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We present a detailed analysis of the rest-frame optical emission line ratiosfor three spectroscopically confirmed galaxies at $z>7.5$. The galaxies wereidentified in the James Webb Space Telescope (JWST) Early Release Observationsfield SMACS J0723.3$-$7327. By quantitatively comparing Balmer and oxygen lineratios of these galaxies with various low-redshift "analogue" populations (e.g.Green Peas, Blueberries, etc.), we show that no single analogue populationcaptures the diversity of line ratios of all three galaxies observed at$z>7.5$. We find that S06355 at $z=7.67$ and S10612 at $z=7.66$ are similar tolocal Green Peas and Blueberries. In contrast, S04590 at $z=8.50$ appears to besignificantly different from the other two galaxies, most resembling extremelylow-metallicity systems in the local Universe. Perhaps the most strikingspectral feature in S04590 is the curiously high [O III] $\lambda4363$/[O III]$\lambda5007$ ratio (RO3) of $0.048$ (or $0.055$ when dust-corrected), implyingeither extremely high electron temperatures, $\sim3\times10^4$ K, or gasdensities $>10^4\ {\rm cm^{-3}}$. Observed line ratios indicate that thisgalaxy is unlikely to host an AGN. Using photoionization modelling, we showthat the inclusion of high-mass X-ray binaries or a high cosmic ray backgroundin addition to a young, low-metallicity stellar population can provide theadditional heating necessary to explain the observed high RO3 while remainingconsistent with other observed line ratios. Our models represent a first stepat accurately characterising the dominant sources of photoionization andheating at very high redshifts, demonstrating that non-thermal processes maybecome important as we probe deeper into the Epoch of Reionization.

monthly notices of the royal astronomical society

First Insights into the ISM at $z>8$ with JWST: Possible Physical  Implications of a High [O III]$\mathbf{\lambda 4363}$/[O III]$\mathbf{\lambda  5007}$