Astronomers using the James Webb Space Telescope (JWST) have identified the most distant known quiescent galaxy, revealing that some massive galaxies stopped forming stars shockingly early in the Universe’s history—just 700 million years after the Big Bang.
A Record-breaking Discovery In Deep Space
The galaxy, named RUBIES-UDS-QG-z7, was uncovered by the European-led RUBIES survey during JWST’s second observation cycle. It holds more than 10 billion solar masses within a compact region only 650 light-years wide, making it exceptionally dense and small.
What makes RUBIES-UDS-QG-z7 even more extraordinary is that it is quiescent—meaning it had already ceased star formation when the Universe was less than a billion years old. This contradicts long-held beliefs about how long it takes for massive galaxies to form and shut down.
“The discovery of this galaxy, named RUBIES-UDS-QG-z7, implies that massive quiescent galaxies in the first billion years of the Universe are more than 100 times more abundant than predicted by any model to date,”said Andrea Weibel, lead author and doctoral student in the Department of Astronomy at the University of Geneva.
The End Of Star Formation—far Earlier Than Expected
According to the study published in the Astrophysical Journal, in standard models, galaxies grow by pulling gas from the intergalactic medium and converting it into stars. Over time, as mass builds up, this process accelerates. Eventually, star formation stops in a process known as “quenching”. But until now, astronomers assumed this happened over much longer timescales.
With JWST’s powerful infrared instruments, scientists can detect light from extremely distant galaxies and study their spectral fingerprints. The NIRSpec/PRISM data from RUBIES-UDS-QG-z7 show an old stellar population, already devoid of hot, blue stars. Only older, redder stars remain, painting a picture of a galaxy that had already completed its stellar life cycle.
“The discovery of RUBIES-UDS-QG-z7 provides the first strong evidence that the centers of some nearby massive ellipticals may have already been in place since the first few hundred million years of the Universe,”said Anna de Graaff, principal investigator of the RUBIES program and postdoctoral researcher at the Max Planck Institute for Astronomy.
Theoretical Models In Crisis
Until recently, the earliest known quiescent galaxies had been spotted 1.2 billion years post-Big Bang (redshift ~5). RUBIES-UDS-QG-z7 pushes this limit back by half a billion years and undermines current simulations of early galaxy formation.
“Finding the first examples of massive quiescent galaxies (MQGs) in the early Universe is critical as it sheds light on their possible formation mechanisms,” explained Pascal Oesch, co-author and professor at the University of Geneva’s Faculty of Science.
Existing simulations cannot account for how such a dense, massive galaxy could assemble and stop star formation so quickly. Astrophysicists now suspect that stellar winds, black hole feedback, and galactic outflows may play a more significant role in halting star formation than previously thought.
The Ancient Seeds Of Giant Galaxies
Despite its distance and age, RUBIES-UDS-QG-z7’s compact size and stellar density mirror the dense cores of giant elliptical galaxies seen in the modern Universe. Its structure suggests it could be the ancestral core of a present-day massive elliptical.
The implications are profound: massive galaxies like those seen today might have had their central bulges in place within just a few hundred million years of cosmic history.
