Black Holes and Revelations

Chris Lintott

The shadow of a black hole, photographed last year by the Event Horizon Telescope

Black holes have a fearsome reputation. They have imperilled a thousand stricken starship crews in the pages of science fiction, and the language used to describe them even in non-fiction often implies menace. In popular science, they ‘lurk’ at the centres of galaxies, waiting to ‘devour’ passing stars. I’m not sure this sort of imagery is justified, though it can be hard to avoid. A lot of the time black holes are passive, quiet beasts.

For the past twenty years, an annual ritual has given astronomers a view of stars dwelling unmolested in close proximity to our galaxy’s dominant black hole. Each summer, the Keck telescopes on the summit on Mauna Kea in Hawaii swing to the south, pointing at the centre of the Milky Way. The mountaintop observatory enjoys some of the best skies on the planet, but even there the movement of the air above the telescope will cause stars to ‘twinkle’, which would normally blur the image. To correct for this, optics in the telescope flick to and fro, reacting to the changing sky above the dome, and producing a crystal clear view.

Thanks to this technology, individual stars are visible at the galactic centre, a hundred million billion miles away. Compare snapshots taken year after year, and you can see the stars move, tracing out elliptical orbits around an otherwise invisible object. The movement of the stars tells us that the object must have a mass equivalent to four million suns, packed into a volume smaller than our solar system. It must be a supermassive black hole, dense enough that not even light can escape its gravitational pull.

Despite the presence of this behemoth, the stars are in no immediate danger of being dragged into it, just as the Earth on its annual orbit is not going to fall into the Sun anytime soon.

Things are not always so quiet in the galactic centre, however. An object called G2, which seems to be a dust-enshrouded star, was observed surviving a close passage in 2014. And the recent discovery of radio echoes of past activity by a radio telescope called MeerKAT suggests that our galaxy’s central black hole went on what I suppose we should call a violent feeding frenzy a few million years ago. Such episodes may be caused by disturbances in the galaxy’s disk, funnelling material towards the centre and feeding the black hole, and they indicate that understanding the history of galaxies relies on understanding the behaviour of their black holes.

This is a problem, because we don’t understand the black holes themselves. We know that small black holes can be created in the supernovae that mark the death of massive stars, but a black hole produced that way would probably have a maximum mass of maybe fifty suns, a long way from the million-solar-mass supermassive examples at the centres of galaxies. In the 14 billion years since the Big Bang, there hasn’t been enough time for a black hole that starts off small to consume enough material to grow up to be the size of the Milky Way’s supermassive black hole.

The mystery has been compounded by the apparent absence of intermediate-mass black holes, the missing link between the small and supermassive states. Astronomers have been looking for a long time, and this week their search may have borne fruit. In 2006, X-ray telescopes picked up a flare, a burst of radiation that matched predictions of what would happen if a remote star were ripped apart by a medium-sized black hole, but astronomers couldn’t be sure that the burst didn’t come from a more local source. The orbiting Hubble Space Telescope has now pinpointed the origin of the flare, which comes from a cluster on the outskirts of a distant galaxy.

Such a cluster may be the remnant of a dwarf system, a stepping stone between primordial matter and modern galaxies like the Milky Way. If so, then it may have a similarly underdeveloped black hole, meaning that in this new source we see the long distant past of our own system and the black hole at its centre. This kind of astrophysics isn’t unlike archaeology, as it involves the slow sifting of ancient remnants in order to piece together a complex story of the past.

The galactic centre, viewed from Earth, is in the constellation of Sagittarius, which at this time of year rises just before dawn hits the summit of Mauna Kea. As Earth continues on its orbit around the Sun, and spring turns into summer, Sagittarius will be visible all night. The Keck telescopes will once more turn towards this patch of sky and take another high resolution photo of the stars at the galactic centre. As every year, we will learn a little more about the galaxy we call home; and the newly discovered intermediate-mass black hole will help us to understand a little more of the cosmic story in which it plays a part.