Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Sandro Tacchella, Benjamin D. Johnson, Brant E. Robertson, Stefano Carniani, Francesco D’Eugenio
| Summary:
We present an interstellar medium and stellar population analysis of three
spectroscopically confirmed $z>7$ galaxies in the ERO JWST/NIRCam and
JWST/NIRSpec data of the SMACS J0723.3-7327 cluster. We use the Bayesian
spectral energy distribution (SED) fitting code Prospector with a flexible
star-formation history (SFH), a variable dust attenuation law, and a
self-consistent model of nebular emission (continuum and emission lines).
Importantly, we self-consistently fit both the emission line fluxes from
JWST/NIRSpec and the broad-band photometry from JWST/NIRCam, taking into
account slit-loss effects. We find that these three $z=7.6-8.5$ galaxies
($M_{star}approx10^{8}~M_{odot}$) are young with rising SFHs and
mass-weighted ages of 3-7 Myr, though we find indications for underlying older
stellar populations. The inferred gas-phase metallicities broadly agree with
the direct metallicity estimates from the auroral lines. The galaxy with the
lowest gas-phase metallicity ($mathrm{Z}_{rm gas}=0.06~mathrm{Z}_{odot}$)
has a steeply rising SFH, is very compact ($<0.2~mathrm{kpc}$) and has a high
star-formation rate surface density ($Sigma_{rm
SFR}approx38~mathrm{M}_{odot}~mathrm{yr}^{-1}~mathrm{kpc}^{-2}$),
consistent with rapid gas accretion. The two other objects with higher
gas-phase metallicity show more complex multi-component morphologies on kpc
scales, indicating that their recent increase in star-formation rate is driven
by mergers or internal, gravitational instabilities. We discuss effects of
assuming different SFH priors or only fitting the photometric data. Our
analysis highlights the strength and importance of combining JWST imaging and
spectroscopy for fully assessing the nature of galaxies at the earliest epochs.
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