Supermassive black hole gives up its secrets

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More than 50 years ago, scientists saw that there was something very bright at the centre of our galaxy. That bright object was long thought to be the Milky Way’s central supermassive black hole, named Sagittarius A, which has a gravitational pull strong enough to make stars orbit around it very quickly. Stars close to Sagittarius A orbit as fast as 20 years. In contrast, our solar system takes about 230 million years to circle the centre of the galaxy.

Supermassive black hole gives up its secrets

Now, a consortium of European institutions has not only confirmed that the object at the centre of our galaxy is, as has long been assumed, a supermassive black hole, but that there is material orbiting very close to the black hole’s event horizon, the point at which even light can't escape the black hole's grasp. This is first time material has been observed orbiting close to the point of no return, and offers the most detailed observations yet of material orbiting this close to a black hole.

The European Space Organisation’s GRAVITY instrument on the Very Large Telescope (VLT) Interferometer was used to see the activity around Sagittarius A. GRAVITY combines the light from four telescopes from the VLT to create a virtual super-telescope 130 metres in diameter. Considering that we are 26 000 light-years (or 245 trillion kilometres) away, this makes seeing anything an amazing achievement, despite the fact that Sagittarius A has four million times the mass of our sun, meaning that the black hole is about 44 million kilometres across.

While some matter in the accretion disc (the belt of gas orbiting Sagittarius A) can orbit the black hole safely, anything that gets too close is doomed to be pulled beyond the event horizon. There are a few types of gases that make up parts of this accretion disk, and scientists previously have only identified some of the the very hot ones. Because these gases are so hot (about 10 million degrees Celsius), they give off X-rays that researchers could easily detect.  Now the team found that the accretion disk also has cooler hydrogen gas. The radiation in the area causes hydrogen atoms to constantly lose and gain their electrons, an activity that releases weak radio waves.

The cool hydrogen ring is about a hundredth of a light-year away from the black hole's event horizon, and contains an amount of hydrogen equivalent to a tenth of the mass of Jupiter, the scientists said. Because of the Doppler effect, which makes light from objects moving toward our planet look slightly bluer, and light from objects moving away from our planet look slightly redder, the researchers concluded that the gas is rotating around the black hole at roughly 30% of the speed of light.

This research was undertaken by scientists from the Max Planck Institute for Extraterrestrial Physics (MPE), the Observatoire de Paris, the Université Grenoble Alpes, CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the Portuguese CENTRA – Centro de Astrosica e Gravitação and the European Space Agency, and it backs the findings of another team of researchers from the Instituto de Astrofisica de Canarias (IAC) in the Canary Islands. Last year, the second team analysed the gas in the galactic winds blown about by black holes and found that the gas in the winds, known as active galactic nuclei (AGN), can reach velocities of thousands of kilometres per second.

They have theorised that when black holes grow rapidly it is because they consume the material in the galaxy at a high rate. This causes the generation of enormous galactic winds, and particularly powerful AGN's generate quasars which are capable of obliterating the material at the centre of galaxies and preventing new stars from forming there.

Studying the winds of ionised and molecular gas from Sagittarius A’s quasar by using the infrared range, the scientists found that they don't always show similar properties. Their data revealed that the ionised wind is even faster than the molecular wind, reaching velocities of up to 1 200km per second.

While both teams believe these winds have a direct effect on how black holes impact the development of the galaxies around them, they don’t fully understood how. With further study, there’s no doubt that they will be able to draw more theories and correlations, helping us understand our universe a little better.

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