An international team of astronomers has identified three ultra-massive galaxies—each nearly as massive as the Milky Way—already in place within the first billion years after the Big Bang. The discovery is surprising because it indicates that the formation of stars in the early Universe was far more efficient than previously thought, challenging current models of galaxy formation.
The findings, published on November 13 in the journal Nature, were made possible by the James Webb Space Telescope's powerful spectroscopic capabilities at near-infrared wavelengths. The study was led by the University of Geneva, and Garth Illingworth, emeritus distinguished professor of astronomy at UC Santa Cruz, was a co-author.
Illingworth worked on NASA’s James Webb Space Telescope (JWST) for decades during its concept development and construction. He was one of the core group of three people in the mid to late 1980s who helped initiate the concept of the “Next Generation Space Telescope” that became JWST in 2002.
"For three decades since the three of us first developed the concept that led to JWST, we knew that it would provide insights into how the earliest galaxies formed and grew,” Illingworth said. “This remarkable result demonstrates again that JWST is revealing that the earliest galaxies—young Milky Way galaxies—are even more unusual and remarkable than we ever expected. The existence of such massive galaxies at the earliest times poses a real conundrum.”
In standard cosmology, galaxies form gradually within large halos of dark matter that capture gas (atoms and molecules). This gas is converted into stars. But typically, only 20%, or less, of the gas is converted into stars in these primordial galaxies. These findings challenge this view, revealing that massive galaxies in the early Universe converted much more of their gas into stars, and so grew much more rapidly than previously thought. They were much more efficient star builders than galaxies that formed later.
'Red Monsters' revealed
JWST’s unparalleled capabilities have allowed astronomers to systematically study galaxies in the very distant and early Universe, providing insights into massive and dust-obscured galaxies. By analyzing galaxies in the FRESCO survey—short for First Reionization Epoch Spectroscopically Complete Observations—scientists found that most sources fit existing models.
The FRESCO program uses the telescope’s primary near-infrared imager (NIRCam) in combination with a grating-prism spectrograph to measure accurate distances and stellar masses of galaxies. Dan Magee, an applications programmer at the UC Observatories, does data analysis for Illingworth and contributed to this effort by processing data from the FRESCO program.
The three massive galaxies reported on in this study have stellar masses comparable to today’s Milky Way, forming stars nearly twice as efficiently as their lower-mass counterparts and as galaxies at later times. Due to their high dust content, which gives them a distinct red appearance in telescope images, they have been named the three “red monsters.”
“Our findings are reshaping our understanding of galaxy formation in the early Universe,” said Mengyuan Xiao, lead author of the new study and postdoctoral researcher at the University of Geneva.
A Milestone in Galaxy Observations
While these findings do not conflict with the standard cosmological model, they raise new questions about galaxy formation—specifically, the question of “too many, too massive” galaxies in the early Universe. Current models may need to consider different processes that allowed certain early massive galaxies to give birth to stars so efficiently, and thus, form very rapidly, very early in the Universe.
Future observations with JWST and the Atacama Large Millimeter Array will provide further insights into these ultra-massive "Red Monsters" and reveal larger samples of such sources, the team concludes. “As we study these galaxies in more depth, they will offer new insights into the conditions that shaped the Universe’s earliest epochs,” Xiao said. “The 'Red Monsters' are just the beginning of a new era in our exploration of the early Universe.”
