How to Precisely Weight a Black Hole with ALMA?
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By carefully looking at other people, you can guess how much they weigh. But if someone wears Harry Potter’s invisibility cloak, you have no idea – the invisible person could be a skinny boy or a fat lady.
Astronomers have a similar problem. By carefully looking at stars or galaxies, they can guess their mass. But how do you ‘weigh’ a black hole? After all, black holes are invisible: they are so massives that their tremendous force of gravity avoid even the light to escape.
Now, astronomers have used ALMA to put a black hole on the scales. Not by studying the black hole itself, but by measuring its effect on neighboring gas. The result: the black hole, which sits in the core of galaxy NGC 1332, is 660 million times more massive than the Sun. It’s the most accurate mass estimate for a supermassive black hole in another galaxy ever.
The black hole is surrounded by a large disk of cold gas. Up to a distance of some 80 light-years, the motion of this gas is determined by the gravity of the black hole – just like the motion of our own planet Earth is determined by the gravity of the Sun. The more massive the black hole is, the faster the surrounding gas will orbit around it.
Thanks to its superb vision and accuracy, ALMA was able to measure the orbital speed of cold carbon monoxide gas in the central parts of the disk: just over 500 kilometers per second. From this measurement, astronomers could calculate the gravity – and thus the mass – of the central black hole.
Our own Milky Way galaxy also has a supermassive black hole in its core. The Milky Way’s black hole weighs in at just 4.3 million times the mass of the Sun. The black hole in the core of NGC 1332 is some 150 times more massive, but it’s still relatively light-weight, as supermassive black holes go.
For astronomers, it is important to precisely know the mass of supermassive black holes in other galaxies. It will help them to better understand the evolution of galaxies and the growth of black holes in the Universe.
NGC 1332 is a galaxy, containing hundreds of billions of stars. It’s not a spiral galaxy like our own Milky Way, but a so-called elliptical galaxy, looking a bit like a huge M&M seen from the side. It is located in the southern constellation Eridanus, at a distance of 73 million light-years. Astronomers already knew about the supermassive black hole in the core of NHC 1332, but no one knew its mass – earlier estimates ranged from 500 million to 1.5 billion times the mass of the Sun.
The mass of the supermassive black hole in the core of NGC 1332 was determined by a large group of astronomers, led by Aaron Barth. Aaron is an astronomer at the University of California at Irvine in the United States. He worked together with colleagues from all over the country, and one from China.
According to Professor Luis Ho of Kavli Institute of Astronomy and Astrophysics, Peking University, China, “it's so cool that we discovered a humongous black hole with this awesome new telescope: it’s more than a hundred times bigger than the one in the middle of the Milky Way, the galaxy we live in!
The team published their results in an article in the Astrophysical Journal Letters, a professional astronomy magazine.