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Black Holes in the Early Universe

Composite image

This composite image combines the deepest X-ray, optical, and infrared data ever obtained to reveal black holes are common in the early Universe. Yellow circles highlight very young galaxies with aggressively growing black holes. Credit: X-ray: NASA/CXC/UH IfA E. Treister et al.; Optical/Infrared: NASA/STScI/S. Beckwith et al.

Using the most sensitive X-ray image ever taken, IfA astronomer Ezequiel Treister and colleagues have found the first direct evidence that black holes existed when the Universe was less than a tenth of its present age. Between 30 and 100 percent of the 200 distant galaxies they observed contained a central black hole that was voraciously consuming the gas and stars that surrounded them. 

This discovery was made with NASA’s orbiting Chandra X-ray Observatory. By pointing Chandra at a patch of sky for over six weeks, astronomers obtained what is known as the Chandra Deep Field South (CDFS). When combined with very sensitive optical and infrared images from the Hubble Space Telescope, the new Chandra data allowed astronomers to search for the black holes.

“Black holes are objects whose gravity is so strong that not even light can escape from them. Until now, we had no idea what the black holes in these early galaxies were doing—or if they even existed,” said Treister, lead author of the study published in the journal Nature. “Now we know they are there and they are growing like gangbusters.”

“It appears we’ve found a whole new population of baby black holes,” said co-author Kevin Schawinski of Yale University. “We think these babies will grow by a factor of about a hundred or a thousand, eventually becoming like the giant black holes we see today almost 13 billion years later.

A population of very young black holes in the early Universe had been predicted, but not yet observed. Detailed calculations show that the total amount of black hole growth observed by this team is about a hundred times higher than recent estimates.

Because these very young black holes are nearly all enshrouded in thick clouds of gas and dust, optical telescopes frequently cannot detect them.  However, the high energies of X-ray light can penetrate these veils, allowing the black holes inside to be studied.

Two critical issues in black hole physics are how the first supermassive black holes were formed and how they grew. Although evidence for parallel growth of black holes and galaxies has been established at smaller distances, the new Chandra results show that this connection starts earlier than previously thought, perhaps right from the origin of both.

Chandra is capable of detecting extremely faint objects at vast distances, but these black holes are so obscured that relatively few photons can escape and hence they could not be individually detected.  Instead, the team used a technique that relied on Chandra’s ability to very accurately determine the direction from which the X-rays came to add up all the X-ray counts near the positions of distant galaxies and find a statistically significant signal. There are probably many more faint galaxies that Chandra and Hubble cannot detect, so scientists will have to wait for the more-powerful James Webb Space Telescope, now scheduled for launch in 2018, to confirm this.

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