12.4 billion light years away 14 galaxies are merging into a colossus. Astronomers discovered the group using the 394-inch (10-meter) South Pole Telescope and examined it in detail with the Atacama Large Millimeter Array (ALMA) in Chile. Both scopes look like big radio dishes and study the sky in “submillimeter” light just beyond the infrared end of the light spectrum.
All 14 objects are rapidly converting their gas to stars 50 and 1,000 times more quickly than the Milky Way, making them rare “starburst” galaxies. At the same time as they’re birthing billions of suns, the galaxies are drawing ever closer and will ultimately merge like cars approaching an intersection from multiple directions. But while cars maintain their separate identities, the galaxies will not. Pulled together by their mutual gravity, they’ll form the ultimate blended family, a single starry behemoth called an elliptical galaxy. The new star formation is probably stimulated by their mutual gravitational tugs, a result of their proximity to one other.
I shouldn’t speak in the future tense. We see them in our corner of time and space when they were still separate entities 1.4 billion years after the universe began. In current time — what you’d call now for someone present at the site of the merger — they long ago became a giant “everything omelette” with a mass of 10 trillion suns, one of the biggest single structures in the universe.
The group represents a protocluster, a group of galaxies in the early universe that comes together to form either a true cluster, a single object or a temporary group that later falls apart. Witnessing the birth of a galaxy cluster, the largest structures in the universe, is rare enough. What astronomers don’t understand is how such a massive cluster got so big so fast. According to prevailing theory, not enough time has gone by for such a massive group to have formed. It takes at least a couple hundred million years to build a galaxy and even longer to form a cluster.
Only one other similar protocluster, the 10-member SMM J004224, has been observed to date. How do these groups get started and what causes them to grow so quickly? Not that I know anything but I wonder if dark matter, that still-mysterious substance that makes up about 80% of the material universe, might provide the seed. Dark matter’s gravitational pull can sweep up and concentrate ordinary matter, which collapses through gravity to form galaxies. Kind of like raindrops coalescing around bits of dust, soot and salt in the atmosphere.
We know that dark matter can help build individual galaxies. Could a dark matter cloud of sufficient mass have provided multiple sites for a bunch of galaxies to form almost simultaneously? To learn more about our featured protocluster, you can read the research paper that appeared in Nature here.