Improved measurements of the age of JWST galaxies at z=6-10

M. Lopez-Corredoira, C. M. Gutierrez

First listed 2026-01-23 | Last updated 2026-01-23

Abstract

From James Webb Space Telescope (JWST) surveys, 31 galaxies with average redshift 7.3 are selected containing large Balmer break, Lyman-$α$ break (V-shaped SED versus $λ$). Apart from Hubble Space Telescope (HST) and JWST-NIRCam (Near-infrared camera) photometry for these galaxies, there are JWST-NIRSpec (Near-infrared spectrograph) spectra for 13 galaxies and mid-infrared photometry (mostly JWST-MIRI) for 15 of them. Spectroscopical analyses included Balmer emission lines, Balmer + 4000 angstroms breaks or CaII lines. Spectral energy distribution (SED) fitting with photometry include old and young stellar populations, emission lines associated to HII regions, AGN, interstellar dust extinction and intergalactic extinction from neutral hydrogen. By adopting realistic extinction curves and taking into account the V-shaped SED and low emission at near infrared at rest, the analyses show that AGN contribution in these galaxies ('little red dots' most of them) should be small on average in the reddest wavelengths, though important for few of the 31 galaxies. Average age of the 31 galaxies: $0.61\pm 0.31$(95% CL) Gyr, while the average age of the $Λ$CDM universe is 0.70 Gyr. This corresponds to a formation epoch $z_{ form.}>11.2$(97.5% CL). Reddest galaxies present largest ages. One of these very red galaxies gets an age incompatible to be younger than the age of the Universe within $>4.7σ$. TP-AGB effect cannot explain this tension. None the less, there may be other uncertainties in the models, so this tension is a provisional result and further research is needed to confirm it.

Short digest

The authors re-analyze 31 z≈6–10 “little red dots” with V‑shaped SEDs using HST+JWST photometry, NIRSpec spectra for 13, and mid‑IR data for 15, fitting mixed stellar populations, nebular lines, possible AGN, and both ISM and IGM extinction with updated curves. Accounting for the low rest‑NIR fluxes, they infer that AGN contribute little at the reddest wavelengths on average and derive an average oldest‑stellar‑population age of 0.61±0.31 Gyr (95% CL), implying z_form > 11.2 (97.5% CL). The reddest SEDs yield the largest ages, and one extreme source appears older than the Universe at >4.7σ, a tension not explained by TP‑AGB effects. The authors note remaining model uncertainties, so the age–cosmology tension is provisional but sharpened by spectroscopy and the extinction treatment.

Key figures to inspect

• Fig. 1 (extinction curves): Compare the Weingartner & Draine UV extension vs. a Calzetti extrapolation to see how the chosen dust law steepens rest‑UV attenuation and drives the need for an older stellar component while limiting AGN reddening leverage.
• Fig. 2 (IGM attenuation): Inspect the Inoue et al. (2014) intergalactic extinction curves to gauge how Lyman‑series absorption sculpts the Lyα break and the V‑shaped SED, affecting redshift and age–dust degeneracies.
• Fig. 3 (stacked NIRSpec rest‑frame 1000–5500 Å): Use the stack to assess Balmer/4000 Å break strength and search for Hβ and [O III] features; the zoom around 4770–5100 Å tests for broad vs. narrow Hβ and informs how typical the AGN component is across the sample.
• Fig. 4 (GALAXEV index predictions): Map observed break/absorption strengths (e.g., D4000 and Hδ) to the age–metallicity grid to see why the fits favor ~0.5–0.8 Gyr ages and why the reddest SEDs push to even older solutions.