IN 1783 John Michell reasoned they must be out there. In 1916 Karl Schwarzschild calculated how big they would be. In 1930 Subrahmanyan Chandrasekhar showed that big enough stars were doomed to become them. Yet it took until the 1970s to convince holdouts among astronomers that black holes actually exist. That was when studies of a celestial X-ray source called Cygnus X-1 revealed an object so massive that it could be nothing else.
Even before that, though, black holes had caught the imagination of astronomers and public alike. The idea of something so dense that its gravitational attraction can stop light (or anything else) escaping from its surface is mind-boggling. Yet such holes are crucial building blocks of the universe. Most, if not all galaxies have one at their centre. Meanwhile, on Earth, the idea that something or someone has “fallen into a black hole” and thereby, from the speaker’s point of view, vanished, has become proverbial.
In fact, black holes—or, rather, their surroundings—are often not black. The process of attracting and swallowing matter acts like a giant particle accelerator as the matter spins around the hole. As with such accelerators on Earth, this generates electromagnetic radiation, from radio waves to X-rays. A big black hole’s neighbourhood, with lots of spinning matter, can thus be very bright indeed. That neighbourhood is also a place where space and time themselves are warped more intensely than anywhere else in the observable universe.
This chaos, and black holes’ restricted dimensions (even the “supermassive” ones in galactic cores are mere millions of kilometres rather than light-years across), makes studying them hard. But not impossible, as delegates to this week’s meeting of the American Astronomical Society (AAS) in Kissimmee, Florida, have heard.
Blown on the steel breeze
Whether galaxies formed around pre-existing black holes or the holes formed after those galaxies had come into being is not yet known. It is suspected, though, that their central black holes help regulate galaxies’ rates of star formation. Eric Schlegel of the University of Texas, San Antonio, brought some evidence to bear on this question. His instrument of choice is Chandra, a space telescope named after Chandrasekhar which detects X-rays. The object of Dr Schlegel’s interest is NGC 5195, a small companion galaxy of the Whirlpool, a well-known spiral galaxy (see above; NGC 5195 is on the right, dangling from one of the Whirlpool’s spiral arms).
Looking at X-rays from NGC 5195 Dr Schlegel and his colleagues spotted two bright, arc-shaped features in the gas opposite the point where the Whirlpool’s arm reaches into NGC 5195 and material is pulled from it towards the galaxy’s central black hole. The shapes and orientations of these arcs suggest the hole has undergone a pair of explosive events caused by the overwhelming amount of material the Whirlpool is dumping on it. The shock waves from such explosions would sweep away dust and gas that are the raw material from which new stars are built. And NGC 5195 is indeed noticeably bereft of new stars—in contradistinction to the Whirlpool, which is rich in them. Dr Schlegel may thus have found a mechanism by which black holes can switch off star formation in their vicinities.
Julie Comerford, of the University of Colorado, Boulder, described another such mechanism. She observes that galaxies often bash into one another and merge. The result of such a merger will have, at least for a time, two central black holes.
Dr Comerford used data from Chandra and also from the Hubble space telescope, which sees visible light, rather than X-rays, to study such galaxies. She found that, on average, galaxies with two black holes in their cores put out more than ten times as much light as those with one. This extra light is created by the extra quantities of material sucked into their twinned cores. One consequence of all this light is to ionise (ie, to strip the electrons from) much more of the surrounding galaxy’s gas than would usually be the case, and then push this ionised gas out of the way. That provides a second way that the behaviour of a galaxy’s black-hole-inhabited core can switch off star formation in the rest of the star system.
Working out how black holes affect galactic growth, though, is not the same as finding out what is happening in the vicinity of the holes themselves. Here, matter is circling close to a black hole’s point of no return, the “event horizon” whose radius was described by Schwarzschild’s calculations. And next month, JAXA, Japan’s space agency, will launch Astro-H, a telescope that will help to do this. It can detect X-rays of exceptionally high energies. As material slips ever closer to the event horizon, the precise details of its X-ray output are a signal of how it is moving. Astro-H will be able to measure this radiation, and thus infer that motion with unprecedented precision. This will permit researchers to measure unambiguously, for the first time, how fast a black hole is spinning. That, in turn, permits tests of Einstein’s general theory of relativity—the very theory that Schwarzschild used to put black holes on solid mathematical ground—that have remained out of reach until now.
Shadows at night
There is, though, a fundamental limit to such pursuits; there can be no way to peer within the event horizon. The best astronomers can hope for is to snap a detailed picture which shows not only the shadow cast by the horizon—an actual black void within the picture—but also the violent environment just outside it.
That will require yet another bit of kit, and another spiral galaxy: the one that plays host to Earth. Feryal Ozel of the University of Arizona told the meeting about progress on the Event Horizon Telescope (EHT). This is designed to capture an image, built up from radio waves, of the Milky Way’s own supermassive black hole, dubbed Sagittarius A* because it is the brightest radio source in that constellation.
Making such an image is hard. Sagittarius A* (or, rather, its event horizon) should be about 12m km across. That sounds big, but it is a twenty-billionth of the black hole’s distance from Earth. A telescope’s resolving power depends on the width of its aperture. The resolving power needed to see at that distance something even of the enormous size of Sagittarius A* therefore requires an extraordinarily large telescope.
As a result the EHT is not a single facility but rather a collaboration between existing radio telescopes scattered across the world (see map). Together, these instruments perform a trick called very long baseline interferometry. At times when they are acting as part of the EHT, they will be pointed simultaneously at Sagittarius A*. The data thus gathered will then be shipped to a central facility in Cambridge, Massachusetts, on enormous disk drives, and there carefully combined in a way that makes it seem as if they had been collected by a single radio telescope with an aperture nearly as wide as the Earth itself.
This should, if the EHT works as advertised, solve a number of outstanding mysteries about black holes: precisely how material falls into them, what causes the jets of material that sometimes squirt from near their polar regions, and just how good Einstein’s equations are at describing the most warped spacetime it is possible to see. That would be a rich haul of facts for a phenomenon that took two centuries to be taken seriously.