Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Harley Katz, Aayush Saxena, Alex J. Cameron, Stefano Carniani, Andrew J. Bunker
| Summary:
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] $lambda4363$/[O III]
$lambda5007$ ratio (RO3) of $0.047$ (or $0.059$ when dust-corrected), implying
either extremely high electron temperatures, $>3times10^4$ K, or gas densities
$>10^4 {rm cm^{-3}}$. 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 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.
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