Black holes are famous for their enormous appetites, pulling in gas, dust, and even light itself.
But new research shows that even the largest black holes in the universe can be surprisingly picky about when and how they eat.
Using the powerful Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have discovered that supermassive black holes do not always take advantage of the vast amounts of gas delivered to them during galaxy mergers.
The study, published in The Astrophysical Journal, was led by Makoto A. Johnstone, a doctoral researcher at the University of Virginia, and involved an international team of astronomers.
The researchers observed seven nearby pairs of merging galaxies, each hosting two supermassive black holes separated by just a few thousand light-years.
These systems offer a rare opportunity to watch black holes during one of the most dramatic events in galaxy evolution.
When galaxies collide, gravity stirs up enormous flows of cold molecular gas, funneling it toward their centers.
This gas is the main fuel that allows supermassive black holes to grow.
In theory, mergers should trigger intense feeding frenzies, causing both black holes to light up as active galactic nuclei, among the brightest objects in the universe. In reality, astronomers often see only one active black hole—or sometimes none at all.
ALMA’s observations revealed why this puzzle exists. Around many of the black holes, especially the more massive ones, the team found dense, chaotic clouds of gas. This shows that galaxy mergers are very effective at delivering fuel to black holes.
However, the brightness of the black holes, which reflects how fast they are feeding, did not increase with the amount of nearby gas. In other words, even when food is plentiful, most black holes are eating only small amounts.
This suggests that black hole growth during mergers is inefficient and uneven. Rather than steadily consuming gas, black holes seem to feed in short, unpredictable bursts. As a result, astronomers may often catch them during quiet periods between meals, making active black holes harder to detect.
The researchers compared mergers where both black holes were clearly active with systems where only one showed signs of feeding. In some cases, the inactive black hole truly lacked nearby gas. In others, gas was present, but the black hole still appeared dormant, possibly because it was temporarily “off” despite having fuel available.
ALMA also revealed that many active black holes are slightly offset from the centers of their surrounding gas disks.
This displacement may be caused by the violent gravitational forces at work during galaxy mergers, which can knock black holes away from their usual positions and disrupt their feeding process.
Together, these findings show that having plenty of fuel is not enough to guarantee black hole growth. Timing, turbulence, dust, and chaotic motion all play crucial roles.
Thanks to ALMA’s exceptional sensitivity and resolution, astronomers can now see these details for the first time, offering new insight into how black holes and galaxies grow together over cosmic time.
Source: KSR.
