A Baby Galaxy from the Cosmic Noon express
In the deep cosmos, more than 12 billion light-years away, the James Webb Space Telescope has spotted a galaxy that looks like a visitor from the dawn of creation. Its light has been traveling to us since the universe was only about two billion years old — a time when galaxies were growing especially vigorously, a period astronomers call the cosmic noon. The galaxy CAPERS-39810 turned out to be surprisingly 'clean': it contains very few heavy elements, which astronomers for short call metals.
Metals are everything heavier than hydrogen and helium. They are born inside stars and spread out when stars explode, gradually enriching the gas from which new stars and planets later form. In CAPERS-39810, for example, oxygen is about 25 times less abundant than in our Milky Way. This means the galaxy has barely been touched by stellar explosions: its matter remains almost the same as it was right after the Big Bang.
The galaxy's spectrum showed bright emission lines of hydrogen and helium: they glow when young, hot stars ionize the surrounding gas. From these lines, scientists estimated the chemical composition. Such 'primitive' galaxies at large distances are hard to find, but each new discovery is like a window into an era when the universe was just learning to create complex substances.
Original abstract
as written by the authors arXiv:2601.17498
We report the discovery of an extremely metal-poor galaxy at a redshift of z = 3.654, identified through infrared spectroscopy using the James Webb Space Telescope (JWST). This galaxy, CAPERS-39810, exhibits a metallicity of 12 + log(O/H) = $$6.73\pm0.13$$, indicative of its primitive chemical composition, resembling the early stages of galaxy formation in the Universe. We use JWST NIRSpec/MSA for spectroscopic analysis, complemented by photometric data from the COSMOS2025 catalog. Our analysis employs the R3 strong-line diagnostic method to estimate metallicity, due to the lack of auroral lines in the spectrum. The galaxy's emission lines, including Hb, [O III], Ha and He I, are clearly detected. The rest-frame equivalent widths of the strong hydrogen recombination lines are EW_0(Hb) = $$184\pm48$$ \AA and EW_0(Ha) = $$1144\pm48$$ \AA. Furthermore, we perform detailed spectral energy distribution modeling to derive a galaxy logarithmic stellar mass of $$8.02^{+0.22}_{-0.34}$$ $$M_\odot$$. This discovery adds to the growing body of evidence for the existence of very low-metallicity galaxies existed at cosmic noon of $$z\approx3$$, which are crucial for understanding the processes of chemical enrichment and star formation in young galaxies at the cosmic noon.
The word 'metals' in astronomy is misleading: it refers to all elements heavier than helium, including oxygen, carbon, and even nitrogen, which makes up the air we breathe.
In Carl Sagan's novel 'Contact,' the heroine searches for signals from extraterrestrial civilizations, but galaxies like CAPERS-39810 remind us that long before any life, the universe was nearly sterile — only hydrogen and helium.