Black hole hidden within its own exhaust
Almost every galaxy in the Universe has a massive black hole in its core. With its gravity, the black hole sucks in material from its surroundings. It gobbles up clouds of gas and dust, and maybe even complete stars. The black hole’s ‘food’ first piles up in a thin, rotating disk of very hot material, before it plunges into the hole.
But just like Cookie Monster, a black hole is a messy eater. It is spilling a lot of ‘crumbs’. Using ALMA, astronomers have discovered that part of the black hole’s meal is blown back into space. This stuff ends up in a large dark ring of cold gas and dust, shaped like a donut. Eventually, the dark donut can even hide the small, rotating disk around the black hole from view, just like the exhaust from a car engine can hide the car from view.
Astronomers have found many supermassive black holes in other galaxies. Many of them are partly or completely hidden in thick donut-shaped rings of dust. It has always been thought that this dust originated in the surrounding galaxy. The black hole’s gravity would have drawn it inward. But the new ALMA observations tell another story.
ALMA observed the outer parts of the thin, rotating disk that surrounds the central black hole in the galaxy M77. ALMA was able to measure the velocity of cold clouds of gas in these outer regions. It turns out that the clouds are moving with a velocity of 400 to 800 kilometers per second (that’s about 1.5 to 3 million kilometers per hour).
At such high velocities, the clouds are flung away into space – they will never be able to fall into the black hole. Most likely, the gas and dust will end up in the dark donut-shaped ring. That means that the stuff in the obscuring ring is not sucked in from the surrounding galaxy. Instead, it’s cooled-down material that was blown away into space, out of the thin, hot disk that surrounds the black hole.
So the supermassive black hole in M77 is really a bit like an engine. It uses fuel – the gas and dust that is being gobbled up –, but it also produces a lot of dirty exhaust. Future observations by ALMA will hopefully reveal how much material goes in and how much comes out. Also, by studying the supermassive black holes in other galaxies, astronomers hope to find out if all donut-shaped dust rings are produced by the central black holes themselves.
M77 is a large spiral galaxy. It’s about 50 percent larger than our own Milky Way galaxy. It is located at a distance of 47 million light-years, in the constellation Cetus the Whale. In the core of the galaxy (which is also known as NGC 1068) is a supermassive black hole. The black hole probably weighs in at about 15 million times the mass of the Sun. However, the black hole can’t be studied in detail: it’s hidden from view by a thick, donut-shaped ring of dust, with a diameter of some 40 light-years.
The ALMA observations of M77 were carried out by a large group of astronomers, led by Jack Gallimore. Jack is a professor of physics and astronomy at Bucknell University in Lewisburg, Pennsylvania. For the ALMA observations, he teamed up with many other astronomers from the United States, the United Kingdom, Italy, Canada, Germany and Chile. The results have been published in Astrophysical Journal Letters, a professional magazine of astronomy.