Black holes are… er, black. The point of a black hole is that gravity is strong enough to prevent light from escaping its grasp. But matter sucked into a black hole is not at all happy with its fate. The matter gets hot and disturbs and starts to glow very brightly before it reaches the black hole. This creates so-called glowing black holes.
Most black holes are proud of themselves and suck matter in front of our eyes. But others are shy and seem to hide their antisocial behavior, which raises the question of whether they were really there. It turns out that these murderous monsters are hiding behind the gas clouds created by collisions with galaxies. It took a lot of detective work to penetrate the fog.
Introduction of the eyewitnesses
Astronomers have long known that not everything in the universe happens slowly. Sure, our sun will be stable for billions of years, but when things go wrong, it quickly goes downhill (use your remaining eight minutes wisely). Likewise, when something huge is sucked into a black hole, it sends out a last-ditch SOS in the form of a bright X-ray flash.
The Burst Alert Telescope (BAT) monitors these events and captures the flash faithfully. However, BAT is not very good at seeing details. For that you need something bigger, like one of the Keck telescopes in Hawaii.
The researchers used Keck to watch these outbursts, hoping to spot supermassive black holes munching on matter. Unfortunately, they found only a giant cloud of interstellar gas, obscuring both the black hole and the environment that may have led to its creation. Image quality was further degraded by the Earth’s atmosphere. The swirling air above Keck blurs the details already obscured by the gas.
However, the Keck has adaptive optics. Essentially, scientists can measure the atmosphere’s effect on light from distant objects and maneuver parts of the telescope’s optics to compensate for some of the atmospheric blur. However, this only works for a small field of view, so you only get details in the center of the image. The bit that is clear has a resolution about 10 times better than the uncorrected image, revealing some hidden details.
Even using adaptive optics, most galaxies were too far away to reveal anything. However, a few were close enough to reveal a secret. The galaxies had two bright centers of gravity, meaning the “system” was actually two galaxies colliding. These weren’t small galaxies falling into large galaxies either – we’re talking about equally sized, gaseous behemoths colliding vigorously with each other.
To verify their data, the researchers turned to the archive of the Hubble Space Telescope. Hubble, while smaller than Keck, has both a large field of view and very good resolution. They examined photos of the same galaxies and similarly found evidence of two centers of mass glowing brightly.
Officer, I accidentally ate that galaxy
Even with this evidence, the researchers weren’t sure. Maybe this was just an accident – we just happened to have a few additional mergers around. Perhaps it is not necessary to go through a traumatic merger to generate a hidden, murderous black hole.
To find out, the researchers examined two sets of nearby galaxies. One set had active nuclei (selected by BBT-detecting X-ray flashes), while the other set had no active nuclei. They found that 17 percent of active core galaxies had a recent merger in their history and a hidden black hole surrounded by the dying glow of its victims. In contrast, only one percent of the stationary galaxies had black holes obscured.
For additional confirmation, the researchers modeled galaxy fusions. They found that supermassive black holes are likely produced when merging galaxies are of similar size and have a lot of gas floating around. The calculated black holes also had accretion disks that glowed brightly.
To determine whether the black hole would be eclipsed, the researchers used the merged galaxy’s matter distribution and temperature to calculate the galaxy’s light emission. From this, the researchers calculated what the results would look like if they had been imaged by Hubble. The results resembled images of actual observations.
So, if you’re in charge of a galaxy and don’t want it to be ravaged by hidden voracious supermassive black holes, avoid colliding with neighboring galaxies.
Nature2018, DOI: 10.1038/s41586-018-0652-7 (About DOIs).