Astronomers at the European Southern Observatory (ESO) in Chile have discovered a clump of six galaxies surrounding a supermassive black hole. The six galaxies are among the most distant ever observed. And, because looking across cosmic distances is also like looking back in time, these galaxies are also among the most ancient objects we know of. According to Science Daily, we are observing them at a time less than a billion years after the Big Bang, close to 13 billion years ago.
All of which means that the black hole in their midst — estimated to have about a billion times the mass of the sun — is also close to 13 billion years old.
Like a very delayed loop of crime scene surveillance video, the observations from the ESO’s Very Large Telescope (VLT) reveal an ancient mystery while also pointing to a potential suspect.
Black Holes, Massive and Supermassive
The only property of black holes that we can directly determine is their mass, which we can calculate based on their gravitational effect on nearby objects. Ordinary, garden-variety black holes have masses ranging from about ten times heavier than the sun upward to a few hundred solar masses. These black holes are believed to be formed by the collapse of massive stars.
But some, called supermassive black holes, are much heavier. According to Phys.org, the nearest supermassive black hole, at the center of our own Milky Way galaxy, weighs in at some four million suns. By cosmic standards, this makes it a lightweight. The newly discovered supermassive black hole is too distant for its gravity to be measured directly, but researchers believe it is a good billion times more massive than the sun.
Dining on Stars
The central mystery here is how supermassive black holes get to be supermassive. Ordinary black holes are formed by the collapse of a single massive star, but the supermassive ones are thousands to millions of times more massive than any star ever observed or believed to be even theoretically possible.
But the intense gravity of even an “ordinary” black hole can continue to capture matter from the space around it, allowing the black hole to fatten up. The more massive it gets, the more powerful its gravity becomes, and supermassive black holes have been seen dining on entire stars that got caught in their gravity.
Astronomers recently captured views of one such stellar meal, as CBS News reports. The black hole shredded the star into thin streams of glowing matter and slurped them down — a process vividly described as “spaghettification.”
This process intensely heats the “spaghettified” stellar material as it falls toward the black hole. Hence, the seeming paradox that supermassive black holes can be among the brightest objects in the universe.
At the Dawn of Time
The supermassive black hole discovered by the ESO team underlines the mystery of how a black hole could find enough “food,” whether as stars or in the form of gas and dust, to grow to supermassive size — especially in the early universe’s timeline, when galaxies themselves were only just coalescing and there wouldn’t have even been enough time yet for a leisurely meal.
This is where the six galaxies seen clumped around the supermassive black hole provide crucial evidence from the scene of the crime (or at least of the meal). Astronomer Barbara Balmaverde, co-author of the discovery report, notes to Astronomy magazine that these six galaxies are among the faintest yet discovered.
According to Balmaverde, this suggests that many other galaxies, too faint for existing telescopes to observe, also surround the black hole. “We believe we have just seen the tip of the iceberg, and that the few galaxies discovered so far around this supermassive black hole are only the brightest ones,” she says.
Enter Dark Matter
This clumping of galaxies, so early in the history of the universe, suggests to cosmologists that dark matter provided the original source of gravitational attraction that compressed gas in the early universe sufficiently to trigger the formation of galaxies.
Dark matter has nothing directly to do with black holes. The similar names testify to the fact that the universe is a weird place, filled with many things we can’t directly see. In this case, dark matter may have provided a primordial “halo” that allowed stars and galaxies to coalesce, producing a black hole and the surrounding material it dined on to grow to supermassive status.
The universe may be weird, but sometimes it turns out to fit together pretty neatly.