London, Oct 14 : A new research has suggested that some of the Milky Way's fastest stars may be immigrants from an orbiting dwarf galaxy that merged with our own.
"Hypervelocity" stars are racing at breakneck speeds out of our galaxy. The prevailing view holds that they are natives of the Milky Way's crowded centre, and that they were catapulted to such extreme speeds by the colossal black hole thought to lurk there.
In that scheme, a pair of stars wanders too close to the supermassive black hole, and one star gets captured while the other gets flung outwards at up to 4000 km/sec.
But since the first hypervelocity star was discovered in 2004, astronomers have begun to notice a curious pattern that, on the face of it, seems to challenge the predictions of the black hole model.
More often than not, the speedy stars appear in or near the constellation Leo in the sky.
According to a report in New Scientist, Warren Brown of the Smithsonian Astrophysical Observatory and colleagues have found 14 of the 16 known hypervelocity stars.
Eight of those 14 stars, or nearly 60 percent, lie in the region around Leo - too many to be mere coincidence.
"They're not randomly distributed in the sky," Brown told New Scientist.
Now, Mario Abadi of Argentina's Cordoba Observatory and colleagues suggest a reason why. Based on computer simulations of merging galaxies, they say the stars may have all been ejected at the same time from a dwarf galaxy that was being assimilated into the Milky Way.
In their scenario, the dwarf galaxy was orbiting the Milky Way on a "radial" path that plunged towards and away from the crowded galactic centre.
When the dwarf was closest to the galactic centre, it felt intense gravitational tugs from the massive region. These tugs boosted the energy of some its stars so much that they broke free of the dwarf galaxy entirely and were flung into space.
According to team member Julio Navarro of the University of Victoria in Canada, "Because each star leaves the dwarf with slightly different energy, they start separating as they drift into space."
"The ones that gained the most energy travel the fastest and reach the farthest," he added.
The hypervelocity stars in Leo are the fastest and farthest to have been ejected from their parent galaxy, he says.
Based on the stars' speeds and distances, the team calculates that the stars were probably kicked out of the dwarf galaxy between 100 million and 200 million years ago.