Nebular continuum in high-redshift galaxies with JWST

Abstract

The study of 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. Since the start of operations of the Space Telescope (JWST), a wealth of high-quality spectroscopic data have been gathered for high-redshift galaxies, opening the possibility of carrying out these studies in the younger Universe. James Webb We aim to estimate the nebular continuum contribution (X_ neb ) of high-redshift star-forming galaxies, and correlate it with different established tracers and physical properties. Furthermore, we test the previously established threshold for significant nebular contribution (X_ neb $=$ 8%) and characterise the physical and evolutionary properties of galaxies above and below this limit. We selected a sample of 54 star-forming galaxies from the DAWN JWST Archive that meet quality criteria required to perform optical spectral fitting with the population spectral synthesis code FADO, namely R ∼ 1000 and a median signal-to-noise ratio per pixel above 5. The selected galaxies cover the redshift range 1.7 $<$ z $<$ 3.9 and we fitted their rest-frame optical spectra to estimate the stellar and nebular continuum emission, plus the physical and evolutionary properties. We show that the Hα and Hβ equivalent widths remain suitable tracers of X_ neb at high redshift. Additionally, our results are consistent with the threshold of X_ neb $=$ 8%, above which the nebular continuum contribution significantly affects the modelling of galaxies when using purely stellar population spectral synthesis codes. We find that galaxies above this threshold are generally less massive, younger, and they exhibit higher star-forming activity and lower dust extinction relative to those below the threshold. Finally, the results suggest that galaxies with X_ neb $>$ 8% are dominated in both light and mass by stellar populations younger than 20 Myr, although the inferred mass fraction of these young stellar populations is likely overestimated due to the outshining effect. Considering the rapidly growing amount of high-quality JWST spectroscopic data and the arrival of future instruments targeting the peak epoch of star formation in the Universe, such as MOONS, this work underscores the importance of properly accounting for both the stellar and nebular continuum emission when performing optical spectral fitting of high-redshift star-forming galaxies. Accurately treating these components is essential for a reliable estimation of their stellar populations and physical properties, thus improving our understanding of galaxy evolution.