According to the most widely accepted cosmological model, the first stars and galaxies began forming around 300-400 million years after the Big Bang. These formed from the clouds of neutral hydrogen gas that permeated the Universe, leading to rapid star formation for about 1 billion years. With the deployment of the James Webb Space Telescope (JWST), astronomers and cosmologists finally got a chance to observe some of these early galaxies. While they believed that only star-forming galaxies existed then, Webb's observations revealed something rather surprising.
In a recent study, an international team led by astronomers from the University of Geneva (UNIGE) presented spectra obtained by the Revealing the Universe with the James Webb Space Telescope (RUBIES) program. Their findings indicate that when galaxies were thought to be still growing rather rapidly, one galaxy appeared to have "quenched" this process already. Their findings indicate that star formation occurred faster than expected, deepening the "tension" between previous cosmological models and Webb's observations of the early Universe.
The study was led by Andrea Weibel, a Ph.D. astronomy student at the University of Geneva and a member of the Galaxy Build-up at Cosmic Dawn research group. He was joined by researchers from the Max Planck Institute for Astronomy (MPIA), the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), the Cosmic Dawn Center (DAWN), the Niels Bohr Institute, the International Centre for Radio Astronomy Research (ICRAR), the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), the NSF's National Optical-Infrared Astronomy Research Laboratory (NOIRLab), and multiple universities.
Three spectra taken by the JWST/NIRSpec. The record galaxy is shown in the middle. © NASA/CSA/ESA, A. Weibel, P. A. Oesch (University of Geneva), RUBIES team: A. de Graaff (MPIA Heidelberg), G. Brammer (Niels Bohr Institute), DAWN JWST ArchiveBased on decades of observations, astronomers and cosmologists have noted a pattern in galactic evolution. New stars form by accreting dust and gas from the intergalactic medium (IGM), causing galaxies to grow and leading to a more efficient accretion process and star formation. Eventually, galaxies experience "quenching," which refers to how they stop forming new stars and become "quiescent." As younger, brighter stars die off, these galaxies become dominated by red dwarf stars, leading to the term "red and dead" galaxies.
The causes of quenching are still debated, with theories ranging from stellar winds and outflows to supermassive black holes (SMBH). However, standard models indicate that galaxies would need a very long time to reach this phase in their evolution. Thanks to advances in near-infrared (NIR) spectroscopy, astronomers have found examples of quiescent galaxies earlier and earlier in cosmic history, but the earliest ones have remained elusive.
The discovery of these galaxies has challenged conventional cosmological models since the quenching process is believed to take a very long time. "Finding the first examples of massive quiescent galaxies (MQGs) in the early Universe is critical as it sheds light on their possible formation mechanisms," said co-author Pascal Oesch, an associate professor in the Department of Astronomy at the UNIGE Faculty of Science. This tension has only been exacerbated thanks to Webb, which has found evidence of MQGs as early as 1.2 billion years after the Big Bang.
Based on their analysis, Weibei and his colleagues found that these galaxies formed even earlier and more rapidly than previously thought. These observations were part of Cycle 2 of Webb's General Observation (GO) program, one of the largest European-led programs. The campaign relied on Webb's Near-Infrared Spectrometer (NIRSpec) instrument to obtain spectra from thousands of galaxies at redshift values of z > 7 - when the Universe was about 1 billion years old.
Using the Hubble Space Telescope, astronomers have identified a relatively nearby relic galaxy, one that has remained unchanged for the past 10 billion years or so. Credit: NASA, ESA, M. BeasleyAmong those observed, scientists found the most distant MQG to date (RUBIES-UDS-QG-z7), which existed about 700 million years after the Big Bang. Detailed modeling indicates that this galaxy accumulated more than 10 billion solar masses of matter before rapidly becoming quiescent. This suggests that key mechanisms in the accepted model of cosmology need some revising. "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 Weibel.
RUBIES-UDS-QG-z7 also measures just ~650 light-years in diameter, implying a higher stellar mass density than quiescent galaxies in the local Universe. These galaxies are theorized to evolve into the cores of massive elliptical galaxies observed today. "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 of the Max Planck Institute for Astronomy and the principal investigator of the RUBIES program.
Further Reading: University of Geneva, The Astrophysical Journal