A Black Hole Is Eating a Web of Galaxies on the Edge of Spacetime

It has taken virtually the entire lifespan of the universe for the light from the 'Web of the Giant' to reach Earth.
October 1, 2020, 1:00pm
​Image: ESO/L. Calç​ada
Image: ESO/L. Calçada

Scientists have peered back in time to the early universe and witnessed an extremely rare sight: A sextet of galaxies entangled in a cosmic web around a young supermassive black hole.

The distant vision sheds light on longstanding questions about how supermassive black holes bulked up so rapidly after the universe was born. It may be that tight-knit groups of galaxies, known as “galaxy overdensities,” provided a steady food source that fueled the impressive growth spurts of these objects.

At least six galaxies, located more than 12 billion light years away, are caught in the “web of the giant” as it is described in a study published on Thursday in Astronomy & Astrophysics. The new observations offer the “first spectroscopic identification of a galaxy overdensity around a supermassive black hole in the first billion years of the universe,” the team added.

“The importance of our work is that we have been the first group to discover these faint galaxies that inhabit the web” because they are “extremely difficult to observe and identify,” said Marco Mignoli, an astronomer at the National Institute for Astrophysics in Bologna, Italy who led the research, in an email.

“But many research teams are pursuing the same ‘hunt,’ and it is reasonable to think that in the next few years, many similar systems will be discovered and studied,” he continued.

Supermassive black holes are unimaginably dense objects that can grow to be billions of times as massive as the Sun. In the modern universe, they are commonly found at the center of large galaxies, including the Milky Way, which contains a black hole that clocks in at four million times the mass of the Sun.

These bizarre objects grow by feeding off any matter that happens to fall into them—be it stars, gas, dust, or, perhaps, unfortunate alien civilizations. But it takes time for a black hole to get swole.

The notion that supermassive black holes had already formed by the time the universe had reached its one billionth birthday “presents a severe challenge for extragalactic astronomy,” according to the new study.

“There is so much we don't know about the formation and evolution of supermassive black holes!” said Mignoli. It’s weird enough for supermassive black holes to exist in our modern universe, he added, but to see them emerge in the first billion years after the Big Bang is “even more difficult to explain” because “the time to grow is extremely limited.”

For eight years, Mignoli and his colleagues have been searching for missing pieces of this puzzle with some of the most sophisticated telescopes on Earth, including the Large Binocular Telescope in Arizona, the Keck Observatory in Hawaii, and the Very Large Telescope (VLT) in Chile.

“It is difficult to identify a precise moment when we had the feeling that we had been successful,” he said. Instead, he described it as “a roller coaster of emotions” complete with a disappointing snowy night in Hawaii that thwarted their Keck observations, as well as an exhilarating single night at the VLT that snagged “the confirmation that three faint galaxies were in the supermassive black hole’s neighborhood.”

That neighborhood is so far away that it has taken virtually the entire lifespan of the universe for its light to reach Earth. As a consequence, we see the structure as it was about 900,000 million years after the Big Bang.

Despite its young age, the supermassive black hole in the system has already gobbled up a billion solar masses. The galaxies around the black hole are packed into a space about 300 times bigger than the Milky Way. That may sound big to puny humans like us, but is a snug fit for a collection of galactic structures.

Galactic runoff is likely trickling toward the black hole through a structure called the cosmic web, providing it with plenty of matter to gorge on. Scientists think that this web, which is made of a mysterious substance called dark matter, scaffolds the universe. Dark matter doesn’t emit light, rendering it invisible to telescopes, but its gravitational influence on nearby objects is very noticeable.

The observations from Mignoli’s team reveal that some supermassive black holes sprout so fast because they are embedded in immense halos of dark matter.

“Now, we have at least observationally confirmed what the theories predicted,” said Mignoli. “These supermassive black holes, regardless of how they initially formed, must grow in special places of the universe, rich in gas and galaxies, from which it is possible to obtain the material to grow so prodigiously.”

While it’s challenging to observe objects that are so far from Earth, new observational techniques and facilities will enable scientists to gaze at the edge of spacetime in much sharper detail. Watching the universe take shape in this byone age will help us understand the evolution of supermassive black holes and the galaxies they occupy, like our own Milky Way.

“We can reasonably assume that the galaxies we have discovered so far are the brightest of those trapped in the web,” concluded Mignoli. “There are many more faint galaxies that have not yet been identified. We are planning to obtain deeper images in order to identify these new galaxies, which can only be observed with future telescopes, such as the Extremely Large Telescope and James Webb Space Telescope.”