Astronomers using data from ESA’s star mapping Gaia mission to look for hypervelocity stars being kicked out of our Milky Way Galaxy were surprised to find stars instead sprinting inwards — perhaps from another galaxy.
The positions and reconstructed orbits of 20 high-velocity stars, represented on top of an artistic view of the Milky Way Galaxy. The seven stars shown in red are sprinting away from the Galaxy and could be traveling fast enough to eventually escape its gravity. Surprisingly, the study revealed also thirteen stars, shown in orange, that are racing towards the Milky Way: these could be stars from another galaxy, zooming right through our own. Image credit: NASA / ESA / Hubble / Marchetti et al.
The Milky Way contains over a hundred billion stars. Most are located in a disc with a dense, bulging center; the rest are spread out in a much larger spherical halo.
Stars circle around the Milky Way at hundreds of miles per second, and their motions contain a wealth of information about the past history of the Galaxy.
The fastest class of stars in our Galaxy are called hypervelocity stars, which are thought to start their life near the Galactic center to be later flung towards the edge of the Milky Way via interactions with the black hole.
Only a small number of hypervelocity stars have ever been discovered, and Gaia’s second data release provides a unique opportunity to look for more of them.
“Of the seven million Gaia stars with full 3D velocity measurements, we found twenty that could be traveling fast enough to eventually escape from the Milky Way,” said team member Dr. Elena Maria Rossi, an astronomer at Leiden Observatory.
“Rather than flying away from the Galactic center, most of the high velocity stars we spotted seem to be racing towards it,” added team leader Dr. Tommaso Marchetti, also from Leiden Observatory.
“These could be stars from another galaxy, zooming right through the Milky Way.”
It is possible that these intergalactic interlopers come from the Large Magellanic Cloud, a relatively small galaxy orbiting the Milky Way, or they may originate from a galaxy even further afield.
If that is the case, they carry the imprint of their site of origin, and studying them at much closer distances than their parent galaxy could provide unprecedented information on the nature of stars in another galaxy.
“Stars can be accelerated to high velocities when they interact with a supermassive black hole,” Dr. Rossi said.
“So the presence of these stars might be a sign of such black holes in nearby galaxies. But the stars may also have once been part of a binary system, flung towards the Milky Way when their companion star exploded as a supernova. Either way, studying them could tell us more about these kinds of processes in nearby galaxies.”
An alternative explanation is that the newly identified hypervelocity stars could be native to our Galaxy’s halo, accelerated and pushed inwards through interactions with one of the dwarf galaxies that fell towards the Milky Way during its build-up history.
Additional information about the age and composition of the stars could help the astronomers clarify their origin.
“A star from the Milky Way halo is likely to be fairly old and mostly made of hydrogen, whereas stars from other galaxies could contain lots of heavier elements,” Dr. Tommaso said.
“Looking at the colors of stars tells us more about what they are made of.”
The findings were published in the Monthly Notices of the Royal Astronomical Society.
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T. Marchetti et al. Gaia DR2 in 6D: Searching for the fastest stars in the Galaxy. MNRAS, published online September 20, 2018; doi: 10.1093/mnras/sty2592
