Technology & Science

Most distant black holes ever found reveal secrets

Black holes further away from Earth than any ever found before have given astronomers a peek at how galaxies were evolving 13 billion years ago — less than a billion years after the Big Bang.
The new study used images from the Chandra X-Ray Observatory, a space telescope that orbits the Earth, to measure the growth rates of the black holes at the centre of galaxies that existed as early as 700 million years after the Big Bang, such as the black hole shown in this artist's drawing. (A. Hobart/CXC/NASA)
Black holes further away from Earth than any ever found before have given astronomers a peek at how galaxies were evolving 13 billion years ago — less than a billion years after the Big Bang.

"We're really looking at first baby growth spurts of the black holes that live at the centre of galaxies today, including our own Milky Way," said Kevin Schawinski, one of the five co-authors of the study published Wednesday in Nature.

Schawinski, an astrophysicist at Yale University, noted that the Milky Way's black hole was formed at about the same time as the black holes observed in the study, and likely went through a very similar infancy.

The Big Bang

The "Big Bang" is the name given to the time around 13.7 billion years ago when the universe began expanding from a single point in space. It continues to expand to this day. The most obvious evidence is the fact that galaxies are moving away from us, and those farther away are moving away more quickly – a phenomenon noticed by U.S. astronomer Edwin Hubble in 1929.

The study was led by Ezequiel Treister, an astronomer at the University of Hawaii and the Universidad de Concepcion. It showed that black holes in the early universe grew in tandem with the galaxies that surround them, just as black holes at the centre of all galaxies — including our own Milky Way — do today.

Co-author Priyamvada Natarajan, a Yale University cosmologist, said the observations show that extremely massive black holes already existed as early as 700 to 800 million years after the Big Bang —"which suggests that either they were born massive to start with, or they experienced rapid growth bursts," she added in a statement.

Schawinski said at a NASA news conference that it also renders another important question irrelevant: "This chicken-and-egg problem of what was there first — the galaxy or the black hole — has been pushed all the way to the edge of the universe."

The study also showed that these early black holes were shrouded in thick clouds of dust and gas from their galaxies, suggesting they were not responsible for a major event in the history of the universe that ended a period called the Dark Ages.

When astronomers look at very distant objects, they are observing light that has travelled vast distances at the speed of light. In some cases, the light left the star or galaxy that was its source billions of years ago. When astronomers turn their telescopes on that source, they are looking far back in time.

Astronomers know particularly little about the first billion years of the 13.7-billion-year-old universe, when the first stars and galaxies formed. Before the study published Wednesday, the only black holes detected from that period were massive quasars, which are nearly fully grown with masses a billion times that of the sun.

The new study used images from the Chandra X-Ray Observatory, a space telescope that orbits the Earth, to measure the growth rates of the black holes at the centre of galaxies that existed as early as 700 million years after the Big Bang.

The researchers painstakingly processed images of 250 galaxies using the Chandra X-Ray Observatory. They used a technique called "stacking," in which multiple images are piled on top of each other, to multiply very weak X-ray signals emitted by the black holes as they devoured gas and dust nearby. Black holes were detected in at least 30 per cent of the galaxies. Only the light with the very highest energy could be detected, suggesting that the rest was swallowed up nearby by dust.

Lighting up the Dark Ages

That explains why the distant black holes have been so hard to see up until now, and also answers a question that had puzzled astrophysicists for some time — whether black holes or stars ended a period in the early universe called the Dark Ages.

That was a time when the universe was opaque and therefore invisible. It ended with an event called reionization that began about 500 million years after the Big Bang and stripped electrons from the hydrogen atoms in the early universe, gradually restoring the universe's transparency over the next 500 million years.

Astrophysicists thought black holes might have existed at the time of reionization and that their light may have caused the process to occur.

"What we now can say is that the first black holes — yes, they were there and they were putting out lots of light, but it was completely eaten away by the gas and dust," Schawinski said. "They couldn't have reionized the universe. It must have been something else —  most likely the first stars."

Galaxies so far away that they are viewed 950 million years after the Big Bang or less are circled in this image from the Chandra X-Ray Observatory. The inset shows X-rays detected in some of the galaxies. (E.Treister/U.Hawaii/NASA; G.Illingworth/UC Santa Cruz/NASA;S.Beckwith/STScI/NASA)