ALMA puts a black hole on the scale

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How do you weigh a black hole?

For a person, it’s easy. You just step upon a scale and read off your weight. But no scale is large
enough to carry a black hole! Even if it were, it would be gobbled up by the black hole’s incredible
gravitational pull in no time.

Nevertheless, astronomers have now precisely determined the mass of a black hole in a distant
galaxy. Not with a scale, but by measuring the rotational velocity of a disk of cold gas around the
black hole, using ALMA.

The principle is well-known in astronomy. Take our own sun, for example. You can’t put it on a scale,
but still, astronomers know how much it weighs. They only need to measure the orbital velocities of
the planets. Mercury, the nearest planet, orbits the sun at some 48 kilometers per second. The Earth,
though, only travels at 30 kilometers per second. And distant Neptune slowly crawls around at just
some 5.5 kilometers per second.

If you know both a planet’s orbital velocity and its distance to the sun, it’s very straightforward to
calculate the mass of the sun. The same is true for gas orbiting a black hole. Close to the black hole,
where gravity is strong, the gas will move faster than far away from the black hole, where gravity is
weaker.

ALMA was trained at a huge elliptical galaxy in the southern sky. From earlier measurements, it was
already known that the galaxy harbors a massive black hole in its very core. Thanks to its sharp vision,
ALMA could discern a rotating disk of cold gas surrounding the black hole. And by measuring the
millimeter radiation of carbon monoxide molecules in the gas, the astronomers could determine the
rotational velocity at various distances from the black hole. From the observations, the astronomers calculated that the black hole in the galaxy’s core must weigh in at a stunning 2.25 billion times the mass of our own sun. It’s the highest black hole mass ever determined by ALMA.

About one out of every ten elliptical galaxies has a rotating disk of gas in its very center. Therefore,
the astronomers hope that they will be able to weigh many more black holes, in other galaxies, using
the same clever technique.

What?

The large elliptical galaxy studied by ALMA is known as NGC 3258. It is at least 100 million light-years
away from the Earth, in the southern constellation Antlia, the Air Pump. The galaxy was discovered in
1834 by astronomer John Herschel, who studied the southern sky from South Africa. Like all elliptical
galaxies, NGC 3258 has a supermassive black hole in its core. The black hole is surrounded by a
rotating disk of cold molecular gas. The new ALMA measurements revealed that the outer rim of this
disk, some 500 light-years away from the black hole, moves around at about one million kilometers
per hour. However, much closer in, at a distance of just 65 light-years from the black hole, the gas whirls around at well over three million kilometers per hour. From these measurements, it follows
that the black hole’s mass must be some 2.25 billion times the mass of the sun.

Who?

The ALMA observations of NGC 3258 were carried out by a team of astronomers led by Benjamin
Boizelle of the Texas A&M University in the United States. Benjamin worked together with his
American colleagues Aaron Barth, Jonelle Walsh, David Buote, Andrew Baker and Jeremy Darling, and
with Luis Ho from Peking University in China. The team has published their results in a paper in The
Astrophysical Journal.

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