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arXiv · 2605.20931

Interpretations of galaxy spectra at high redshift. \ The H_gamma/H_beta excess

M. Contini
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Abstract

Spectra from the cosmic dawn obtained with JWST/NIRSpec (James Webb Space Telescope near-infrared spectroscopy) in the SMACS0723 Early Release Observations are now available. Analyses carried out by different teams indicate poor to extremely low oxygen metallicities (log(O/H)+12<8.0), a characteristic feature of pristine galaxies. In this work, we present new modelling of spectra emitted by objects in the redshift range 2.16<=z<= 8.68, including their recently corrected spectra in the z = 2-9 range. The models account for both photoionisation and shock processes. Our aim is to identify similarities and differences with respect to local galaxies by searching for possible remnants of pristine galaxies among low-z objects. We analyse selected emission-line ratios in relation to elemental abundances and physical parameters. We find that the gaseous clouds within galaxies at cosmic dawn have preshock densities at least a factor of 100 higher than those in local galaxies, but comparable to those calculated for local metal-poor galaxies at 0.005 0.5, indicating high temperatures (>10^5K) in the emitting gas. Clear affinities are evident between the high-z galaxy spectra and those of local metal-poor galaxies at 0.005 6 and z<0.05, whereas some cloud remnants of pristine galaxies survived and are now found embedded, for example, at 0.005<z<0.05.

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M. Contini. 2026-05-20. Interpretations of galaxy spectra at high redshift. \ The H_gamma/H_beta excess. https://arxiv.org/abs/2605.20931

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