Over the last couple of decades, we have discovered many high-velocity stars, stars so fast that they can quickly exceed their birth cluster and even leave the disk of their galaxy. PG1610+062 is one such object. It left the Sagittarius spiral arm in the Milky Way with a velocity of 550 kilometers (342 miles) per second, almost enough to escape the gravitational pull of our galaxy.
Researchers believe the star was kicked out from its original birthplace by an interaction with a mid-mass black hole (MMBH), an object bigger than a black hole formed by a supernova but certainly not as big as Sagittarius A*, the supermassive black at the center of the Milky Way.
As reported in Astronomy & Astrophysics, using the European Space Agency’s Gaia spacecraft the scientists reconstructed the region of origin of the star and it is nowhere near the center of the galaxy. The team was also able to estimate the acceleration that PG1610+062 must have experienced. This couldn’t have been produced by a supernova or another a binary system giving a kick to this star. These two facts strongly suggest that a MMBH was responsible for the acceleration.
“Now, PG1610+062 may provide evidence that MMBHs could indeed exist in our galaxy. The race is on to actually find them,” lead author Andreas Irrgang of the Friedrich-Alexander University of Erlangen-Nuremberg in Germany, said in a statement.
MMBHS or intermediate-mass black holes are expected to have masses between 100 and 100,000 times that of our Sun. They are giants from our human perspective but still a fraction of the size of supermassive black holes. For comparison, Sagittarius A* is over 4 million solar masses and there are black holes weighing billions of times our Sun. Few of these candidate objects have been found but given that black holes are by definition black, it has been very difficult to confirm their presence with certainty.
PG1610+062 was also studied using the Keck Observatory in Hawaii. Researchers first thought that this was an old small star, which is pretty typical for the halo, the spherical region around the Milky Way. The data obtained instead revealed a young massive star, between four and five solar masses.
While this star is moving really fast, faster stars are known. Some hyper-velocity stars move at almost double the velocity registered by PG1610+062.