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

TL;DR

This work reassesses the ages of JWST-selected high-redshift galaxies by combining deeper photometry (10 filters to 26 μm) with JWST-NIRSpec spectroscopy for a larger sample (31 galaxies, median z ≈ 7.3). Using SED fitting with two SSPs plus an AGN component and updated extinction prescriptions (including intergalactic attenuation), the authors find a mean old-population age of about 0.61 Gyr, implying a formation redshift $z_{ m form.} > 11.2$ at 97.5% CL. Notably, reddest systems show ages approaching or exceeding the Universe’s age at their redshifts in some fits (e.g., one case with >4.7σ tension), a result that remains provisional given model uncertainties such as TP-AGB treatment and emission-line strength. Overall, the data favor modest AGN contributions in most galaxies and a substantial older stellar component, providing a cosmological test bed and highlighting areas where additional data and improved modeling are needed to resolve remaining tensions.

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.

Figures (8)

• Figure 1: Log-log plot of interstellar extinction curves used in W24, Paper I and this paper. All of them are based on Cal00 curve (only valid for $\lambda \ge 1200$ Å ), but for Paper I and herein Wei01-$R_V=4$ curve for $\lambda _{\rm rest}<1200$ Å is used, whereas for W24 Cal00 curve is extrapolated for $\lambda _{\rm rest}<1200$ Å and added a factor $\left(\frac{\lambda }{5500\ \AA}\right)^{-1.53}$.
• Figure 2: Log-linear plot of intergalactic extinction curves derived by Ino14.
• Figure 3: Top panel: Stacked spectrum (black solid line) corresponding to the weighted average of the 13 galaxies (normalized each of them at 4000 Å ) with available spectrum, in the range $\lambda _{\rm rest}=$1000-5500 Å at rest. Resolution $\Delta \lambda =10$ Å . Bottom panel: Zoom of the top panel in the range $\lambda _{\rm rest}$=4770-5100 Å ; dotted lines stand for the individual 13 spectra.
• Figure 4: Prediction of $D(4000)=\frac{F_\lambda(4160-4290)}{F_\lambda(3500-3630)}$ and $D_n(4000)=\frac{F_\lambda(4022-4072)}{F_\lambda(3900-3950)}$ for a single stellar population in GALAXEV stellar population synthesis model, as a function of age and metallicity.
• Figure 5: Prediction of the equivalent width of CaII-K-3933 Å in absorption (negative flux) line for a single stellar population in GALAXEV stellar population synthesis model, as a function of age and metallicity.