In the later universe, supermassive black holes are easy to spot.
It seems that almost every galaxy has a supermassive black hole at its core. Based on the presence of extremely bright objects early in the universe’s history, it seems that this relationship dates back to the very beginning of the galaxy: galaxies appear to have been built around these monstrous black holes.
But this poses a bit of a problem. There is a limit to how fast black holes can grow, and they shouldn’t have reached the supermassive stage so quickly. There have been a few models suggesting how they could grow fast enough, but it’s hard to get data on what’s going on so early in the history of the universe. Now, however, a team is announcing some of the first observational support for a single model: the direct collapse of gas into a black hole without first bothering to form a star.
Most black holes form from the collapse of a star with tens of times the mass of the sun. The resulting black holes eventually become several times as massive as our local star. But supermassive black holes are an entirely different breed, with masses ranging from 100,000 times to a billion times that of the sun.
It is technically possible for a stellar black hole to grow to that size by attracting surrounding matter, but the process takes time. Part of that is just getting so much mass near the black hole in the first place. But black holes are also messy eaters. As material spirals in, it heats up and emits radiation, which can rebound against any further matter that falls into it. This process puts a limit — called the Eddington limit — on how fast material can enter the black hole.
To build a supermassive black hole fast enough, a stellar black hole must be pushing against the Eddington limit almost from the moment it forms. Most researchers consider that unlikely, so they’ve come up with various models (such as repeated black hole mergers) to explain how supermassive black holes form fast enough. But it’s hard to recognize objects in the early universe, let alone understand what they are and what environment surrounds them. So there was no good way to differentiate between these models.
But that may be changing. In recent years, researchers have built a model of one possible explanation for supermassiveness: direct collapse. Instead of building a star, blowing it up and then spiraling material into it, collapsing black holes form directly when a huge cloud of gas collapses under its own weight. Because material doesn’t spiral inward, the gas can avoid the Eddington limit and fall into the black hole in a straight line, encouraging rapid growth.
While this works on paper (or on a supercomputer), no examples of it have been found in the real world. But the international team behind this model also calculated what something like this would look like. While the matter closest to the black hole would emit photons at very high energies, all the surrounding gas would absorb most of these photons and gradually re-emit them at lower energies. By the time we would see them, they would be in the infrared region of the spectrum, with rapidly increasing intensity in the redder region of the spectrum.
This model gave a signal that they could look for. So they looked for objects that fit that description in data from a multi-instrument observation campaign called CANDELS GOODS-S. To find the objects originally associated with a high-energy event, they checked whether the objects could be detected by the Chandra X-ray Observatory.
A pair of sources came through this screen, named Object 29323 and Object 14800. To generate the emission from Object 29323 simply by forming stars, the authors estimate they would need to form stars at a rate 5,000 times that of our Milky Way. That’s twice the speed of the fastest star-forming object we’ve ever observed, a huge, mature starburst galaxy. Since that kind of speed is unlikely to say the least, they argue that these objects are likely black holes that are collapsing on the fly.
To find out if they are right may take a few years of waiting. Closer to launch, the James Webb Space Telescope is designed to image some of the earliest objects in the Universe.
The arXiv. Abstract number: 1603.08522 (About the arXiv). Will be published in Monthly communications from the Royal Astronomical Society.