Confirmed: a monster black hole at the heart of the Milky Way
Long suspected, the existence of a supermassive black hole has now been ‘resoundingly’ proven. Nick Carne reports.
The evidence is now almost irrefutable that there is a supermassive black hole with a mass four million times that of the sun lurking in the middle of the Milky Way.
New observations by the European Space Observatory (ESO) show clumps of gas swirling around at about 30% of the speed of light on a circular orbit just outside what astronomers conclude is the black hole’s event horizon.
It’s the first time material has been seen orbiting close to the point of no return – and “a resounding confirmation of the massive black hole paradigm", according to study leader Reinhard Genzel, of the Max Planck Institute for Extraterrestrial Physics (MPE) in Germany.
"This always was one of our dream projects, but we did not dare to hope that it would become possible so soon," he says.
The findings are published in the journal Astronomy & Astrophysics.
Scientists from a consortium of European institutions used the GRAVITY instrument on the ESO’s Very Large Telescope (VLT) Interferometer in Chile to observe flares of infrared radiation coming from the accretion disc around Sagittarius A*, a bright and compact astronomical radio source at the heart of the Milky Way.
While some matter in the accretion disc – the belt of gas orbiting Sagittarius A* at relativistic speeds – can orbit the black hole safely, anything that gets too close is doomed to be pulled beyond the event horizon.
The closest point to a black hole that material can orbit without being irresistibly drawn inwards by the immense mass is known as the innermost stable orbit, and it is from here that the observed flares originate.
“It’s mind-boggling to actually witness material orbiting a massive black hole at 30% of the speed of light,” says the MPE’s Oliver Pfuhl. “GRAVITY’s tremendous sensitivity has allowed us to observe the accretion processes in real time in unprecedented detail.”
The instrument combines the light from four telescopes in ESO’s VLT to create a virtual super-telescope 130 metres in diameter.
Earlier this year, the same team used GRAVITY and SINFONI, another instrument on the VLT, to accurately measure the close fly-by of the star S2 as it passed through the extreme gravitational field near Sagittarius A*and for the first time revealed the effects predicted by Einstein’s general relativity in such an extreme environment. During S2’s close fly-by, strong infrared emission was also observed.
“We were closely monitoring S2, and of course we always keep an eye on Sagittarius A*,” says Pfuhl. “During our observations, we were lucky enough to notice three bright flares from around the black hole; it was a lucky coincidence!”
This emission, from highly energetic electrons very close to the black hole, exactly matches theoretical predictions for hot spots orbiting close to a black hole of four million solar masses. The flares are thought to originate from magnetic interactions in the very hot gas orbiting very close to Sagittarius A*.