News Space 12 January 2017
2 minute read 

Distant stars in Milky Way may be stolen from another galaxy


The Sagittarius dwarf was robbed of a third of its stars and 90% of its dark matter as it moved around the much larger galaxy.


An artist's impression of the Milky Way galaxy.
NASA / JPL

Five of the farthest stars in our galaxy may have been nicked from a nearby dwarf galaxy, according to modelling performed by a pair of Harvard University astrophysicists.

Marion Dierickx and Avi Loeb simulated the movements of the Sagittarius dwarf galaxy, which orbits the Milky Way galaxy, over the past eight billion years.

They found stars were torn off the smaller galaxy with each lap, creating streams of stars stretch up to a million light-years long. A few of these stars were roped into the Milky Way's clutches.

The work, published on Arxiv, has been accepted for publication in The Astrophysical Journal.

Our own galaxy, the Milky Way, sits within a bunch of more than 50 galaxies called the Local Group. The two biggest galaxies in the Local Group are Andromeda and the Milky Way, each of which has their own system of satellite galaxies.

The Sagittarius dwarf is one of dozens around the Milky Way and one of the nearest. It was discovered in 1994 by astrophysicists in the UK who found a large group of stars moving around the far side of the Milky Way.

It appears to be elongated towards the Milky Way, suggesting that it's being stretched by the larger galaxy's gravitational tug.

To see how the Sagittarius dwarf has behaved over the past eight billion years, Dierickx and Loeb simulated the full trajectory of the little galaxy. They tweaked its velocity and approach angle to see what best fit current observations.

In this computer-generated image, a red oval marks the disk of our Milky Way galaxy and a red dot shows the location of the Sagittarius dwarf galaxy. The yellow circles represent stars that have been ripped from the Sagittarius dwarf and flung far across space. Five of the 11 farthest known stars in our galaxy were probably stolen this way.
Marion Dierickx / CfA

The Sagittarius dwarf started off about 10 billion times the mass of our sun, or about 1% of the Milky Way's mass. But over time, the dwarf lost around a third of its stars and a 90% of its dark matter.

This produced three streams of stars, torn off by the larger galaxy's gravitational pull, that reach as far as a million light-years from the Milky Way's centre.

They also found five of the 11 most distant stars in the Milky Way – around 300,000 light-years from Earth – are located and travelling at a velocity that matches what's expected of stars stripped from the Sagittarius dwarf.

While the other six don't seem to be from Sagittarius, they may have been nabbed from a different dwarf galaxy.

"More interlopers from Sagittarius are out there just waiting to be found," Dierickx says.

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Belinda Smith is a science and technology journalist in Melbourne, Australia.