A tiny galaxy observed as it existed 13 billion years ago has revealed the lowest oxygen abundance ever measured in a galaxy of its kind. Using the James Webb Space Telescope (JWST) together with the natural magnifying effect of gravitational lensing, astronomers have obtained the clearest picture yet of one of the universe’s most chemically primitive galaxies.
The discovery provides a rare opportunity to examine a galaxy that formed shortly after the first stars began enriching the cosmos with heavier elements. It also strengthens the proposed link between the earliest galaxies and theultra-faint dwarf galaxies that still orbit the Milky Way today.
The study, published in Nature, presents the most detailed characterization yet of LAP1-B, an ultra-faint galaxy investigated by an international team led by Kimihiko Nakajima of Kanazawa University, with Masami Ouchi from the National Astronomical Observatory of Japan (NAOJ) and the University of Tokyo.
A Chemical Composition Unlike Any Other
Just after the Big Bang, the universe consisted almost entirely of hydrogen and helium. Heavier elements, including oxygen and carbon, were produced later inside the first generations of stars before being released into space through stellar explosions.
The research team found that LAP1-B contains an oxygen abundance equal to only 1/240th that of the Sun, setting a new record for a galaxy observed at such an early stage of cosmic history.
“I was instantly thrilled by the extreme lack of oxygen,” said Kimihiko Nakajima. “Finding a galaxy in such a primitive state is astonishing. It’s a chemical signature that clearly indicates a primordial galaxy caught in the moments shortly after its formation.”
The observations also revealed a high carbon-to-oxygen abundance ratio. The paper notes that this elemental signature closely matches theoretical predictions for material expelled by explosions of the universe’s first-generation stars.
The Natural Telescope That Made It Possible
The breakthrough relied on gravitational lensing, a phenomenon in which the gravity of a massive galaxy cluster bends and magnifies the light from a much more distant object.
In the case of LAP1-B, its light was amplified by roughly 100 times, allowing JWSTto collect enough information for a detailed chemical analysis. The research team observed the galaxy for more than 30 hours, producing data that had previously been out of reach for objects this faint.
The observations showed that combining the telescope’s sensitivity with the natural magnification of gravitational lensing made it possible to study one of the smallest and faintest galaxies known from the reionization era.
A Clue to the Milky Way’s Ancient Past
The team also determined that LAP1-B contains less than 3,300 solar masses, indicating that most of the galaxy consists of an invisible dark matter halo.
Its low stellar mass and unusual chemical composition closely resemble those of the ultra-faint dwarf galaxies (UFDs)located near the Milky Way. These galaxies contain very few stars, are extremely dim, and host stellar populations that are more than 12 billion years old.
Masami Ouchi explained that astronomers had long suspected these systems were relics of the universe’s earliest galaxies because of their lack of heavy elements, although no direct observational link had been established.
“UFDs are not only the faintest galaxies; they are composed of ancient stars over 12 billion years old and are often described as ‘fossils of the universe,” he said.
He noted that while astronomers had suspected these ancient, chemically simple objects originated from the universe’s first galaxies, the missing connection was only established with the discovery of LAP1-B.
“It is a profound surprise to find that LAP1-B looks exactly like the ‘ancestor’ we had only imagined in theories. This helps us solve the mystery of why these cosmic fossils have survived in their current form to the present day.”
