The James Webb Space Telescope Observes the Earliest Galaxies — GN-z11

Discovery

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Technical Explanation

1. Cosmological redshift and the Lyman-alpha line. The hydrogen Lyman-alpha line (emitted at λ₀ = 1,216 Å in the rest frame) is shifted by the expansion of the Universe into the near-infrared. At z = 10.6, this line appears at λ_obs = λ₀ × (1 + z) = 1,216 × 11.6 ≈ 14,100 Å (1.41 µm). The JWST's NIRSpec spectrograph, covering 0.6–5.3 µm with a resolution of R ≈ 2,700, detects this line and metallic emission lines (C III] at 1,909 Å → 2.21 µm) with a signal-to-noise ratio > 10.

2. JWST optics: segmented mirror and the L2 Lagrange point. The 6.5 m primary mirror consists of 18 hexagonal beryllium segments coated with gold (gold thickness: 100 nm). The collecting area is 25.4 m² — 6.25× that of Hubble (4.0 m²). The telescope orbits the L2 Lagrange point, 1.5 million km from Earth, where the 5-layer Kapton sunshield keeps the instruments at ~40 K (−233 °C). This temperature is critical: thermal radiation from the telescope itself above 50 K would drown out infrared signals from distant galaxies.

3. Extreme star formation and the mass problem. GN-z11 exhibits a star formation rate (SFR) of ~25 M☉/year, measured via rest-frame UV luminosity at 1,500 Å. To build up 10⁹ M☉ in only ~300 million years (the time available between the first stars and the observation epoch), an average SFR of at least ~3 M☉/year is required — but the observed rate of 25 M☉/year points to intense episodes of star formation (starbursts). The initial mass function (IMF) and gas-to-star conversion efficiency under these conditions remain debated.

4. Metallicity and early chemical enrichment. The detected C III] (1,909 Å) and C IV (1,549 Å) emission lines indicate metallicity of ~0.1 Z☉ (one-tenth the solar abundance of heavy elements). This implies that at least one generation of massive stars (Population III, > 100 M☉) had already lived, synthesized carbon via the triple-alpha process (3 ⁴He → ¹²C), and exploded as supernovae — all within < 300 million years. This timeline constrains models of first-star formation.

Why It Worked

Hubble had already measured GN-z11's redshift in 2016 by grism spectroscopy (Oesch et al., …Measured with Hubble Space Telescope Grism Spectroscopy), but with a far larger uncertainty — its WFC3/IR spectrograph covered only up to 1.7 µm with insufficient sensitivity. The JWST's NIRSpec, with ~100× greater sensitivity in the near-infrared and coverage up to 5.3 µm, detected 5 distinct emission lines, eliminating all residual ambiguity.

JWST's position at L2 — where the thermal shield eliminates infrared background from Earth, Moon, and Sun — enables deep exposures of > 28 hours on a single target, reaching a limiting magnitude of ~31.5 AB (flux of ~1 nJy).

Causal Chain

Big Bang (13.8 Ga) → Recombination (z ≈ 1,100, 380,000 years) → Dark Ages → First Pop III stars (z ≈ 20–30, ~100 Ma) → GN-z11 formed (z ≈ 10.6, ~430 Ma) → Reionization complete (z ≈ 6, ~1 Ga) → Hubble Deep Field (1995, z < 6) → Hubble measures z for GN-z11 by grism spectroscopy (2016) → JWST confirms z = 10.6 by spectroscopy (2023) → Discovery of even more distant galaxies (JADES-GS-z14-0, z ≈ 14.2, 2024)

Historical Anecdote

During the deployment of JWST on December 25, 2021, all 344 "single points of failure" (mechanisms whose malfunction would have ended the mission) functioned flawlessly. The 18 mirror segments required 3 months of alignment using actuators with 7 axes each, achieving a surface precision of < 25 nm RMS — i.e., λ/10 at 2.5 µm, the diffraction limit. The total program cost (1996–2022) was $10 billion, making it the most expensive scientific instrument ever built.

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Sources

References verified during the August 2026 fact-checking audit: these are the pages
against which this bulletin's claims were checked.

1. JADES NIRSpec spectroscopy of GN-z11 — Bunker et al., A&A 677, A88 (2023)
2. A Remarkably Luminous Galaxy at z=11.1 Measured with HST Grism Spectroscopy — Oesch et al. 2016
3. MoM-z14, galaxie la plus lointaine confirmée (z = 14,44), janvier 2026