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What Is a Black Hole, and How Are They Formed?

It’s been a big week for black holes. One study this week detected ancient black holes growing vigorously at the centers of their galaxies.  Another found powerful gamma ray flashes from a giant black hole consuming a star.

A good time, we thought, for some basics on black holes. A black hole is an object that has collapsed under its own weight to a point, creating an object that is fantastically small, yet enormously dense.  It sucks in everything it can absorb, and once formed, nothing, not even light, can escape its gravitational pull. 

“They are the most voracious eaters in the universe,” said Kevin Schawinski, a Yale university astrophysicist. “You can only go inside, and you can never come back out.”  

There are two main types of black holes. There are black holes born from the death of stars, which are roughly a few times the mass of our sun. These stars end their lives when the hydrogen fuel that makes up the star’s interior burns off, causing the star to collapse. 

Then, there are supermassive black holes, which range in mass from a few hundred thousand times the mass of our sun to a few billion times that mass, and exist at the center of galaxies.  The black hole that lives at the center of the Milky Way is four million times the mass of our sun.

There are two schools of thought on how these supermassive black holes form. It’s possible that they are seeded from the death of the earliest stars of the universe, which were massively large.

Another, recent theory involves discs of gas that swirl and funnel like a tornado.  The early universe was filled with gas and radiation.  In some spots, gravity caused gas to fall into halos of dark matter and form into gas discs, Priya Natarajan, a theoretical astrophysicist explained.  Instabilities in these discs caused the swirling effect, in which the gas begins to funnel from the outside in.

The funneling can be likened to tornado formation. “The flow is like a tornado vortex,” Natarajan said. “Very rapid, dramatic and violent. And it can happen quickly.”

Scientists this week announced the discovery of a population of hyperactive, baby black holes growing with their host galaxies. They represent the oldest black holes ever found, and are possibly caused by this funneling phenomenon, said Natarajan, also one of the authors of the study.  The black holes date back to 800 million years after the Big Bang — that’s 12.7 billion years ago.

This is extremely young in cosmic time. To put it in perspective, our universe is now 13.7 billion years old — these black holes existed in its infancy.

The black holes are of the supermassive variety, though they haven’t yet reached full mass.  And data indicates that they are closely linked to the formation of their galaxies. 

“We believe that the growth of black holes and the growth of galaxies are symbiotically linked,” said Schawinski, also a study author.  “We’re pushing all the way to the very, very beginning of the universe and asking questions about how this relationship works and how it began.” 

The finding, released this week in the journal, Nature, is based on computer models using data from NASA’s Chandra X-ray Observatory.  Scientists pointed Chandra at the exact patch of sky where the Hubble Space Telescope had spotted baby galaxies, and observed it for 45 days. 

They searched for X-ray emissions as markers to detect active, or feeding, black holes. As matter falls into dark holes, it emits energetic X-rays. Schawinski calls them  “the final death scream.”

It turns out more than 30 percent of distant galaxies contained black holes, but it took years to find them, because they were shrouded in cosmic dust. “It took seven years to detect the first signatures of growing supermassive black holes,” said Ezequiel Treister, the study’s lead author.

This is the first time that we’re pinpointing when these black holes were forming and growing, and “we’re also getting the first clues as to how these black holes grew,” said Mitchell Begelman of the University of Colorado, Boulder, who was not involved in the study, but participated in a June 15 NASA presser on the subject.

Still in question, though, is how these black holes formed.  Was it from massive discs of gas collapsing under their own gravity straight down into a black hole, or was it from the death of the first generation of stars, believed to be much more massive than the typical star?

The hope, scientists say, is to use Hubble and Chandra to push even farther back into the past to answer these questions, and figure out how these things formed.

This finding is not going to make anybody’s life better, but it’s important for human discovery and curiosity, Natarajan said. Cosmologists, she said, are the armchair explorers of our generation.  And it’s fun work, she added.

“I can’t imagine doing anything else,” she said.  “I really can’t imagine doing anything else.”

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