We present a detailed analysis of the rest-frame optical emission line ratios for three spectroscopically confirmed galaxies at z>7.5. The galaxies were identified in the James Webb Space Telescope (JWST) Early Release Observations field SMACS J0723.3−7327. By quantitatively comparing Balmer and oxygen line ratios of these galaxies with various low-redshift "analogue" populations (e.g. Green Peas, Blueberries, etc.), we show that no single analogue population captures the diversity of line ratios of all three galaxies observed at z>7.5. We find that S06355 at z=7.67 and S10612 at z=7.66 are similar to local Green Peas and Blueberries. In contrast, S04590 at z=8.50 appears to be significantly different from the other two galaxies, most resembling extremely low-metallicity systems in the local Universe. Perhaps the most striking spectral feature in S04590 is the curiously high [O III] λ4363/[O III] λ5007 ratio (RO3) of 0.048 (or 0.055 when dust-corrected), implying either extremely high electron temperatures, ∼3×104 K, or gas densities >104cm−3. Observed line ratios indicate that this galaxy is unlikely to host an AGN. Using photoionization modelling, we show that the inclusion of high-mass X-ray binaries or a high cosmic ray background in addition to a young, low-metallicity stellar population can provide the additional heating necessary to explain the observed high RO3 while remaining consistent with other observed line ratios. Our models represent a first step at accurately characterising the dominant sources of photoionization and heating at very high redshifts, demonstrating that non-thermal processes may become important as we probe deeper into the Epoch of Reionization.
@article{arxiv.2207.13693,
title = {First Insights into the ISM at $z>8$ with JWST: Possible Physical Implications of a High [O III]$\mathbf{\lambda 4363}$/[O III]$\mathbf{\lambda 5007}$},
author = {Harley Katz and Aayush Saxena and Alex J. Cameron and Stefano Carniani and Andrew J. Bunker and Santiago Arribas and Rachana Bhatawdekar and Rebecca A. A. Bowler and Kristan N. K. Boyett and Giovanni Cresci and Emma Curtis-Lake and Francesco D'Eugenio and Nimisha Kumari and Tobias J. Looser and Roberto Maiolino and Hannah Ubler and Chris Willott and Joris Witstok},
journal= {arXiv preprint arXiv:2207.13693},
year = {2022}
}