As astronomers were reading the first chapters of the universe’s history, they discovered a group of supermassive black holes that appear to have matured much faster than scientists expected.
Priyamvada Natarajan is something of a cosmic biologist. She studies the lives of giant black holes, objects so dense they trap all matter and light in their grasp. As an astronomy graduate student, Natarajan was one of the first to look at black holes as populations rather than individual objects, and studied their general classification and evolution as if they were bats in a rainforest. Now an astrophysicist at Yale University, Natarajan continues to study the behavior of these beasts, and has turned her focus to understanding how they are born.
Traditionally, black holes are thought to form after a massive stellar explosion, and grow in mass as they feed on nearby gas reserves. But some observations of supermassive black holes in the very early universe have suggested there is more to the picture. In 2006, Natarajan and his colleagues proposed a revolutionary new explanation for how disks of gas could collapse directly into unusually massive baby black holes without any stars forming. Last year, a joint observation by the James Webb Space Telescope (JWST) and the Chandra X-ray Observatory spotted a distant, luminous black hole that finally appears to verify Natarajan’s prediction.
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“This is definitely a very strong case in favour of these massive black hole seeds,” says astronomer Raffaella Schneider of Sapienza University of Rome. “(Natarajan) proposing this idea has really helped the community to broaden our view of the different possibilities.”
Natarajan spoke scientific American about how recent observations support his proposal for “direct-contraction black holes” and what they tell us about the ancestry of these creatures.
,An edited transcript of the interview follows.,
How did you become interested in studying black holes and their origins?
I have always been fascinated by the invisible elements in the universe. My work has mainly been about trying to understand at a fundamental level these dark components of the universe – dark matter and dark energy, as well as the nature of black holes. I find these objects incredibly fascinating and mysterious. They are a reminder of the limits of our knowledge, the places where the known laws of physics break down.
Over the past few decades, black holes have gone from being a purely mathematical concept to real objects we can observe, and they have now become a focal point in our understanding of how galaxies form. The universe is filled with black holes of all sizes. They are a significant part of our cosmic inventory, so understanding how they form is a fundamental open question.
What don’t we know about black hole formation?
Typically, black holes are born when stars die. When the most massive stars undergo gravitational collapse, the little piece they leave behind is a black hole. This is a very clearly established origin story.
The most distant black hole ever detected in X-rays is located in the galaxy UHZ1, imaged by NASA’s Chandra X-ray Observatory (Purple) and infrared data from NASA’s James Webb Space Telescope (Red, Green, Blue,
X-ray: NASA/CXC/SAO/Ákos Bogdán; Infrared: NASA/ESA/CSA/STScI; Image processing: NASA/CXC/SAO/L. Frattare & K. Arcand
But about two decades ago, when we started looking farther and farther into the universe with missions like the Sloan Digital Sky Survey, we found very massive black holes — with masses about a billion times the mass of the Sun — when the universe was only one to two billion years old. Given the rate at which we know black holes like to feed, there simply wasn’t enough time for the tiny seeds from the explosions of the first stars to grow into these giant black holes. Over the next few years, we started to see that these weren’t just a few strange objects; there was actually a whole population of supermassive black holes in the very early universe. And that’s where the puzzle began.
Some people started to explore whether there might be ways for black holes to get food much faster than the known limit. Theoretically there is, but we haven’t found any solid observational indications of this yet. So I started thinking, what if we just started with bigger seeds? My team and I realized that if a gas disk is radiated by stars from a nearby galaxy, it could bypass the star-formation process and collapse directly into a black hole. This direct-collapse black hole would be very large at birth – 1,000 to 100,000 times the mass of the Sun. That black hole could then merge with a nearby galaxy and easily grow to the size we see.
How was this proposal received by the community?
We had a lot of people who were opposed to this. They said, “The physics is beautiful, and it makes sense, but is this process efficient enough to actually happen in the universe?” At that time, these eras of the universe were not accessible for observation. To see these early seeds forming, we have to look at the first billion years after the formation of the universe.
That’s why the promise of the JWST was so tempting; it inspired us to keep working on it. We started thinking about what signals we could look for as evidence of a direct-collapse black hole, and we came up with an idea. In nearby galaxies, the mass of all the stars is often 1,000 to 10,000 times the mass of the central black hole. But in these direct-collapse scenarios, for a short time, the mass of the black hole can actually equal the mass of the stars. That means you should see an extremely bright, actively accreting black hole that essentially outshines all the stars in the galaxy. If we were able to see one of these galaxies in both X-ray and infrared light, we might see distinct signals of a supermassive black hole at its center.
However, even with JWST and Chandra, we can’t see far enough to directly see the formation of early black hole seeds. But I realized that if nature was kind to us, one of these galaxies could be hiding behind a magnifying glass: a galaxy cluster rich in dark matter that acts as a dramatic gravitational lens. I was working on mapping some of these gravitational lenses with the Hubble Space Telescope, and I suggested we focus our new telescope on this very complex cluster called Abell 2744. I knew every part of that dark matter map inside and out. I was hopeful, but it was really like shooting in the dark.
And how did you benefit from it?
Early last year I got a call from my colleague, astrophysicist Akos Bogdan, who had been watching the Chandra observations of galaxies behind the Abell 2744 lens. He said, “Are you sitting down? I think we’ve found something.” By chance, the spectrum of one galaxy incredibly matched the prediction plot we had made for a hypothetical detection in 2017. It was astonishing. It checked out every predicted property. This is very compelling evidence that direct-collapse black holes formed in the early universe. This is no longer just a speculation.
Now, there may be other ways to seed a black hole. That’s what I’m working on next: trying to find other pathways and what their unique observational signatures might be. It opens up a whole Pandora’s box of exciting questions.
I can imagine. How did you feel when you finally found evidence for your idea in nature?
This is what I find so exciting about being an astrophysicist—I want to see theoretical ideas encounter observational data. We are in this amazing time in history where you can make a prediction and have it validated or invalidated in your lifetime. This is why people say we are living in the golden age of cosmology. I am so grateful.
