An international team that was led by the University of Geneva (UNIGE) and includes Professor Stijn Wuyts from the University of Bath in the UK has identified three ultra-massive galaxies - each nearly as massive as the Milky Way - that had already assembled within the first billion years after the Big Bang.
The researchers' results indicate that the formation of stars in the early Universe was far more efficient than previously thought, challenging existing galaxy formation models.
The surprising discovery - described today in the journal Nature - was made by the James Webb Space Telescope (JWST) as part of the JWST FRESCO programme.
The programme set out to systematically analyse a complete sample of emission-line galaxies (ELGs) within the first billion years of cosmic history. ELGs exhibit strong emission lines in their spectra (a spectrum is the range of different wavelengths of light emitted). These emission lines appear as bright lines at specific wavelengths, standing out against the darker background of the spectrum.
The presence of emission lines enabled the team to accurately pin down the distances to the galaxies in the sample. In turn, precise knowledge of the distances and emission line strengths allowed the researchers to reliably measure the amount of stars contained within the galaxies. Three stood out by their large stellar content.
"Finding three such massive beasts among the sample poses a tantalising puzzle", said Professor Wuyts, co-author of the Nature study and Hiroko Sherwin Chair in Extragalactic Astronomy at Bath's Department of Physics.
"Many processes in galaxy evolution have a tendency to introduce a rate-limiting step in how efficiently gas can convert into stars, yet somehow these Red Monsters appear to have swiftly evaded most of these hurdles."
Fast growing Red Monsters
Until now, it was believed that all galaxies formed gradually within large halos of dark matter. Dark matter halos capture gas (atoms and molecules) into gravitationally bound structures. Typically, 20% of this gas, at most, is converted into stars in galaxies. However, the new findings challenge this view, revealing that massive galaxies in the early Universe may have grown far more rapidly and efficiently than previously thought.
Detail in the FRESCO study was captured through 'slitless spectroscopy' with JWST's Near Infrared Camera, a surveying method that allows light to be captured and unravelled into its constituent wavelengths for all objects in a field of view. This makes it an excellent method for measuring accurate distances and physical characteristics of galaxies.
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 analysing galaxies included in the FRESCO survey, scientists found that most galaxies fit existing models. However, they also found three surprisingly massive galaxies, with stellar masses comparable to today's Milky Way.
These are forming stars nearly twice as efficiently as lower mass galaxies from the same epoch or ordinary galaxies at later times in cosmic history. Due to their high dust content, which gives these three massive galaxies a distinct red appearance in JWST images, they have been named the three Red Monsters.
Dr Mengyuan Xiao, lead author of the new study and postdoctoral researcher at UNIGE, said: "Our findings are reshaping our understanding of galaxy formation in the early Universe."
Dr David Elbaz, director of research at CEA Paris-Saclay and collaborator on this project, said: "The massive properties of these Red Monsters were hardly determined before JWST, as they are optically invisible due to dust attenuation."
A Milestone in Galaxy Observations
Pascal Oesch, associate professor in the Department of Astronomy at the UNIGE, and principal investigator of the observation programme, said: "Our findings highlight the remarkable power of NIRCam/grism spectroscopy. The instrument on board the space telescope allows us to identify and study the growth of galaxies over time, and to obtain a clearer picture of how stellar mass accumulates over the course of cosmic history."
While these findings do not conflict with the standard cosmological model, they raise questions for galaxy formation theories, specifically the issue of 'too many, too massive' galaxies in the early Universe.
Current models may need to consider unique processes that allowed certain early massive galaxies to achieve such efficient star formation and thus form very rapidly, very early in the Universe. Future observations with JWST and the Atacama Large Millimeter Array (ALMA) telescope will provide further insights into these ultra-massive Red Monsters and reveal larger samples of such sources.
Dr Xiao said: "These results indicate that galaxies in the early Universe could form stars with unexpected efficiency. As we study these galaxies in more depth, they will offer new insights into the conditions that shaped the Universe's earliest epochs. The Red Monsters are just the beginning of a new era in our exploration of the early Universe."
Professor Wuyts added: "That is what is so great about astronomy, we're constantly being surprised by new discoveries. Already in its first few years of operation, JWST has thrown us a couple of curveballs. In more ways than one, it has shown us that some galaxies mature rapidly during the first chapters of cosmic history."
