The smallest, dimmest galaxies in the early universe were doing all the heavy lifting to light up the cosmos.
April 29, 2026
Original Paper
A GLIMPSE of the 99%: a census of the faintest galaxies during the epoch reionization and its implications for galaxy formation models
arXiv · 2604.23823
The Takeaway
Observations of the faintest galaxies from 13 billion years ago show no sign of the slowdown predicted by current theories. Standard models suggested that the growth of these tiny galaxies should have been stunted by the intense radiation of their neighbors. Instead, they appear to be numerous and active, meaning they played a much larger role in ionizing the early universe than previously thought. This discovery challenges our best computer simulations of how the first structures in space began to form. It suggests that the history of our universe was written by its smallest members rather than just its giants.
From the abstract
We present a comprehensive study of the galaxy UV luminosity function (UVLF) at $z=6-9$ leveraging deep JWST observations from the GLIMPSE survey. Thanks to gravitational lensing, we probe the UVLF to an unprecedented depth of $M_{\text{UV}} = -12$ mag, approximately three magnitudes deeper than previous robust constraints. Our UVLF determination incorporates a rigorous end-to-end uncertainty framework, including statistical and systematic lensing uncertainties. We find that the $z \sim 7$ UVLF