Black Holes Once Stirred a Seething Cosmic Cauldronby
The real work of modern science often starts with pixels on a screen. In one case, in the spring of 2003, a display of pixels on my desk in New York conveyed the message that Chandra, the X-ray telescope orbiting high above the Earth’s surface, had obtained a precious cargo of data.
For the previous couple of days, the finest mirrors and instruments that humans could produce had pointed toward a small patch of the cosmos, close to the constellation of Auriga -- the Charioteer. In this direction we could see all the way to 4C41.17, a mysterious structure deep in the cosmic past.
As I squinted at the noisy spread of pixels, I noticed within it a shape: a pinpoint of X-ray light, along with something else. I sent the image to a printer. Subdued by the heat-fused ink, the noisy features dissipated, and there in stark relief was the extraordinary light of something unknown. It looked like a dragonfly’s wings.
At the center of the image was the bright pinpoint of light, easy enough to identify as the intense X-ray emission from a super-massive black hole. But what was this other mysterious stuff, the wings of light, spanning more than 300,000 light-years?
I pored over articles in astrophysical journals and cautiously brought up the data with other colleagues. A few ideas bounced around. Then I noticed two papers that helped shed light on the problem. To explain them requires first giving a little background in the relevant physics.
Out in the cosmos, the jets of matter squirting from black holes can accelerate particles such as electrons to huge velocities. These particles carry an exceptional amount of kinetic energy, and as their speed increases, they become more massive and more energetic -- like a sponge getting larger as it soaks up water. When a photon happens to bounce or scatter off one of these speedy electrons, it takes up some of the energy. Incredibly, electrons moving fast enough can boost a photon from the microwave domain all the way up to the X-ray and even gamma-ray domain. That’s like taking a cup of coffee and boosting its temperature high enough to drive the steam turbines of a power plant.
This could explain the X-ray photons I was seeing. A super-massive black hole was already thought to live in the system. But what about the low-energy photons? In the present-day universe there are about 410 cosmic-microwave-background photons in every cubic centimeter of space. This was nowhere near enough to account for the huge output in energy I was seeing. I twiddled my thumbs and stared out the window, trying to imagine myself in that distant place, and suddenly the pieces started fitting together.
The explanation hinged on time. The universe was very different 12 billion years ago than it is today. Back then, not yet 2 billion years after the Big Bang, the cosmos was much smaller. Space time was quite literally more compact; the distance between galaxies was less than a quarter of what it is now. This also meant that the cosmic-background photons had not yet been stretched as much as they would be over the next 10 billion years, so they were almost five times more energetic. Here could be the answer! The thick sea of photons would bounce and scatter off the electrons pouring from around a black hole, and would light up the region with X-rays.
This meant something else, too -- something wonderful. What I seemed to be witnessing was a black hole in the center of a young galaxy cluster blowing bubbles just as its descendants do in the present-day universe. Except the bubbles were lit up as they boosted photons into the X-ray band -- a beautiful and elegant manifestation of fundamental physics. Bursting with excitement, I called Ian Smail, at Durham University in northern England, the colleague I was working with. We might not have seen the hot gas of a baby galaxy cluster directly, but we had found the stuff inside that gas. We had found the glowing bubbles driven by the massive heart of a black hole.
Now we wanted to know exactly what was happening in this extraordinary environment. The black hole was squirting out relativistic particles and inflating bubbles within an unseen medium of gas, and this gas was being gathered up in the growing gravity well of the system. But it also had to be getting dense and cool enough to produce all the big and short-lived stars, and it had to be feeding the black hole. We were missing a crucial piece that would show us exactly where the rest of the matter was in this system.
A few weeks went by and serendipity raised its head again. This time it was in the form of a chance meeting with the Dutch-born astronomer Wil van Breugel, from the Lawrence Livermore National Laboratory and the University of California. Van Breugel had access to the two great Keck telescopes that are perched on Mauna Kea in Hawaii. At an altitude of 13,000 feet, these instruments were the perfect tools for capturing visible light from ancient structures.
Van Breugel told us he and his colleagues had captured our object, 4C41.17. It had taken one of the Keck telescopes, with its more than 30-foot-diameter mirror, more than seven hours of exposure time to produce the image, and the view stunned us. There was the hydrogen gas, recovering from some as-yet-unknown buffeting, cooling off by emitting photons of ultraviolet light. In the center were bright clumps and specks, each thousands of light-years across. There was an hourglass shape to the whole thing. It gave the impression of matter both coming and going, flowing inward, but also being propelled outward, a 12 billion-year-old tempest.
Especially intriguing to me was the transfer of energy from the jet-driven particles of the black hole to the cosmic photons in order to produce the glowing X-ray forms. Here was a new mechanism for a black hole to reach out and tweak the cosmic structure. It would happen only in the youthful universe, where space time was still compact enough for the photons to efficiently seed this transfer. It seemed we had found a new way for black holes to sculpt and mold the world around them, to disrupt the formation of new stars and structures.
This evidence indicates that black holes regulate the production of new stars in the giant galaxies of today’s universe, and have likely done so throughout cosmic time. And in the very distant past, they could limit how big those galaxies could ever hope to grow, acting like farmers keeping weeds under control.
While our interpretation seems reasonable, there is always room for future investigation. Regardless, the most remarkable aspect for me was my own shifting sense of what a massive galaxy in the young universe might be like. I had thought that a galaxy would form serenely and majestically from the gentle condensation of material out of the pristine sea of hydrogen, helium and dark matter.
In contrast, 4C41.17 was revealed to be a maelstrom. In a billion or 2 billion years this would indeed be another giant galaxy, almost certainly an elliptical fuzzball of hundreds of billions of stars. But we were witnessing an early stage when it was a pit of seething radiation and particles, driven by the thrashing forces of a growing super-massive black hole.
There is now little doubt that super-massive black holes have played a major, likely dominant, role in sculpting the cosmic forms that we see. Twelve billion years ago, and even earlier, they stemmed the flood of new stars as matter cooled and condensed. And they have continued to hold matter at bay.
Elsewhere, in other galaxies, super-massive black holes also make their presence felt. But in these other places, the interplay between construction and destruction is more complex. We happen to find ourselves living in a very large spiral galaxy, the Milky Way. It’s natural to wonder what influence black holes have had on this place, and what role they might continue to play.
(Caleb Scharf, an astrophysicist, is the director of Columbia University’s Astrobiology Center and author of the blog Life, Unbounded. This is the third of five excerpts from his new book, “Gravity’s Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars and Life in the Cosmos,” which will be published on Aug. 14 by Farrar, Straus and Giroux. The opinions expressed are his own. Read part 1, part 2, part 4 and part 5.)
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