NOVA

We grow up hearing about black holes; in science fiction, in novels. They're sort of common-day things. But the more I study black holes, the more mysterious, in some ways, they become. I'm Chung-Pei Ma. There are two types of black holes. One is the end stage of massive stars. Stars 10 times, 20 times, the mass of the sun, eventually die as a black hole. It is a spectacular death. Those are much more common. There are hundreds of millions of them in our galaxy and every galaxy out there. Those are easy to make. There's a second type of black hole, the type of black holes we're studying, supermassive black holes. They get up to 10, 20 billion times the sun's mass. They seem to live at the center of every galaxy. The one at the center of our Milky Way is 4 million times the Sun's mass. But we're targeting the most massive black holes at 10 to 20 billion times the sun's mass Clearly, they have been eating something to grow to that size. One possibility is direct collapse. It skips the cycle of stars. The universe, when it was younger, contained a lot more gas. If you were able to get a cloud of gas that had a mass of 100,000 suns, it collapses under gravity into a black hole. In one shot. At the center of two galaxies, we found supermassive black holes, about 20 billion suns. They are both bigger galaxies. When the galaxies are bigger, the black holes are bigger. And generally speaking, we find bigger galaxies in larger clusters. Clusters of hundreds to thousands of galaxies. They're like people, they live in cities. Like, you know, you look at the Earth at night you see San Francisco, you see New York, but then there's vast regions with very little light. Same thing with the sky. We also see places where there's hardly any galaxies. One would expect to find a giant back hole at the center of a giant galaxy in a spectacular cluster. Like finding a skyscraper in Manhattan. The one we found five years ago is a skyscraper Manhattan. It's in a big cluster. The one we found a few months ago, NGC 1600, also about 20 billion suns, is like finding a skyscraper in a small town in the Midwest. Galaxy cannibalism. Two galaxies come together, merge, making a bigger galaxy. Forming this overwhelmingly large black hole at the center of 1600. By its shape. It's an elliptical galaxy. When you put two disks together, when they merge, they form an elliptical galaxy, the most massive galaxies. So, now we 're pretty sure that you have to have mergers of galaxies. Give them enough time to form bigger and bigger ones and maybe over 13 billion years of cosmic history, perhaps they can grow super massive black holes 20 billion times the Sun's mass.