Image: X-ray: NASA/CXC/Penn State/B. Luo et al
An international team of astrophysicists led by researchers in Penn State unveiled the above image at the American Astronomical Society meeting last week, which features the highest concentration of black holes ever seen.
Each of the colored points in the image represent X-ray emissions picked up by NASA’s orbital Chandra X-ray observatory, with the lowest energy X-rays being seen as red whereas the highest energy X-rays are a bright blue. Gas becomes hotter the closer it gets to the black hole’s event horizon (the so-called “point of no return”), producing the bright X-ray emissions observed by Chandra. As Penn astrophysicist Neil Brandt explained to Motherboard, approximately 70 percent of those points of light represent supermassive black holes and the rest are X-ray emissions from objects like stellar mass black holes, neutron stars, and clouds of hot gas.
The entire image covers an area of the sky that is equal to about two-thirds of a full moon. The center of the photo contains the highest concentration of supermassive black holes ever seen.
“I’ve been working on this for about 17 years, but even I was surprised at how many additional supermassive black holes we detected as we looked deeper,” Brandt told Motherboard.
The image is the result of 11 and a half weeks of Chandra staring into the “deep field south,” a window to the universe that is relatively unobscured by the clouds of neutral hydrogen that make up the Milky Way. It is the deepest X-ray image ever taken, allowing astronomers and astrophysicists an unprecedented glimpse at how black holes formed in the early universe. Chandra was basically set to a 7 million second exposure to allow for a highly sensitive reading—indeed, the faintest X-ray sources detected in the image were the result of a single photon reaching the satellite’s detectors about once every ten days.
The Chandra X-Ray Observatory, which launched in July, 1999. Image: NASA
The black holes observed in this photograph have masses that range from 100,000 to 10 billion times the mass of our Sun and some of the X-rays are from black holes in galaxies that are about 12.5 billion light years from Earth. This allows astronomers to explore the formation of black holes just one or two billion years after the Big Bang, offering key insights into a longstanding question in astrophysics: how are black holes able to grow so massive so quickly?
“We know that supermassive black holes existed very early in the history of the universe,” said Brandt. “The question is: how did they come to be?”
According to the researchers, this image and related data suggest that the seeds of early supermassive black holes likely have masses that are 10,000 to 100,000 the mass of the Sun, as opposed to other estimates that theorized that supermassive black holes grew from seeds that were only about 100 times the Sun’s mass. Just what constitutes the “seed” of a black hole is still a matter of debate—some physicists think that supermassive black hole are the result of “light” seeds while others think they resulted from “heavy” seeds.
According to Brandt, light seeds are formed after a massive star collapses, resulting in a black hole that has a mass only about 10 times the mass of our sun. The idea is that these light seeds then began accumulating mass by devouring gas, dust and stars over the course of the universe, eventually growing into a supermassive black hole.
The heavy seed camp, on the other hand, thinks that early supermassive black holes were created when huge gas clouds collapsed, creating a supermassive black hole that has between 1,000 and 100,000 times the mass of our sun and then grew very slowly over time.
“The big questions we're trying to get at with this work is what are the nature of the seeds and what were the exact mechanisms by which these seeds grew to become these supermassive black holes?” said Brandt. “We think we can explain it, but there are so many ways it could've happened that we need to narrow it down.”