We’ve all heard of supernovas, those events in which a star explodes and briefly blasts out more energy than an entire galaxy. That involves just one star. A very different kind of violence is happening in some galaxies. It involves the black hole that lives at the center of each galaxy. It’s called the galactic nucleus. In about 10 percent of galaxies, the nucleus is on a rampage, pulling in huge regions of the solar system orbiting it. The rest are like our Milky Way with its well-behaved black hole resting quietly, its gravitational force peacefully holding together the hundreds of billions of stars in its orbit. That includes, of course, our favorite star, the sun.
But here’s the surprising part: Based on a complex body of research that I won’t go into here, astrophysicists have seen evidence that the rampaging black holes, which they call active galactic nuclei, or AGNs, turn themselves on and off over intervals of time ranging from 10,000 years to 10 million years. During the few decades since astronomers discovered evidence of this, however, they had never actually seen a black hole emerge from its slumber and begin feeding on nearby matter. Now they have done so.
For the first time, scientists believe they have witnessed the birth of an AGN, and it happened while they were watching. Or, at least, while their telescopes were watching and recording the pictures. When researchers saw hints of this, they went back into the archive of images, and found the "before" images to compare with current "after" pictures.
After lining up the images, what scientists saw was the quiet heart of a distant galaxy, 300 million light years away, explode into action. It began pulling in vast quantities of gas and dust that had been quietly orbiting the black hole. As these particles of matter fall into the black hole, they move so fast and rub together with so much friction that they turn white hot and even hotter, emitting the full spectrum of electromagnetic energy. For a few years, these monsters become the brightest steady light source in the universe. Once the particles cross the "event horizon," the bits of matter disappear as they add their mass to that of the black hole. The hole itself is, of course, black, emitting no light. It is an infinitesimal point containing the mass (and gravitational force) of everything that ever fell into it.
The first clue to this event came from a telescope called the Zwicky Transient Facility at the Palomar Observatory near San Diego, California. (Fritz Zwicky was an astronomer at nearby Cal Tech who first suggested the existence of dark matter.) This is a conventional telescope built in 1948 but recently modified to produce instant digital images. The new upgrades, including computer-controlled pointing, allow it to sweep and swivel to see and photograph the entire northern hemisphere night sky every night. Computers compare each night’s output with the previous night’s to see if anything has changed.
Then the researchers cross matched those findings with a catalog of 2.4 million quiet galaxies maintained by the Sloan Digital Sky Survey. That’s a huge program involving some 300 scientists at 25 universities around the world who combine their data into the world’s biggest and most detailed map of the universe. The survey found one that showed the biggest change from its first sighting in 2003 and the group’s new observation in 2022. The change suggested it was a rapidly developing AGN.
"We could not see the activation of the black hole in real time," team member Paula Sánchez Sáez of the European Southern Observatory told the journal Science. "We just know that the galaxy was not active 18 years ago, and now it is active."
But that’s still not capturing the beginning of the process.
Then the researchers noticed that in 2019 the Zwicky computers had spotted one particular galactic nucleus, some 300 million light years away, that showed the first hint of brightening. That was one catalogued as SDSS1335+0728. The first follow-up observations didn’t show much change, but in 2022, it could be seen brightening suddenly and spectacularly. It showed all the signs of becoming an AGN. They estimated its mass at roughly one million times greater than that of our sun.
But had the black hole actually been totally dormant before researchers saw those early signs of brightening? That question sent them deeper into the astronomy archives, looking for even older pictures to see if they showed this galaxy as normal, lacking an active nucleus. Sure enough, they found their galaxy, and it looked as normal as the Milky Way. They were able to gather data on it in all the usual electromagnetic spectra from x-rays and ultraviolet to optical and infrared.
That’s something like having a sick adult human with totally mysterious symptoms but also having access to all that person’s lab records going back to prenatal time. With that data, it might be possible to trace findings and phenomena over the course of a lifetime.
Philip Best, an astronomer at the University of Edinburgh, told Science that it was only a matter of time before researchers saw an AGN in its formative years.
"It’s interesting that we are now beginning to identify these objects," says Best, who was not involved in this research. He says, "studying how the nature of the active nucleus changes across its lifetime can help to reveal the physical processes that lead to the fueling of the AGN."
With that new knowledge, he says, it might be possible to say whether the black hole at the center of our own Milky Way might one day rise up and begin swallowing its galaxy.
If you made it this far in the column, you may be an astronomy nerd. If so, you would enjoy NASA’s Astronomy Picture of the Day. It’s at apod.nasa.gov. Have a look.
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