JWST spectroscopic insights into the diversity of galaxies in the first 500 Myr: Short-lived snapshots along a common evolutionary pathway

article JWST spectroscopic insights into the diversity of galaxies in the first 500 Myr: Short-lived snapshots along a common evolutionary pathway published yes Guido Roberts-Borsani author Pascal A Oesch author Richard Ellis author Andrea Weibel author Emma Giovinazzo author Rychard Bouwens author Pratika Dayal author Adriano Fontana author Kasper E Heintz author Jorryt J Matthee author 7439a258-f3c0-11ec-9501-9df22fe067200000-0003-2871-127X Romain A Meyer author Laura Pentericci author Alice Shapley author Sandro Tacchella author Tommaso Treu author Fabian Walter author Hakim Atek author Sownak Bose author Marco Castellano author Yoshinobu Fudamoto author Takahiro Morishita author Rohan P Naidu author Ryan L Sanders author Arjen van der Wel author JoMa department We investigate the nature and spectroscopic diversity of early galaxies from a sample of 41 sources at $z\geqslant 10$ with James Webb Space Telescope (JWST)/NIRSpec prism observations. We compare the properties of strong ultraviolet (UV) line emitters, traced by intense C iv emission, with those of more ‘typical’ sources with weak or undetected C iv. The more typical (or ‘C iv-weak’) sources reveal significant scatter in their C iii] line strengths, UV continuum slopes, and physical sizes, spanning C iii] equivalent widths (EWs) of $\sim$1–51 Å, UV slopes of $\beta \sim -1.6$ to $-2.6$, and half-light radii of $\sim$50–1000 pc. In contrast, C iv-strong sources occupy the tail of these distributions, with C iii] EWs of 16–51 Å, UV slopes $\beta \lesssim -2.5$, compact morphologies ($r_{\rm 50} \lesssim 100$ pc), and elevated star formation surface densities ($\Sigma _{\rm SFR} \gtrsim 100\, M_\odot \, \mathrm{yr}^{-1}\, \mathrm{kpc}^{-2}$). These properties suggest concentrated starbursts that temporarily outshine the host galaxy. Comparing average properties from composite spectra, we find the diversity of the sample is primarily driven by bursty star formation on very short time-scales ($\le$3 Myr), with strong C iv emitters observed at the apex of the bursts and sources devoid of emission lines during relative inactivity. An apparent association between strong C iv and enhanced nitrogen abundance suggests both may be modulated by the same duty cycle, reflecting a generic mode of star formation. We show that active galactic nuclei are unlikely to contribute significantly to this duty cycle based on UV line diagnostics and photoionization models. Our results support a picture whereby brief bursts and lulls can explain the spectral diversity and early growth of bright galaxies in the first 500 Myr. https://research-explorer.ista.ac.at/download/21898/21902/2026_MNRAS_RobertsBorsani.pdf application/pdfno https://creativecommons.org/licenses/by/4.0/ Oxford University Press2026 eng Monthly Notices of the Royal Astronomical Society 0035-8711 1365-2966 2508.2170810.1093/mnras/stag701 5483 G. Roberts-Borsani, P.A. Oesch, R. Ellis, A. Weibel, E. Giovinazzo, R. Bouwens, P. Dayal, A. Fontana, K.E. Heintz, J.J. Matthee, R.A. Meyer, L. Pentericci, A. Shapley, S. Tacchella, T. Treu, F. Walter, H. Atek, S. Bose, M. Castellano, Y. Fudamoto, T. Morishita, R.P. Naidu, R.L. Sanders, A. van der Wel, Monthly Notices of the Royal Astronomical Society 548 (2026). Roberts-Borsani G, Oesch PA, Ellis R, et al. JWST spectroscopic insights into the diversity of galaxies in the first 500 Myr: Short-lived snapshots along a common evolutionary pathway. Monthly Notices of the Royal Astronomical Society. 2026;548(3). doi:<a href="https://doi.org/10.1093/mnras/stag701">10.1093/mnras/stag701</a> Roberts-Borsani G, Oesch PA, Ellis R, Weibel A, Giovinazzo E, Bouwens R, Dayal P, Fontana A, Heintz KE, Matthee JJ, Meyer RA, Pentericci L, Shapley A, Tacchella S, Treu T, Walter F, Atek H, Bose S, Castellano M, Fudamoto Y, Morishita T, Naidu RP, Sanders RL, van der Wel A. 2026. JWST spectroscopic insights into the diversity of galaxies in the first 500 Myr: Short-lived snapshots along a common evolutionary pathway. Monthly Notices of the Royal Astronomical Society. 548(3), stag701. Roberts-Borsani, G., Oesch, P. A., Ellis, R., Weibel, A., Giovinazzo, E., Bouwens, R., … van der Wel, A. (2026). JWST spectroscopic insights into the diversity of galaxies in the first 500 Myr: Short-lived snapshots along a common evolutionary pathway. Monthly Notices of the Royal Astronomical Society. Oxford University Press. <a href="https://doi.org/10.1093/mnras/stag701">https://doi.org/10.1093/mnras/stag701</a> Roberts-Borsani, Guido, et al. “JWST Spectroscopic Insights into the Diversity of Galaxies in the First 500 Myr: Short-Lived Snapshots along a Common Evolutionary Pathway.” Monthly Notices of the Royal Astronomical Society, vol. 548, no. 3, stag701, Oxford University Press, 2026, doi:<a href="https://doi.org/10.1093/mnras/stag701">10.1093/mnras/stag701</a>. Roberts-Borsani, Guido, Pascal A Oesch, Richard Ellis, Andrea Weibel, Emma Giovinazzo, Rychard Bouwens, Pratika Dayal, et al. “JWST Spectroscopic Insights into the Diversity of Galaxies in the First 500 Myr: Short-Lived Snapshots along a Common Evolutionary Pathway.” Monthly Notices of the Royal Astronomical Society. Oxford University Press, 2026. <a href="https://doi.org/10.1093/mnras/stag701">https://doi.org/10.1093/mnras/stag701</a>. G. Roberts-Borsani et al., “JWST spectroscopic insights into the diversity of galaxies in the first 500 Myr: Short-lived snapshots along a common evolutionary pathway,” Monthly Notices of the Royal Astronomical Society, vol. 548, no. 3. Oxford University Press, 2026. 218982026-05-20T14:34:29Z2026-05-21T06:16:04Z