Lensed Arc at z≈10: A New Window to the Era of the First Stars ✦
Context
The era of cosmic dawn, a few hundred million years after the Big Bang (the birth of the universe), saw the appearance of the first galaxies. These objects ionized intergalactic hydrogen, ending the dark ages. Yet detecting such faint systems at high redshifts (a measure of distance) remains a daunting challenge: their light is too dim for direct observation. Here, gravitational lensing comes to the rescue—an effect predicted by Albert Einstein and developed by Fritz Zwicky. According to gravitational lensing, massive objects like galaxy clusters bend and magnify light from distant sources, offering a peek into an otherwise unreachable epoch.
Methods
The key instrument was the James Webb Space Telescope (JWST) with its NIRCam camera, which performed deep photometry (brightness measurement in different filters) of the Abell S1063 cluster in 11 bands from 0.9 to 4.8 µm. To separate the arc's light from a bright foreground galaxy, scientists used a special algorithm to subtract a model of isophotes (lines of equal brightness). Then, a gravitational lensing model of the cluster was built, which allowed estimating a magnification factor μ≈43. The redshift–distance relation is given by Hubble's law, discovered by Edwin Hubble. Finally, to interpret the data, Bayesian modeling of the spectral energy distribution (spectrum SED) was performed using the Prospector code, yielding two competing scenarios: an extremely blue continuum or strong emission lines.
Results
The analysis reveals that the source lies at a redshift z≈10.45–10.75 (depending on the model) and is magnified by the gravitational lens by a factor of 43–120, making it visible. In the first scenario, the excess in the F200W filter is explained by an extremely blue ultraviolet continuum with a slope β=-2.92, indicating a record-low metallicity (abundance of elements heavier than helium)—less than 0.03% of solar. This allows for the presence of Population III stars—the very first lights, made only of hydrogen and helium, which kicked off nucleosynthesis (the creation of heavier elements). In the second scenario, the excess is caused by strong emission lines of HeII, CIV, and CIII], emerging from a very young (1–3 Myr) compact region of star formation with hard ionizing radiation. The object's intrinsic stellar mass is no more than a few million solar masses, making it a typical dwarf galaxy.
Implications
The discovery of GAR10 shows that already 500 million years after the Big Bang, very low-mass galaxies with extreme properties existed. If the Population III scenario is confirmed, it would be the first direct observational evidence for the existence of metal-free primordial stars that initiated nucleosynthesis. In any case, the object underscores the importance of dwarf systems for ionizing the universe, since even a single such galaxy can pack a powerful ionizing punch. This forces a revision of star formation models at the dawn of cosmic history.
Future development
Future observations with JWST's NIRSpec spectrograph will deliver high-resolution spectra and precisely measure lines, breaking the ambiguity. Moreover, new gravitational lensing data and improved cluster mass models will help find even fainter arcs at even higher redshifts. Down the road, next-generation telescopes like the Extremely Large Telescope will be able to directly study the chemical composition and kinematics of such objects, ushering in an era of detailed archaeology of the first galaxies.
Impact
This work will influence theories of star formation in the early universe, models of chemical evolution, and estimates of the contribution of ultra-faint galaxies to reionization. It also spurs the development of analysis methods for spectra of extremely metal-poor systems and the refinement of metallicity models in extreme environments.
Next steps
The top priority is to obtain a spectrum of GAR10 with JWST/NIRSpec to detect the HeII and CIII] lines and determine which scenario is real. In parallel, the lensing model must be refined, accounting for the dark matter distribution in the cluster for a more accurate magnification estimate. A systematic search for similar systems at high redshifts will reveal whether GAR10 is a unique object or one of a large population.
Key open problems
GAR10 is directly tied to several unsolved questions in modern cosmology: what is the nature of the first ionizing sources that drove reionization; how do the very first stars form in mini-halos of dark matter; and what role do dwarf systems play in the overall evolution of the universe. Studying it could help define the boundary between Population III stars and early enriched stars, as well as understand feedback mechanisms at small galactic scales.
🎯 Gravitational lensing is sometimes compared to a natural telescope: a massive galaxy cluster acts like a giant magnifying glass, boosting a distant object's brightness by tens of times. Without this effect, GAR10 would be 40 times fainter and would have remained invisible even to Webb.
Key numbers
- redshift z: 10.45–10.75
- lensing magnification μ: 43 (from 23 to 121)
- UV continuum slope β: −2.92±0.12 (for scenario I)
- metallicity: log(Z/Z⊙) ≈ −3.56 (0.0003 solar)
- stellar mass: ~10^6–10^7 M⊙