A letter published in Nature today announces the first observations of developing black holes in the early Universe. The discovery answers a long-standing question in astronomy: how early on were black holes forming, and what were the earliest ones like? The observations required a clever technique which stretched the capabilities of modern instruments.

We have now imaged many galaxies from early on in the history of the Universe—the first billion years or so after the Big Bang—but evidence for black holes from this time has remained elusive.

Astronomers can spot galaxies from this early period using a phenomenon known as "red shift." As space stretches with the expansion of the Universe, electromagnetic radiation traveling through space is stretched as well. This increases the wavelength, so light that was originally blue will shift toward the red end of the spectrum. By looking at key markers in the spectrum, such as those associated with the element hydrogen, scientists can calculate how much red shift has occurred, and thus, how long the light must have traveled.

Although there's evidence that a black hole exists at the center of every galaxy, their tell-tale signatures have not been detected in these early galaxies. There were two possible explanations for this: one was that the signature was too weak to be detected by current instruments due to complicating factors like its absorption by galactic gas. The other was that these "run-of-the-mill" black holes did not arise until later in cosmic history.

Quasars form at very large black holes, and are the most luminous objects in the Universe. They have been found in a number of these early galaxies because they are easy to detect, but have never been observed in the process of growing, leaving some to wonder if they differed from the smaller black holes we see later on. The new observations essentially eliminate the possibility that early black holes were fundamentally different.

To obtain the observations, the team had to find a way to pick out the extremely faint X-ray signals that would indicate the presence of black holes. Using NASA’s Chandra X-ray Observatory and Hubble Space Telescope, they zeroed in on the locations of galaxies with large red shifts, meaning they were looking at the early Universe.

Taken one galaxy at a time, the signal would be practically impossible to pick out from the background noise, but the researchers "stacked" a couple hundred such galaxies. By doing so, they were able to detect a significant signal from those galaxies, even though they may have picked up as few as 5 X-ray photons from any one of them. Even at these low numbers, the X-rays they see indicate that there are, in fact, black holes in the earliest galaxies.

From the relative abundance of short and long wavelengths of X-ray radiation, they also determined that those black holes are obscured by large amounts of gas and dust (a sign of rapid growth). That helps explain why they were so difficult to detect, but it also has other implications. Astronomers think that stars first began to burn around 400 million years after the Big Bang, emitting ultraviolet radiation that reionized (stripping the electrons from) some of the Universe's hydrogen. That was the start of an important cosmological era known as the Epoch of Reionization

The gas and dust present around the observed black holes, however, would have prevented the escape of any ultraviolet radiation that could have contributed to reionization, meaning that black holes had little to do with it. Conversely, that means that information about the Epoch of Reionization cannot be used to constrain the history of black holes, which the researchers claim will mean that several current hypotheses need to be revised.

These new observations can’t say anything definitive about the origins of the earliest black holes, but they support the idea that black holes and galaxies have coevolved, starting very early on in cosmic history. That may help researchers determine whether the first black holes formed from the collapse of discs of gas and dust or from the collapse of early stars, a lower-mass scenario. Regardless, our picture of what the Universe was like nearly 13 billion years in the past just got a little clearer.

Nature, 2011. DOI: 10.1038/nature10103 (About DOIs).

Listing image by Photo by NASA, ESA, and The Hubble Heritage Team (STScI/AURA)