Fast stars may be joining us from other galaxies
Gaia has helped astronomers spot hypervelocity stars that appear to come from outside the Milky Way
Scientists studying stars travelling at high speeds within the Milky Way believe a good number of these celestial objects could actually be on a journey from another galaxy. Research using data from the European Space Agency’s Gaia mission shows that most observed hypervelocity stars are rapidly moving towards the galactic centre. It had previously been thought such stars moved outwards.
The fresh findings have been reported by scientists at Leiden University in the Netherlands following observations made by the Gaia spacecraft between 25 July 2014 and 23 May 2016. By looking at the positions, distance indicators and motions of some seven million stars with full 3D velocity measurements, the researchers found 20 were fast enough to eventually escape the Milky Way.
In the past such behaviour has been pinned on the black hole at the centre of our galaxy propelling these stars towards the Milky Way’s edge. Here, however, something unusual was happening. “Rather than flying away from the galactic centre, most of the high-velocity stars we spotted seem to be racing towards it,” explains study co-author Tommaso Marchetti. “These could be stars from another galaxy, zooming right through the Milky Way.”
One possibility is that the hypervelocity stars have come from a relatively small galaxy orbiting the Milky Way called the Large Magellanic Cloud. “Stars can be accelerated to high velocities when they interact with a supermassive black hole,” says co-author Elena Maria Rossi. “So the presence of these stars might be a sign of such black holes in nearby galaxies.”
Alternatively, the stars may have been part of a binary system, flung towards the Milky Way when their companion star exploded as a supernova. “Studying them could tell us more about these kinds of processes in nearby galaxies,” Rossi concludes.
An artist’s impression of the positions and orbits of 20 high-velocity stars inthe Milky Way