The Milky Way May Have Just Lost a Whole Bunch of Satellite Galaxies

The space around the Milky Way isn’t vacant. It’s swarming with dwarf galaxies – small, faint, and low in mass, with as few as around 1,000 stars each.

This is not unusual. We know from our observations of other large galaxies that dwarf galaxies often congregate nearby, and can become captured in the gravitational field of the larger object.


Astronomers have so far identified nearly 60 smaller galaxies within 1.4 million light-years of the Milky Way, though it’s likely there’s a lot more hiding in the dark. Most look like they’re hanging around, like fruit flies around a banana.

According to a new analysis of data from the Gaia satellite, however, most of those galaxies are actually relatively new to the area – too new to be orbiting the Milky Way, at least not yet, researchers think.

“We conclude that due to their unequaled high energies and angular momenta, most dwarfs cannot be long-lived satellites, and if they could be bound to the Milky Way, they are at first passage, i.e., infalling less than 2 billion years ago,” researchers write in a new paper led by astrophysicist François Hammer of the Paris Observatory in France.

Dwarf galaxies in the space around the Milky Way. (ESA/Gaia/DPAC, CC BY-SA 3.0)

The Gaia mission is an ongoing project to map the Milky Way with the greatest precision yet, including the three-dimensional positions, motions, and velocities of the stars and objects therein (and a little way outside).

Using measurements of these properties, Hammer and his colleagues used data from the early third data release from Gaia to calculate the movements of 40 dwarf galaxies outside the Milky Way. Then they used parameters such as the three-dimensional velocity of each galaxy to calculate its orbital energy and angular momentum.


The results were really intriguing – because they showed that most of the dwarf galaxies assumed to be Milky Way satellites are moving a lot more quickly than objects known to be in orbit around the Milky Way, such as stars from Gaia-Enceladus and the Sagittarius dwarf spheroidal galaxy.

The Milky Way has repeatedly cannibalized other galaxies over its long history. Gaia-Enceladus, also known as the Gaia Sausage, was subsumed around 9 billion years ago. Its traces remain in a population of stars orbiting at relatively low energies.

The Sagittarius dwarf spheroidal galaxy is currently being disrupted by tidal (gravitational) forces and incorporated into the Milky Way, a process that started around 4 to 5 billion years ago. Those stars are whizzing around a bit faster than the Gaia-Enceladus stars.

The dwarf galaxies are moving even more energetically still. This, the team concludes, means that these dwarf galaxies can’t have been close to the Milky Way long enough for the massive galaxy’s gravitational field to have slowed them down.

This finding could alter our understanding of the interactions between normal galaxies and dwarf galaxies, and the properties of dwarf galaxies, the researchers say.


It’s possible that some of the dwarf galaxies will be captured in the Milky Way’s orbit (although it’s impossible to say which ones), but how long they will remain is an open question.

“The Milky Way is a…

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