Nebular continuum in high-redshift galaxies with JWST

Studying the relevance of accounting for both stellar and nebular continuum emission when performing optical spectral fitting has been mainly limited to galaxies in the local Universe. The high-quality spectroscopy provided by JWST is now opening the possibility of carrying out these studies in the younger Universe. We aim to estimate the nebular continuum contribution (X neb) of high-redshift star-forming (SF) galaxies, explore its relation to common tracers and galaxy properties, and test the previously established threshold for significant nebular contribution (X neb = 8%). We select a sample of 54 SF galaxies from the DAWN JWST Archive meeting quality criteria required for spectral fitting with the population synthesis code FADO. The selected galaxies cover the 1.7 < z < 3.9 range and we fit their rest-frame optical spectra to estimate the stellar and nebular continuum emission, plus the physical and evolutionary properties. We show that the H alpha and H beta equivalent widths remain suitable tracers of X neb at high redshift. We also confirm the X neb = 8% threshold, above which neglecting the nebular continuum can significantly bias spectral modelling. Galaxies above this threshold are generally less massive, younger, exhibit higher SF activity and lower dust extinction relative to those below it. Moreover, these galaxies are light and mass dominated by stellar populations younger than 20 Myr, although their mass contribution is likely overestimated due to the outshining effect. Considering the growing amount of galaxy spectroscopic data at Cosmic Noon from JWST and future facilities, such as MOONS, this work reveals the importance of properly accounting for both stellar and nebular continuum emission when performing optical spectral fitting to obtain reliable stellar population and physical properties estimates.