The prospect of measuring the mass of the most massive known objects in the universe would send most people into a cold sweat, but for astronomers using the monster Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, it's all in a day's work. However, ‘weighing' a supermassive black hole from millions of light-years away is far from being a simple task.
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Supermassive black holes are known to lurk inside the cores of most galaxies. They can have galaxy-wide impacts on star formation and are intimately tied to the billions of years of evolution of their host galaxies. To understand how supermassive black holes grow and impact the health of their interstellar environment is therefore one of the most important studies in modern astrophysics.
So, first things first, like any health check, we need to find a way of measuring the mass of these black hole behemoths.
There are several ways to gauge the mass of a supermassive black hole, but it depends on how far away the block hole is and what kind of galaxy it inhabits.
For the supermassive black hole in the core of our galaxy, the Milky Way, astronomers have been able to zoom into Sagittarius A* - a region bright in radio wave emissions - and track the motions of individual stars around an invisible point using incredibly precise infrared telescopes. This invisible point, of course, is the location of a supermassive black hole that is now known to have a mass 4 million times the mass of our sun.
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Gauging the mass of the closest supermassive black hole to Earth is one thing - it is, after all, ‘only' 25,000 light-years from the nearest observatory - what about measuring black holes in the cores of other galaxies?
Because they are so distant, measuring the velocity of stars in the cores of other galaxies is not possible. So, to measure the masses of these objects, astronomers will be on the lookout for radio-bright objects called ‘megamasers' speeding around the central black hole and use them as a beacon of sorts. Unfortunately, megamasers are very rare.
Alternatively, astronomers will try the next best thing and measure the motion of ionized gases inside the galactic core; the velocity of these gas clouds can reveal the mass of the black hole. But this method is best suited for elliptical galaxies; it cannot be used to gauge the mass of supermassive black holes in the cores of spiral galaxies.
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But now, it seems astronomers have found a way to measure the mass of supermassive black holes in the cores of sprial (and barred) galaxies using the observing power of ALMA.