JWST SEES STRANGE BLACK HOLES IN THE EARLY UNIVERSE
Astronomers are winding back the clock to discover how the first black holes formed.
Since its launch, astronomers have been using the James Webb Space Telescope (JWST) to peer back to a time when the first stars and galaxies were forming. Not only is JWST showing us that early galaxies are different than anticipated, it’s also revealing that black holes in the early universe are behaving unexpectedly as well.
YOUNG HEAVYWEIGHTS “In the local universe there is a supermassive black hole at the center of every massive galaxy,” Fabio Pacucci, a Center for Astrophysics | Harvard & Smithsonian researcher, says. These black holes are millions to billions of times the mass of the Sun. “And there are pretty tight correlations between the mass of the black hole and many properties of the host galaxy.”
But according to Pacucci and colleagues’ research, published in The Astrophysical Journal Letters and presented Jan. 9 at the winter meeting of the American Astronomical Society, that correlation seems to break down at early times.
Typically, a supermassive black hole now is about 0.1 percent the total mass of its host galaxy. But galaxies early in the universe’s history, such as those in the study between 700 million and 1.5 billion years after the Big Bang, have black holes often between 1 percent and 10 percent the mass of the galaxy.
While there had been hints
of such a trend, Pacucci’s study looks at as many high-redshift galaxies as possible from JWST. And “every single supermassive black hole studied by JWST up to that point was very over-massive with respect to stellar mass of the galaxies,” he says.
STARTING LARGE
Some theories posit that early supermassive black holes formed from mergers of smaller black hole “seeds” left behind when the first stars exploded. But in that case, astronomers aren’t sure how to explain the appearance of supermassive black holes in the early universe, when there wasn’t yet time to build them up.
A theory called heavy seeding attempts to solve this. In the early universe, there could have been gas clouds so huge and dense they couldn’t form stars, instead collapsing right into black holes 10,000 times the mass of the Sun or more. In this way, heavy seeding creates small supermassive black holes from the outset, no mergers required.
A galaxy with such a black hole in the center would have found star formation difficult, if not impossible — which makes heavy seeding an even more likely culprit for the galaxies Pacucci’s team saw. “The over-massive black hole is really injecting a lot of energy into the system,” Pacucci says. “It’s heating up the gas. And once the gas is heated up, it’s very hard to form stars.” He says it likely wasn’t until about 2 billion or 3 billion years after the Big Bang that the material outside the black hole started to take on a larger percentage of the galaxy’s mass.
Subsequent work could help piece together the steps between massive seeds and more massive galaxies, as well as whether the find is truly representative of the conditions of the early universe.
HUNGRY BLACK HOLES
In a paper published Jan. 17 in Nature, another group seeking to understand how the first black holes formed used JWST to identify the oldest known supermassive black hole just 400 million years after the Big Bang. It is a few million times the mass of the Sun and lies at the center of the galaxy GN-z11.
Normally, such a large black hole should have taken a billion years to form. Instead, the team says, it’s gotten so large by eating away at its surroundings much faster than predicted.
Black holes grow by pulling in material like dust and gas. The infalling matter creates a glowing accretion disk. But there is a theoretical limit to how fast a black hole can eat, called the Eddington rate. If it tries to eat faster, the accretion disk heats up and radiates so powerfully that it blows itself away, escaping as a wind of energetic particles and limiting the black hole’s growth.
But astronomers have observed black holes occasionally exceeding the Eddington rate without losing their disks. This seems to be happening within GN-z11: Its black hole is eating at five times the Eddington rate. So, GN-z11 seems to show that supermassive black holes can go through short episodes of super-Eddington accretion before calming down again.
The team can’t determine whether GN-z11’s black hole started as a lower-mass black hole or a heavy seed created through direct collapse. But each new black hole that JWST finds helps researchers to piece together the puzzle of how supermassive black holes form.