Hypervelocity stars are racing out of the Milky Way (Illustration: Ruth Bazinet/CfA)
Nearly 60% of the known hypervelocity stars are clustered in or near the constellation Leo in the sky (black ellipse) (Illustration: Abadi et al.)
"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.
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%, 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.
Energy boostIn 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.
Oleg Gnedin of the University of Michigan in Ann Arbor, who is not a member of the team, likens the stars in the dwarf galaxy to spaghetti boiling in a pot.
When the dwarf galaxy gets near the Milky Way's centre, it's like turning up the heat on the already boiling pot. "The water gets too hot, pops the lid, and the spaghetti flies away in all directions," he says. "Some pieces are hotter and will fly a longer distance than the colder pieces that barely leave the pot," he told New Scientist.
Stream of starsTeam member Julio Navarro of the University of Victoria in Canada explains that the stars ejected in this way form a long stream or trail.
"Because each star leaves the dwarf with slightly different energy, they start separating as they drift into space," he says. "The ones that gained the most energy travel the fastest and reach the farthest."
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.
Testable predictionAbadi's team acknowledges that there are weaknesses to its argument. For example, most of the known hypervelocity stars appear to be young and massive, suggesting they came from a galaxy rich in star-forming gas.
But most nearby dwarf galaxies are gas poor, says Brown. "Over time, the gas gets stripped out by interactions with the Milky Way," he told New Scientist. "I just don't see where their gas-rich progenitor comes from."
Future observations could test their idea. Most of the stars in dwarf galaxies should be old and low in heavy elements, while stars catapulted from the galactic centre should be metal rich.
Brown is sceptical that hypervelocity stars arise in dwarf galaxies. "But it's a testable prediction and I like that," he says. "We can answer the question one day."
Heavier modelWhat will happen to the stars in the future? Abadi's group, unlike Brown's, believes that most of the stars will never escape the Milky Way.
They say the Milky Way may be three times more massive than current observations suggest, meaning the stars would have to be moving a lot faster than they are to break free of its gravity.
The mass of the Milky Way can only be measured reliably from the galactic centre out to a few times the orbital distance of the Sun, Navarro says. Beyond that, there is not enough normal matter – such as stars – to make the measurements, and models differ in how much mysterious dark matter they contain.
Using a relatively heavy model for the Milky Way, the team found that only one of the hypervelocity stars around Leo is moving fast enough to eventually escape the Milky Way's clutches.
"The hypervelocity star speeds are unusually high, but not necessarily extreme," the team says in a study submitted to the journal Astrophysical Journal Letters. "This is important, since it suggests that more prosaic dynamical effects that do not rely on supermassive black hole ejection may be responsible for the unusual speeds."
Observatório Astronômico Monoceros