In the earliest days of the universe, the most massive galaxies lit up with the fury of a billion suns—and then stopped almost as abruptly. Researchers have long been baffled by dusty star-forming galaxies that blazed with the intensity of 500 times our sun's annual output, only to fall mysteriously silent within their first billion years. Now a team at Brazil's University of São Paulo, publishing in Astronomy & Astrophysics, has traced this cosmic silence back to a violent origin story written in galaxy collisions.
The puzzle centers on two seemingly unrelated populations of galaxies in the early universe. Dusty star-forming galaxies, or DSFGs, are cosmic powerhouses born roughly three to four billion years after the Big Bang, pumping out stars at a rate of up to 500 solar masses per year—compared to just one solar mass annually in our own Milky Way. Shrouded in thick dust, they're invisible to optical telescopes but blaze with light at infrared and submillimeter wavelengths, making them detectable by instruments like the Atacama Large Millimeter/Submillimeter Array and the James Webb Space Telescope. Then there are the massive quiescent galaxies, the dead zones of the early cosmos—galaxies that formed rapidly, shut down star production almost immediately, and have remained dormant ever since.
The breakthrough came when researchers led by professor Laerte Sodré Júnior tracked these galaxies backward through cosmic time using computer models of galaxy formation, examining them at redshifts between 2 and 4—peering back to when the universe was between three and four billion years old. What they discovered was startling: between 86 and 96 percent of today's dead, quiescent galaxies were once the same raging DSFGs we see in the early universe. Nearly all of these cosmic graveyards had lived violent, prolific childhoods.
The mechanism behind this transformation is equally dramatic. When two massive galaxies of similar size collided head-on in the infant cosmos, the merger triggered two catastrophic processes simultaneously. First came an extreme burst of star formation as the collision compressed vast quantities of gas into the galactic core. Second, material rained down onto the supermassive black hole lurking at the center, feeding it rapidly and releasing tremendous energy outward. That energy had lethal consequences for the galaxy's future: it heated the surrounding halo of gas so intensely that the gas could no longer cool and fall back into the galaxy, cutting off the fuel supply for new stars. Star formation ground to a halt in less than a billion years.
This violent path to extinction differs markedly from how most galaxies evolve. The majority grow gradually through slower mergers and gentler interactions, consuming their gas reserves more leisurely. They continue forming stars for far longer, eventually going quiet only at much later cosmic epochs.
The James Webb Space Telescope's recent observations have both confirmed and complicated this narrative. The telescope has brilliantly mapped dusty star-forming galaxies and revealed that the early universe contains far more massive, quiescent galaxies than existing models predicted—a discrepancy that researchers are still working to resolve. While the galactic merger theory provides a coherent framework for explaining the dramatic transformation from cosmic furnace to stellar graveyard, the full picture remains incomplete, awaiting refinement from future observations and deeper theoretical understanding.