astronomers led by scientists from the University of California. The team have discovered that several small, dwarf galaxies orbiting the Milky Way were snatched away from another satellite galaxy — the Large Magellanic Cloud (LMC).

The pilfered galaxies include Carina and Fornax as well as several ultrafaint dwarfs. Both Carina and Fornax were once satellites of the LMC. The Milky Way was able to snatch these galaxies due to its more powerful gravitational influence — ripping apart the LMC and extracting the satellite galaxies in a daring cosmic heist.

“These results are an important confirmation of our cosmological models, which predict that small dwarf galaxies in the universe should also be surrounded by a population of smaller fainter galaxy companions,” says Laura Sales, an assistant professor of physics and astronomy. “This is the first time that we are able to map the hierarchy of structure formation to such faint and ultrafaint dwarfs.”

The findings shed light on both the formation and evolution of the Milky Way and the total mass of the LMC and, how much of that mass is composed of dark matter. In fact, the research demonstrates that the LMC — and galaxies that are similar to it — may host tiny galaxies that contain no stars at all— only dark matter.

“If so many dwarfs came along with the LMC only recently, that means the properties of the Milky Way satellite population just 1 billion years ago were radically different, impacting our understanding of how the faintest galaxies form and evolve,” explains Sales — who led the study detailed in the journal Monthly Notices of the Royal Astronomical Society.

The team made their discovery by employing data from the Gaia space telescope, which has been studying the motions of several nearby galaxies. They then compared these observations to state-of-the-art cosmological hydrodynamical simulations from the Feedback in Realistic Environments project.

Satellites and Dwarfs

In a way analogous to the Earth orbiting the Sun, our galaxy is orbited by several smaller satellite galaxies. We currently know of at least 50 such galaxies, the largest of which is the LMC — a large dwarf galaxy resembling a faint cloud in the night sky over the Southern Hemisphere.

Dwarf galaxies themselves are relatively small collections of anywhere from a few thousand to a few billion stars. They can either be satellites of larger galaxies, or they can be “isolated,” existing on their own and independent of any larger object. The research seems to show that the LMC used to be isolated but was itself captured by the Milky Way’s gravity — becoming a satellite galaxy.

The results also demonstrate that the ongoing merger with the LMC is infusing the Milky Way with an abundance of dark matter and that the LMC has far more tiny satellites that the team expected.

“The high number of tiny dwarf galaxies seems to suggest the dark matter content of the LMC is quite large, meaning the Milky Way is undergoing the most massive merger in its history,” says Ethan Jahn, a graduate student in the team and the paper’s first author. “The LMC, its partner, is bringing in as much as one-third of the mass in the Milky Way’s dark matter halo — the halo of invisible material that surrounds our galaxy.”

The difficulty in measuring small galaxies means that some ultrafaint dwarf galaxies that have already been catalogued may be satellites of the LMC, Jahn explains. He also believes that new, ultrafaint dwarfs associated with the LMC are yet to be uncovered.

“The LMC hosted at least seven satellite galaxies of its own, including the Small Magellanic Cloud in the Southern Sky, prior to them being captured by the Milky Way,” Jahn adds.

The team now intend to turn their attention to how stars in the satellite galaxies of LMC-sized galaxies form and the role dark matter plays in the process.

Jahn concludes: “It will be interesting to see if they form differently than satellites of Milky Way-like galaxies.”