The most ancient black holes grew fat on cosmic fast food

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In the first billion years after the Big Bang, the universe was a smaller, emptier place, without the huge galaxies that dominate today. But there were already supermassive black holes that had grown shockingly huge by gorging themselves on gas.

Generally speaking, the size of a black hole at the center of a galaxy will be proportional to the size of the galaxy as a whole. This makes sense — after all, a black hole needs a constant source of material to consume in order to grow larger, and a relatively tiny galaxy won't provide much food. That's why it was so shocking when the Sloan Digital Sky Survey (SDSS) detected supermassive black holes that existed just 700 million years after the Big Bang, long before the age of giant galaxies.

Carnegie Mellon physicist Tiziana Di Matteo explains the conundrum that this discovery posed, in a press release:

"The Sloan Digital Sky Survey found supermassive black holes at less than 1 billion years. They were the same size as today's most massive black holes, which are 13.6 billion years old. It was a puzzle. Why do some black holes form so early when it takes the whole age of the universe for others to reach the same mass?"


Today's supermassive black holes are generally the result of galactic collisions, in which smaller black holes were merged together into one larger super-structure. But that explanation doesn't work for these ancient black holes, which predate the earliest galactic collisions and in fact formed at a time when these early, tiny galaxies were far more isolated than their counterparts are today.

The theoretical model and the observational data for these black holes simply didn't line up, which is why the Carnegie Mellon researchers decided to take a new approach. They created an incredibly complex computer simulation called MassiveBlack, which was tasked with recreating the first billion years after the Big Bang. Di Matteo describes the scope of this simulation:

"This simulation is truly gigantic. It's the largest in terms of the level of physics and the actual volume. We did that because we were interested in looking at rare things in the universe, like the first black holes. Because they are so rare, you need to search over a large volume of space."

You can see a part of the simulation in the image on the left, which shows the distribution of gas across the entire volume of space. The image then zooms in three times, each instance increasing the magnification by a factor of ten, to reveal the supermassive quasar on the right side, with huge streams of gas flowing into it - but more on that in a moment.


Of course, even the most colossal simulation can't make these black holes just magically appear — or at least that wouldn't be considered a particularly useful result. MassiveBlack still had to follow the known laws of physics, and that's why what happened next is so cool.

Fellow researcher Rupert Croft explains:

"We didn't put anything crazy in. There's no magic physics, no extra stuff. It's the same physics that forms galaxies in simulations of the later universe," said Croft. "But magically, these early quasars, just as had been observed, appear. We didn't know they were going to show up. It was amazing to measure their masses and go 'Wow! These are the exact right size and show up exactly at the right point in time.' It's a success story for the modern theory of cosmology."


So just how did these ancient black holes get so massive? It's all about the movement of gas. In today's galaxies, cold gas flows towards the central black hole, but en route it slams into other gas. This temporarily heats up and slows down the gas, a process known as shock heating, which slows the rate of black hole growth.

But these ancient black holes weren't surrounded by massive, fully formed galaxies, and so the gas was able to flow directly along filaments, which are the vast thread-like structures that link together different parts of the cosmos. With nothing to slow the gas down, the black holes were able to eat this constant diet of cold, fast food, growing exponentially faster than black holes do today. This new discovery may also help us understand better the formation of the first galaxies, which likely sprang up around these engorged black holes.


Via the Astrophysical Journal Supplemental Series. Top image by NASA/JPL. Simulation image courtesy of Yu Feng.