A Galaxy That Slept for Half a Billion Years express
The James Webb Space Telescope looked back to a time when the universe, just after the Big Bang, was only a billion years old, and found the 'Sleepy' galaxy. Unlike its neighbors, which shone with young stars, it looked tired and dim.
Splitting light into its colors, scientists realized: in its first 300 million years, Sleepy blazed to life, piling up a mountain of stars, and then fell asleep for half a billion years. This slumber left a mark — a hydrogen dip in the glow, a sharp cutoff in the spectrum. It turned out to be three times stronger than in any other galaxy of that era, like the echo of a suddenly cut-off scream.
The discovery flips the script. Previously, it was thought that bright early galaxies inevitably grow into giants. Sleepy, instead, became a dim dwarf, and perhaps the majority of these 'expectation-defying' sleeping beauties exist.
Original abstract
as written by the authors arXiv:2512.03154
JWST has revealed unexpectedly bright galaxies in the first 500 Myr after the Big Bang. Their overabundance suggests that they are preferentially observed during burst phases, where their star formation rates increase dramatically. In cosmological simulations, such bursts transition into short ($$\approx 40$$ Myr) periods without star formation or naps. Using JWST/NIRCam medium-band observations, we report the discovery of the galaxy CANUCS-A370-2228423 ($$z = 5.95 \pm 0.06$$, $$\log(M_\ast/M_{\odot}) = 9.14 \pm 0.09$$), dubbed The Sleeper. Its star formation history indicates rapid assembly in the first 300 Myr ($$z \gtrsim 14$$), where it formed a $$\log(M_\ast/M_{\odot}) = 8.7^{+0.3}_{-0.4}\ M_{\odot}$$ progenitor, comparable in stellar mass to the few spectroscopically confirmed galaxies at those redshifts. Unexpectedly, this is followed by several hundred million years of suppressed star formation, in stark contrast to nappers. This results in a remarkably strong hydrogen Balmer break, exceeding that of any galaxy observed within the first billion years by a factor of $$\approx 3$$. Furthermore, Sleeper-like systems are overabundant in the observed survey volume compared to theory, as the probability of finding such galaxies in simulations is $$< 0.2\%$$. The discovery of The Sleeper therefore disrupts the current narrative that all luminous galaxies in the first few hundred million years grow into massive descendants. Instead it presents an alternative evolutionary pathway in which these unusually luminous galaxies fade into inefficient dwarfs after an early starburst, revealing greater diversity in the first stages of galaxy evolution.
By today, Sleepy would have become so dim that no modern telescope could see it — half a billion years of slumber have made it nearly invisible.