At the edge of the galactic black hole

Astronomers are observing the immediate environment of this gravational giant

A team of researchers – including the Max Planck Institute for Extraterrestrial Physics in Garching – have gained astounding insights into the galactic centre: The astronomers have spotted gaseous clouds which are spinning around the assumed black hole at the heart of the Milky Way at a speed of around 30 percent of the speed of light. The gas is moving in a circular orbit outside the innermost stable path and can be identified through radiation bursts in the infrared range. This discovery was made possible by the Gravity Instrument, which combines the light of all four eight-metre mirrors of the Very Large Telescope at the European Southern Observatory (ESO). Thanks to this technology, which is called interferometry, Gravity generates the power of a virtual telescope with an effective diameter of 130 metres.

This unusually compact object sits right in the middle of the Milky Way and generates radio emissions: Astronomers call it Sagittarius A*. It is highly probable that this is a black hole with the mass of approx. four million suns. But this is by no means certain, and scientists are always devising new tests to support this thesis. Researchers have now used the Gravity Instrument to take a close look at the edges of the alleged black hole.

According to this theory, the electrons in the gas approaching the event horizon should speed up and therefore increase in brightness. The region of only a few light hours around the black hole is very chaotic, in a similar way to thunderstorms on Earth or radiation bursts on the Sun. Magnetic fields also play a part here, because the gas conducts electricity making it a plasma. The latter should ultimately show up as a flickering “hot spot” circling the black hole on the final stable path.

In fact, the astronomers registered such radiation bursts on the so-called accretion disc – a gaseous ring with a diameter of only ten or so light minutes which circles the galactic centre at extremely high speed. Matter can circle safely so long as it does not come too close to the black hole; matter within the event horizon can no longer escape the enormous gravity. The radiation bursts (flares) that have now been observed originate from matter in an orbit near this event horizon.

“We saw a total of three such flares. They all had the same radius and the same orbital periods,” says Reinhard Genzel, Director of the Max Planck Institute for Extraterrestrial Physics in Garching and Study Leader. A simple orbit model can be used to explain the motion of these three hot spots in the galactic centre, whose radius is three to five times that of the event horizon. In addition, the measurements exactly confirm the theoretical predictions for gas circling near the innermost stable orbit.

“Taking all of our observations into account, we have clear evidence that this really is matter in an orbit near the event horizon of a black hole with a mass of four million suns,” confirms Jason Dexter from the Max Planck Institute in Garching, one of the main authors of the paper published in the journal Astronomy & Astrophysics. And Dexter’s colleague Oliver Pfuhl adds: “Gravity and its enormous sensitivity enabled us to observe the accretion processes in real time – and at a level of detail never seen before.“

The very high angular resolution and measurement precision of Gravity as well as the precision spectroscopy using the integral field camera Sinfoni at the Very Large Telescope had already made it possible for the same team to measure the flight of the star S1 near the galactic black hole earlier in the year. This revealed a gravitational redshift as predicted by the general theory of relativity.

“We have always dreamed of making such observations. But we did not dare hope that they could actually become a reality – with such clear results,” Reinhard Genzel says. So is Sagittarius A* actually a supermassive black hole? “Our result it seems overwhelmingly confirms this assumption.”


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