Milky Way ‘kidnapped’ and consumed several dwarf galaxies

11 Oct 2019

Clouds of gas in the Large Magellanic Cloud. Image: ESO

Astronomers have discovered evidence that our own Milky Way ‘kidnapped’ several dwarf galaxies around 1bn years ago.

Researchers have discovered that several of the dwarf galaxies that orbit our Milky Way do not hail from these parts. In a paper published to the Monthly Notices of the Royal Astronomical Society, a team of astronomers, led by scientists at the University of California, Riverside, was able to find evidence that they may have been ‘kidnapped’ from the Large Magellanic Cloud (LMC).

This large dwarf galaxy resembles a faint cloud in the southern hemisphere night sky. As well as consuming several bright and well-known satellite galaxies, the Milky Way also took with it several ultra-faint dwarfs.

More than 50 satellite galaxies have been discovered orbiting the Milky Way, but the number may be higher than previously estimated.

The discovery was made using the Gaia space telescope with data on the motions of several nearby galaxies that were contrasted with the latest cosmological hydrodynamical simulations. The team used their positions in the sky to predict the velocities of material such as dark matter, showing that at least four ultra-faint dwarfs and two classical dwarfs, Carina and Fornax, used to be satellites of the LMC.

‘The most massive merger in its history’

As part of the ongoing merging process, the Milky Way used its powerful gravitational field to tear apart the LMC to steal these satellites. The team said the findings have important implications for the total mass of the LMC and the formation of the Milky Way.

“If so many dwarfs came along with the LMC only recently, that means the properties of the Milky Way satellite population just 1bn years ago were radically different, impacting our understanding of how the faintest galaxies form and evolve,” said Laura Sales, lead of the research team.

The fact that there are a high number of tiny dwarf galaxies suggests significant amounts of dark matter in the LMC and that “the Milky Way is undergoing the most massive merger in its history”, said first author Ethan Jahn.

While once isolated, the LMC was eventually captured by the gravity of the Milky Way to become its satellite. Now, the team said it wants to study how the satellites of LMC-sized galaxies form their stars and how that relates to how much dark matter mass they have.

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

Colm Gorey was a senior journalist with Silicon Republic