Black holes are… um, black. The point of a black hole is that the force of gravity is strong enough to prevent light from escaping its grasp. But the matter that is being sucked into a black hole is not at all happy about its fate. The matter gets hot and bothered and starts to glow very brightly before it reaches the black hole. This produces what are called luminous accreting black holes.

Most black holes are proud of themselves, sucking down matter right before our very eyes. But others are shy and seem to hide their antisocial behavior, raising questions about whether they were actually there. It turns out that these murderous monsters are hiding behind the gas clouds created by galaxy collisions. It took a serious amount of detective work to penetrate the fog.

Introducing the eyewitnesses

Astronomers have long recognized that not everything in the Universe happens slowly. Sure, our Sun will be stable for billions of years, but when things start to go wrong, they go downhill quickly (use your remaining eight minutes wisely). Likewise, when something big gets sucked into a black hole, it sends a last desperate SOS in the form of a bright X-ray flash.

The Burst Alert Telescope (BAT) keeps an eye out for these events and faithfully records the flash. However, BAT isn’t very good at seeing details. For that you need something bigger, like one of the Keck telescopes in Hawaii.

The researchers used Keck to look at these bursts, hoping to view supermassive black holes munching on matter. Unfortunately, all they found was a gigantic cloud of interstellar gas, obscuring both the black hole and the environment that may have led to its creation. The image quality was made even worse by the Earth’s atmosphere. The roiling air above Keck blurs the details that are already obscured by the gas.

The Keck, however, has adaptive optics. Essentially, scientists can measure the effect of the atmosphere on light from distant objects and maneuver parts of the telescope’s optics to compensate for some of the atmospheric blurring. This only works for a tiny field of view, though, so you only get detail right in the center of the image. The bit that is clear has a resolution that is about 10 times better than the uncorrected image, revealing some hidden details.

Even with the help of 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 mass, meaning that the "galaxy" was actually two galaxies that had collided. These weren’t little galaxies falling into big galaxies, either—we are talking equally sized, gassy behemoths vigorously smashing into each other.

To check their data, the researchers turned to the Hubble Space Telescope’s archive. Hubble, although smaller than Keck, has both a large field of view and very good resolution. They examined pictures of the same galaxies and, similarly, found evidence of two centers of mass glowing brightly.

Officer, I accidentally ate that star system

Even with this evidence, the researchers were not sure. Maybe this was just an accident—we could have a few extra mergers nearby by chance. Maybe it isn’t 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 BAT-detecting X-ray flashes), while the other set did not have active nuclei. They found that 17 percent of the active nuclei galaxies had a recent merger in their history and a hidden black hole surrounded by the dying glow of its victims. By contrast, only one percent of quiescent galaxies had obscured black holes.

For additional confirmation, the researchers modeled galaxy mergers. They found that supermassive black holes are likely to be produced when galaxies that merge are similarly sized and have a lot of gas floating around. The calculated black holes also had accretion disks that glowed brightly.

To determine if the black hole would be obscured, the researchers used the merged galaxy’s matter distribution and temperature to calculate the light emission from the galaxy. From that, the researchers calculated what the results would look like if it had been imaged by Hubble. The results looked similar to images from actual observations.

So, if you are in charge of a galaxy and don’t want it to be infested with hidden ravenous supermassive black holes, avoid collisions with neighboring galaxies.

Nature, 2018, DOI: 10.1038/s41586-018-0652-7 (About DOIs).