People I speak to are often surprised to hear me talk about imaging black holes. “But how can you see something that is just a black hole, something that has such powerful gravity that no light can escape it, against the blackness of space?” is a question they want to ask, but don’t for fear of looking like a fool. But it’s actually a really good question and the answer is that you actually don’t see it directly at all. Instead, you look for the spectacular effects that black holes have on their surroundings. First of all, since black holes suck things in through good old fashioned gravity, nothing just falls straight in — it whips around the black hole instead, spiraling inwards as momentum is lost to tidal forces. But since that gravity is so immensely powerful, those tidal forces are very strong indeed – anything physical spiraling down towards the black hole gets torn apart, more and more aggressively into ever finer pieces as it gets closer to the event horizon. As the spiral gets closer in, the incoming material starts piling up. This process generates immense internal friction and heat, resulting in very bright emissions of high-energy radiation. It is this characteristic pattern that we look for, and frequently find.
But until recently, we’d never managed to directly image material actually in the process of falling into a black hole — a rather important step, as it would provide final proof that we’re actually right about where these emissions are coming from. NASA’s Chandra X-Ray orbital observatory has now taken these observations a step further: While studying the supermassive black hole at the centre of NGC 3115 (a large galaxy some 35 million light years away), it has imaged hot gas at different distances from the black hole, showing all stages of the process of being consumed. Happily for astronomers, this new data confirms a lot of what we were pretty sure we knew about what happens around black holes – the models don’t have to change!