In the early Universe, things were quite different. The first stars were much more massive than stars today, and contained mostly Hydrogen. Astronomers have good ideas about how they formed, but other objects from around this time, namely black holes, are much tougher to account for. Early black holes were huge, with no explanation for how they grew so large. “Early” means “first Billion years after the Big Bang,” but even in that time, it’s hard to determine how observed black holes could grow as large as 100,000 solar masses.
I say 100,000 solar masses, because that is the mass of two ‘seed’ black holes, discovered last week by Italian astronomers working with data from the Chandra X-ray observatory, the Spitzer Space Telescope (SST), and the Hubble Space Telescope (HST).
Supermassive black holes in the cores of galaxies like the Milky Way can weigh millions of times the mass of the Sun, but how did they grow so large? Even with 13.7 Billion years of life in the universe, the problem is finding ways to supply them with enough mass in the first place.
There are two main theories for how black holes formed and grew in the early universe. The first involves the first stars collapsing into black holes that weigh 10 to 100 solar masses. These black holes merge quickly and form a much larger black hole that can feed on gas and dust as it forms a galaxy around it. Mergers and accretion through the life of the universe then allows them to grow to their current proportions. The other theory is that a massive cloud of gas, present in the early universe, rapidly collapses down to a single seed black hole, and then continues to merge and accrete.
If the recent finding is confirmed, it will help piece together this early history of black holes. The new seed black holes give strength to the second theory, that a cloud collapsed into a single massive black hole that could then grow over the life of the universe.
“There is a lot of controversy over which path these black holes take,” said co-author Andrea Ferrara of Scuola Normale Superiore in Pisa, Italy. “Our work suggests we are converging on one answer, where black holes start big and grow at the normal rate, rather than starting small and growing at a very fast rate.”
Follow up observations are still needed to confirm that the discovery is legitimate, that they are in fact these seed black holes and not a false positive in their analysis. But it does remind us that black holes are difficult to understand in throughout the universe’s history, and not just at the present time. They are the most extreme objects in the universe, and the more we learn about them, the more we realize just how important they are to a foundation of understanding the universe we are a part of.