Problem: galaxies die earlier than expected 🌀

An international team led by UNIGE shows that red and dead galaxies are present just 700 million years after the Big Bang, indicating that star formation stops earlier than anticipated.

Scientists long believed that only star-forming galaxies should be observable in the young Universe. The James Webb Space Telescope now reveals that some galaxies stopped forming stars earlier than expected.


A recent discovery by an international team led by the University of Geneva (UNIGE) highlights the tension between theoretical models of cosmic evolution and observations. Among hundreds of spectra obtained through the Webb RUBIES program, the team identified a record-breaking galaxy that had already stopped forming stars at a time when galaxies are typically still growing. This study is published in the Astrophysical Journal.


In the early Universe, about half of the observed galaxies have stopped forming stars: they are "dead" and no longer growing. Astronomers refer to them as "quiescent," "quenched," or "red and dead" galaxies. They appear red because they no longer contain young, bright blue stars—only older, smaller stars with reddish hues remain.

Key factors in theoretical models need to be reassessed.

A significant portion of quiescent galaxies are found among massive galaxies, often observed with elliptical morphologies. The formation of these red and dead galaxies normally takes a long time, as they must first accumulate a large number of stars before star formation stops completely. The exact cause of quenching remains a mystery.

"Finding the earliest examples of massive quiescent galaxies (MQGs) in the early Universe is crucial because it sheds light on their possible formation mechanisms," explains Pascal Oesch, associate professor at the Department of Astronomy in UNIGE's Faculty of Science and co-author of the study. The search for these systems has been a major goal for astronomers for years.

Observations contradicting models

Thanks to technological advances, particularly near-infrared spectroscopy, astronomers have gradually been able to identify massive quiescent galaxies (MQGs) at increasingly distant cosmic epochs. Their abundance contradicts theoretical models, which predicted a longer formation time.

With the James Webb Telescope (JWST), this contradiction between theory and observations has intensified, as several MQGs have been confirmed at a redshift of 5, corresponding to 1.2 billion years after the Big Bang—a relatively short period on an astronomical timescale. Now, the new study led by UNIGE reveals that they formed even earlier and faster.

In JWST Cycle 2, the RUBIES program (The Red Unknowns: Bright Infrared Extragalactic Survey), one of the largest European programs dedicated to extragalactic research with the NIRSpec instrument, obtained spectroscopic observations of several thousand galaxies, including hundreds of newly discovered sources from JWST's first images.

The record for the most distant "dead" galaxy

Among these new spectra, scientists identified the most distant massive quiescent galaxy (MQG) ever observed, with a spectroscopic redshift of 7.29, corresponding to about 700 million years after the Big Bang. The spectrum obtained with NIRSpec/PRISM reveals an unexpectedly old stellar population in a still-young Universe.

Detailed modeling of the spectrum and imaging data shows that this galaxy formed a stellar mass of over 10 billion (10¹⁰) solar masses within the first 600 million years after the Big Bang, before abruptly stopping star formation, confirming its quiescent state.

"The discovery of this galaxy, named RUBIES-UDS-QG-z7, implies that massive quiescent galaxies in the first billion years of the Universe are over 100 times more abundant than current models predict," explains Andrea Weibel, a PhD student at UNIGE's Department of Astronomy and lead author of the study. This suggests that key factors in theoretical models—such as the effects of stellar winds or the intensity of material flows induced by star formation and massive black holes—must be reassessed.

New insights into galactic cores

Finally, RUBIES-UDS-QG-z7 is only about 650 light-years in size, implying a high stellar density comparable to the highest central densities measured in quiescent galaxies at slightly lower redshifts. These galaxies are likely to become the cores of the oldest and most massive elliptical galaxies observed in the local Universe.

"The discovery of RUBIES-UDS-QG-z7 provides the first solid evidence that the centers of some nearby massive elliptical galaxies may have already been in place within the first few hundred million years of the Universe," concludes Anna de Graaff, a postdoctoral researcher at the Max Planck Institute for Astronomy in Heidelberg, initiator of the RUBIES program and second author of the paper.

This research is published in Astrophysical Journal.
DOI: 10.3847/1538-4357/adab7a