Improved measurements of the age of <i>JWST</i> galaxies at <i>z</i> = 6 − 10

ABSTRACT From James Webb Space Telescope (JWST) surveys, 31 galaxies with average redshift 7.3 are selected containing large Balmer break, Lyman-$\alpha$ break (V-shaped SED versus $\lambda$). Apart from Hubble Space Telescope (HST) and JWST-NIRCam (Near-infrared camera) photometry for these galaxies, there are JWST-NIRSpec spectra for 13 galaxies and mid-infrared photometry (mostly JWST-MIRI) for 15 of them. Spectroscopical analyses included Balmer emission lines, Balmer + 4000 Å breaks or Ca ii lines. Spectral energy distribution (SED) fitting with photometry include old and young stellar populations, emission lines associated to H ii regions, active galactic nucleus (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 per cent CL) Gyr, while the average age of the Lambda cold dark matter ($\Lambda$CDM) universe is 0.70 Gyr. This corresponds to a formation epoch $z_{\rm form.}\gt 11.2$ (97.5 per cent 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 $\gt 4.7\sigma$. 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.