Astronomy: Black hole in satellite galaxy hurls stars into Milky Way
At the edge of the Milky Way, there are a surprising number of hyper-fast stars that originate from outside. They apparently betray a supermassive black hole.
The Milky Way (as an arc-shaped band) and the Large Maggellanic Cloud (between the telescopes) above the Paranal Observatory
(Image: ESO/Y. Beletsky)
There is apparently a supermassive black hole in the Milky Way's largest satellite galaxy that is hurling hyper-fast stars into our home galaxy. This is what a research group led by Jiwon Jesse Han from the Harvard & Smithsonian Center for Astrophysics (CfA) believes to have discovered. Their research is currently under review. According to the study, the group counted only seven stars in a selection of 16 stars speeding through the Milky Way that were accelerated to these high speeds in the galactic center. Nine, on the other hand, are likely to originate from the Large Magellanic Cloud and the vicinity of a significantly more massive black hole than was assumed there.
Much faster than their surroundings
Hypervelocity stars (HVS) are stars that are significantly faster than the escape velocity of their surroundings. They can be formed, for example, by the so-called Hills mechanism, in which a binary star system is separated at a black hole and one of the stars is hurled away at over 1000 km/s. There are a small number of these stars at the edge of the Milky Way, which the research group has focused on. From the properties of some of them, the team was able to determine not only how strongly they were accelerated in the Large Magellanic Cloud, but also how massive the black hole responsible for this is.
The number of discovered hyper-fast stars with an origin in the neighboring galaxy in relation to the number of those from the Milky Way can only be explained by a supermassive black hole hiding there, the team writes further. It would therefore have to have 6 × 105 solar masses. Although this is smaller than the 4.3 × 106 solar masses of Sagittarius A*, the supermassive black hole at the heart of the Milky Way, it is “significantly more” than previously assumed, according to the study. For a smaller black hole, simply too many hyper-fast stars have been discovered that originate from there.
The research work, which has not yet been published in a specialist journal, adds another piece of the puzzle to the picture of the Milky Way's largest satellite. The Large Magellanic Cloud accounts for around 10 percent of the mass of our Milky Way, but it is not yet clear whether it actually orbits the Milky Way or merely passes through it. Only last fall, a research team explained that the only reason the Large Magellanic Cloud does not lose too much gas for the formation of stars due to its “catastrophic interaction” with the Milky Way is because it is comparatively large. If it were smaller, all that would have remained of it would be “a collection of aging red stars” – with a fairly massive black hole at its center.
(mho)