ALMA sees two-way traffic around a black hole

Read time: 3 minutes

Here’s an easy experiment you can do at home. Fill the kitchen sink with water. Then pull out the
plug, and carefully watch how the water flows down the drain. You will notice that it doesn’t
disappear in a straight line. Instead, the water flows away in a rotating, spiral pattern.
You’ll notice something else, too. No matter which way the water rotates – either clockwise or anti-
closkwise – all the water spirals in the same direction. You don’t see water close to the drain flow in
one direction, and water further out in the opposite direction – that would be weird
Yet, that is exactly what astronomers are witnessing in the core of a galaxy known as M77. The galaxy
has a cosmic ‘sink’ in its very center: a supermassive black hole. Nearby gas clouds are falling into the
black hole, because of its strong gravity. And just like the water in the kitchen sink, the gas doesn’t
disappear in a straight line. Instead, it first piles up in a rotating ‘accretion disk’ around the black
hole.
So far, so good. But now, new ALMA observations have revealed something weird and unexpected.
The inner part of the disk is rotating in one direction, and the outer part of the disk is moving in the
opposite direction! Something like that has never been seen before.
The astronomers think that the accretion disk must have been disturbed somehow; maybe because
the large galaxy has consumed a smaller companion in the past. Without such a disturbing influence,
it’s unclear how two parts of the same disk could ever rotate in opposite directions.
The ALMA discovery may help solve another puzzle. Astronomers have discovered that massive black
holes were already around when the universe was quite young. It’s hard to explain how a black hole
can grow so rapidly – it’s as if you have a jar with tadpoles, and two days later they have already
grown into adult frogs.
But if the accretion disks around black holes can move in unexpected ways, like in the case of M77,
the disks cannot be stable in the long run. Eventually, much more gas will fall into the black hole in a
short period of time. Maybe that’s how black holes in the early universe were such fast growers!

What?

M77 (also known as NGC 1068) is a large spiral galaxy in the constellation Cetus, the Whale. The
galaxy was discovered in 1780 by French astronomer Pierre Méchain. It is located at a distance of
some 47 million light-years. The core of the galaxy harbors a black hole that is almost 17 million times
more massive than the Sun. Earlier ALMA observations already revealed that gas is falling into the
black hole at an enormous rate. ALMA has now also studied the cold, molecular gas in the so-called
‘accretion disk’ around the black hole. The inner part of the disk (within 4 light-years distance from
the black hole) is spinning in the same direction as the galaxy. However, the outer part of the disk
(out to a distance of 22 light-years) is rotating in the opposite direction.

Who?

The ALMA observations of the accretion disk around the black hole in the core of the galaxy M77
were carried out by an international team of astronomers. The team was led by Violette Impellizzeri of the Joint ALMA observatory in Santiago, Chile. Violette worked together with ten colleagues from
Chile, the United States, Canada, Germany, the United Kingdom, and Italy. The team published their
results in The Astrophysical Journal Letters, a professional astronomy magazine.

ALMA URL