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A massive comfort

There are bigger things in the Universe than our earthly worries. Much bigger.

People have always looked to the heavens for inspiration and distraction from everyday life. Some see their futures in the stars and others talk to the man in the Moon. But for the really big problems, you need a really big distraction. So for readers who feel especially gloomy, perhaps about the latest bout of political shadowboxing on global warming in Durban, South Africa, or the perilous state of European finances, let Nature introduce you to two newly identified black holes. These are not the ordinary, two-a-penny stellar black holes, mind, but their supermassive relatives — the big beasts of the cosmos.

On page 215 of this issue, astronomers from across the United States present their discovery of the two most massive black holes ever found. The larger of the pair would tip the scales at some ten billion times the mass of our own Sun. Heavy stuff indeed.

In some respects, it is surprising that these black holes have managed to hide for so long. The curious twinkling of some quasars dated to the earliest days of the Universe had made astronomers pretty sure that the giants were out there somewhere, yet the previous heavyweight champion, in the giant elliptical galaxy Messier 87, is a relatively puny 6.3 billion solar masses.

Measuring the motion of stars from two different galaxies that are the biggest in their clusters, NGC 3842 and NGC 4889, Nicholas McConnell of the University of California, Berkeley, and his colleagues found that NGC 3842 has a central black hole with a mass of 9.7 billion solar masses, and NGC 4889 has a black hole about the same size or even larger. (All massive galaxies with a spheroidal component, such as the bulge of the Milky Way, are thought to harbour a supermassive black hole at their centre.)

As celestial inspirations go, it is true that the giant twins of darkness at the heart of NGC 3842 and NGC 4889 are somewhat distant companions. The nearest is about 98 megaparsecs from Earth — about 320 million light years. And it is true that McConnell's team was able to look to the sky with more than a shrug of resignation — the researchers could use integral-field spectrographs at the giant Gemini North and Keck telescopes perched on Hawaiian mountains. But what these supermassive black holes lack in touchy-feely access, they surely gain in being really, really big. And if it helps those of us weighed down by terrestrial matters, they are out there, somewhere.

As supermassive black holes go, they are perhaps just a little too supermassive, certainly a bit larger than astronomers would have predicted given what they know about their host galaxies. Perhaps the processes that influence the growth of the largest galaxies and their black holes are somehow different from those in smaller galaxies? As the authors note, better adaptive optics instruments on telescopes and very-long-baseline radio interferometry are finding black holes in ever more exotic galaxies, and will help to answer that question.

It now seems plausible that, within a decade or so, astronomers will be able to capture the first direct image of a black hole. Plans are afoot to link a network of instruments, stretching from the high desert plains of Chile and the South Pole to potential sites in Africa and New Zealand. Pointed at the centre of our own galaxy, such a telescope should be able to see SgrA*, the supermassive black hole believed to sit there. Astronomers already have an image in mind: a bright ring spun around a dark shadow cast by the black hole, because no light escapes its event horizon.

Widely described as the point of no return, the event horizon of SgrA* would be the largest in our skies, but still just 30 microarcseconds across — the apparent size of a tennis ball on the Moon when viewed from Earth. To capture its image would be a stunning technical achievement in itself, but it would also open the door to further studies of how black holes spin and gather material, as well as probing some fundamental aspects of space-time and general relativity. And the first picture taken of our local supermassive black hole — the most enigmatic and charismatic of all the wonders of the Universe — would surely be one of the defining images of the time. It might even knock everyday trouble and strife from the front pages, and perhaps even, for a while, from people's minds.

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A massive comfort. Nature 480, 152 (2011). https://doi.org/10.1038/480152a

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