Quasars are the brightest objects in the Universe, spewing prodigious amounts of light and relativistic particles out of jets powered by supermassive black holes at the centers of galaxies. Because of their power output, they're some of the most distant (and therefore oldest) bodies yet detected. And that creates a problem. All our models of early galaxy formation suggest that it would take a long time to collect enough mass to form one of these black holes, and yet they appear to be present very early in the Universe's history. A paper that will be released by Nature today provides a possible explanation for the rapid appearance of these black holes: galaxy collisions created unstable gas clouds that formed black holes directly, without the need for star formation.

Normally, black holes form through the collapse of stars that are significantly more massive than the sun. Even with the massive, unstable stars that are thought to have formed early in the Universe's history, however, this process takes hundreds of thousands of years. Both star formation and the production of black holes also create prodigious amounts of radiation, which tends to drive away further gas. Thus, even though it's possible to form black holes on a tight timeline, it's hard to feed them enough additional matter to turn them into supermassive ones. And, even in cases where gas is funneled into galactic centers, simulations suggest that most of the gas would end up forming stars long before it got to the central black hole.

There were a very few specific scenarios that could produce appropriately sized black holes at galactic centers, but the conditions required have been considered pretty unlikely, leading researchers to continue looking for a mechanism that might trigger their formation.

But, as the authors of the new paper note, most of these simulations simply treated the galactic core as existing in isolation. Currently, however, the consensus is that most current galaxies are the product of collisions and mergers of the first, primordial galaxies. So, realistically, the earliest black holes probably formed against this more violent backdrop, in which tidal forces and shockwaves can shift massive amounts of gas around, potentially feeding a black hole's growth.

It's not that nobody thought to make a more realistic model; it's simply that doing so was too challenging. "Addressing this issue," the authors state, "requires a three-dimensional simulation following gas dynamics across an unprecedented range of spatial scales, from tens of kiloparsecs to below a parsec, which has been computationally too demanding until now."

Now, however, the authors could send two disk-shaped galaxies spinning into each other, and computers can keep up. The galaxies were given typical metal contents and dark matter halos, and hydrodynamic equations are used to simulate the collision, cooling, and star formation. Once the cores merge, the resolution of the simulation was increased to provide more details on what happens in the core.

Initially, the merged cores produce a disk of gas with 2 billion solar masses, spread across about 80 parsecs (about 260 light years). The disk, however, is gravitationally unstable, and quickly forms two spiral arms that transfer mass inward to the center of the disk, while dissipating angular momentum out. In less than 10,000 years, over 108 solar masses are at the center of the disk, and more is streaming in at the rate of over 104 solar masses a year. As matter streams in, this core collapses into a distance smaller than the ones used for the simulation.

It's hard to tell precisely what happens at this point, but the authors suggest two possibilities: either a black hole forms at the very core and grows rapidly (consistent with the steep density profiles seen in the simulation), or a dense "quasi-star" forms that quickly collapses into a black hole. In either case, it's all over in less than 100,000 years following the merger, well within the rapid schedule suggested by our observations, and fast enough to be over before much star formation could take place.

There's no way to test this model at the moment, but the authors suggest we may not have long to wait. The Laser Interferometer Space Antenna should be able to search for gravity waves from these events, and that instrument has been made a major priority for the US space program.

Nature, 2010. DOI: 10.1038/nature09294 (About DOIs).

Listing image by ESO/L. Calcada