First insights into the ISM at z > 8 with JWST: possible physical implications of a high [O III] λ4363/[O III] λ5007

Harley Katz, Aayush Saxena, Alex J. Cameron, Stefano Carniani, Andrew J. Bunker, Santiago Arribas, Rachana Bhatawdekar, Rebecca A. A. Bowler, Kristan N. K. Boyett, Giovanni Cresci, Emma Curtis-Lake, Francesco D'Eugenio, Nimisha Kumari, Tobias J. Looser, Roberto Maiolino, Hannah Ubler, Chris Willott, Joris Witstok

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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 >104 cm−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 characterizing 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.

Original languageEnglish
Pages (from-to)592-603
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Publication statusPublished - 20 Sept 2022


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