ALMA puts a distant black hole on the scales

ALMA puts a distant black hole on the scales

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Using ALMA observations, astronomers have weighed a black hole in the core of a distant galaxy. It turns out to be 140 million times more massive than the sun.

Almost every single galaxy in the universe has a large black hole in its core. Black holes are mysterious objects. They have so much gravity that they suck in gas clouds and stars that come too close. That's why they're called holes. Moreover, to escape the gravitational field of a black hole, you would need to travel faster than light, which is impossible. As a result, nothing – not even light – can escape from a black hole. That's why they're called black.


Our own Milky Way galaxy also has a black hole in its core. Luckily, it's at a safe distance of some 27,000 light years. Large ground-based telescopes, like the Keck telescope on Mauna Kea, Hawaii, and the Very Large Telescope on Cerro Paranal in Chile, have discovered stars that orbit the black hole at very high velocities. From these orbital velocities, astronomers were able to deduce the mass of our Milky Way's central black hole. It's about 4 million times more massive than the sun.

But this technique doesn't work for other galaxies. They are so far away that you can't see individual stars whirling around their central black holes, not even with the largest telescopes. For instance, the galaxy NGC 1097 is at a distance of 45 million light years. That's almost 1,700 times further away than the centre of our own Milky Way.


So how can you weigh a black hole in another galaxy? Astronomers have developed a couple of tricks, but these only work in special cases. Now, they have found a new way to put distant black holes on the scales, using observations by ALMA.

ALMA can't see individual stars in the central parts of other galaxies, but it can observe cool clouds of gas that orbit the black hole at a larger distance. In the case of NGC 1097, ALMA measured the distribution and the velocity of hydrogen cyanide and formylium gas. From these measurements, the astronomers were able to 'weigh' the black hole: it is 140 million times the mass of the sun – 35 times more massive than the black hole in our own Milky Way.

NGC 1097 is a so-called barred spiral galaxy – a spiral with an elongated 'bar' of stars in its centre. It is at a distance of 45 million light years, in the southern constellation Fornax the Oven. The galaxy was first discovered by English astronomer William Herschel in 1790. Hidden deep in the very centre of NGC 1097's bar is a supermassive black hole. Just how massive it is has been unknown – until now.

The ALMA observations of NGC 1097 were carried out by Kyoko Onishi of the Graduate University for Advanced Studies in Japan. She worked together with Japanese astronomers Satoru Iguchi and Kotaro Kohno, and with American radio astronomer Kartik Sheth. The team has published their results in The Astrophysical Journal.