New light on galaxies' hearts of darkness
At the centre of just about every galaxy lies a supermassive black hole, swallowing nearly any star or material in its reach.
Or so the theory goes. But they swallow light, too - making them all but impossible to observe directly. Instead, astrophysicists, including some based in Abu Dhabi, resort to looking for kinks in the orbits of nearby stars as they fight the black holes' immense gravitational pull, and the illuminated "halo" around the hole created by the gas and dust continuously feeding it.
But in more than two-thirds of galaxies, there is nothing - just darkness - suggesting that either there is no supermassive black hole, or that one exists in an empty space, with no material to gobble up.
For nearly four decades, cosmologists have been looking for evidence to prove that supermassive black holes are galaxy staples. Occasionally, a star manages to narrowly escape the enormous pull but rips apart in the process.
As it does so, huge quantities of gas are released, glowing brightly as they in turn are sucked into the void. With X-ray and ultraviolet satellites, these tidal disruption flares (TDFs) can be observed. But they are rare. For every 1,000 supernovae - the flare seen when a star explodes at the end of its life - there is just one TDF. And the distinction between a supernova and a TDF is subtle.
To pin down the differences, an international group of scientists has studied a decade's worth of archived telescopic data of 2.6 million galaxies, tirelessly examining the tiny differences between consecutive images. Of the 342 intense flares they found, there were just two TDFs.
The research, led by New York University's Centre for Cosmology and Particle Physics, was recently published in the peer-reviewed Astrophysical Journal. The hope is that establishing just how often TDFs happen - the current best guess is about once in 100,000 years in each galaxy - will help provide a new window into general relativity.
"It's a matter of going through thousands upon thousands of images in one patch of the sky, and subtracting them from each other," said Dr Joseph Gelfand, a radio astronomer and assistant professor of physics at New York University Abu Dhabi, who was one of the study's authors.
"Even when we see in a tiny fraction any change, a new object or source of light, there could be many explanations.
"We needed to determine whether they were massive stars exploding in bright flashes of light, flares completely unrelated to stars falling into black holes, or stars ripped to shreds in this extraordinarily rare event."
So they first had to look at the exact pattern of wavelengths produced by millions of stars. Different gases emit a slightly different spectrum of light, under different circumstances.
The wavelengths produced by gas around a black hole are different from the light emitted by a supernova, or a flare. Examining the spectrum from a particular type of star, or a particular event, gives a "fingerprint" that scientists can then be on the lookout for elsewhere.
Because supernovae are relatively common, scientists are familiar with their ultraviolet glow, which tends to be red in colour. Of the 342 flares observed, 340 were red.
But two were not. Instead, the light from them was more blue. The scientists suspected they might have found their prize - the light from the hot gas of a star being ripped apart by a supermassive black hole.
Still, that was not enough. So they looked at how the brightness of those two flares peaked and then faded. Supernovae tend to get brighter over a few weeks and fainter over a period of months.
TDFs happen much more quickly, becoming brighter over just a few days as they continue in a chaotic orbit, crashing into themselves and producing intense radiation. Then, in a matter of weeks rather than months, they become fainter and disappear as the superhot gas is sucked into the black hole. And this quicker pattern was what they found.
But this was still not enough. So Dr Gelfand turned to the radio waves emitted by the flares. Supernovae often produce X-ray and radio waves, while TDFs do not.
Fortunately, the events had been witnessed by the US National Radio Astronomy Observatory's Very Large Array radio telescope in New Mexico. They had been flagged as a possible supernovae, but never analysed, the data sitting disregarded in a digital archive.
When Dr Gelfand looked for radio frequencies, he found nothing. And that, at last, was enough to confirm the team's belief. What they were looking at were TDFs, not the remnants of supernovae.
"That makes this one of the strongest cases that these types of explosions exist," he said.
Most importantly, according to Sjoert van Velzen, the Dutch doctoral student who led the study, it is a step towards answering questions about how black holes gather material - and "to test the fundamental theory of general relativity that describes black holes".
And that could change the way we see the role of black holes, how they feed and grow in host galaxies - and perhaps even science's view of how the universe came to be.