Galaxies in the early cosmos typically drew in gas from intergalactic space, fueling star formation and building up mass in a self-sustaining cycle. Yet this growth eventually ends through a process known as "quenching."
In the nearby Universe, nearly half of all observed galaxies have quenched, no longer producing new stars. These galaxies, often referred to as "red and dead," appear reddish due to the absence of hot, young blue stars and are dominated by older, cooler stellar populations.
Such quiescent galaxies are more commonly found among massive systems, especially those with elliptical shapes. Their formation is thought to be gradual, requiring prolonged star formation before activity is shut down. However, the precise causes of quenching remain a topic of debate. "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, associate professor at UNIGE's Department of Astronomy and co-author of the study. Identifying these early systems has long been a key objective in astrophysics.
Advanced spectroscopic techniques, especially in the near-infrared, have enabled astronomers to confirm the presence of MQGs at increasingly earlier cosmic times. Their prevalence poses a significant challenge to current models, which suggest that such galaxies should require more time to form. Observations with JWST have now pushed the boundary to a redshift of 5, corresponding to just 1.2 billion years after the Big Bang. The new study reveals that galaxy formation and quenching may have occurred even sooner.
As part of JWST Cycle 2, the RUBIES program (Red Unknowns: Bright Infrared Extragalactic Survey) utilized the NIRSpec instrument to gather spectra from thousands of galaxies, including numerous new discoveries based on early JWST imaging.
Among these, scientists pinpointed the most distant MQG yet detected, designated RUBIES-UDS-QG-z7. With a redshift of 7.29, this galaxy had already ceased star formation a mere 700 million years post-Big Bang. Spectral data showed it contained an unexpectedly mature stellar population. Modeling of its light revealed that it amassed over 10 billion solar masses of stars within the first 600 million years, then abruptly quenched its star formation.
"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, a PhD student at UNIGE and lead author of the study. This suggests that aspects of galactic evolution models, such as feedback from black holes or stellar winds, may need serious revision. Current theories do not account for galaxies quenching this early.
RUBIES-UDS-QG-z7 also exhibits extreme compactness, measuring only about 650 light-years across. Its dense stellar makeup mirrors that of quiescent galaxies observed at lower redshifts between 2 and 5.
Astronomers believe such systems may represent the dense cores of today's massive elliptical galaxies. "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," concluded Anna de Graaff, principal investigator of the RUBIES program and a postdoctoral researcher at the Max Planck Institute for Astronomy.
Research Report:RUBIES Reveals a Massive Quiescent Galaxy at z = 7.3
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