Importance of modelling the nebular continuum in galaxy spectra

Neglecting to model stellar and nebular emission has been shown to have a significant impact on the derived physical properties of galaxies experiencing high levels of star formation. This impact has been seen at low redshifts for galaxies in a period of extremely significant star formation, the so-called extreme emission-line galaxies. It has also been suggested as a more general phenomenon among star-forming galaxies at high-redshifts. Even though various studies have approached the issue, a clear limit for the relevant effect of nebular contribution to the total optical emission has not been established. We aim to correlate the nebular contribution in the optical regime with different tracers and to define a threshold, in terms of the nebular contribution, above which there is a significant impact on the estimation of physical properties of galaxies. Additionally, we want to investigate the implication of the results for high-redshift galaxies. We selected a sample of galaxies from SDSS-DR7 with a wide range of star-forming activity levels and analysed their spectra with two conceptually distinct spectral fitting tools: one that self-consistently models stellar and nebular emission and ensures that the best-fitting star formation and chemical enrichment history obtained reproduces the observed nebular characteristics of a galaxy (FADO), and another that lacks such a self-consistency concept (STARLIGHT) and was applied using a purely stellar base. We estimated the nebular contribution and correlate it with different tracers. Then, we compared the stellar properties estimated by the two spectral fitting tools for different degrees of optical nebular contribution. Additionally, we estimated the stellar properties using FADO in pure-stellar mode to further strengthen the robustness of our results. The rest-frame Hα and Hβ equivalent widths (EWs) show a strong linear correlation with the optical nebular contribution and are suitable tracers. We find that for an optical nebular contribution above 8%, which corresponds to EW(Hα)≃500 Å, and EW(Hβ)≃110 Å, there is a significant impact on the estimated physical properties and underlying stellar populations of a galaxy. Given the different definition of FADO for the continuum, this threshold actually corresponds to EW(Hα)≃375 Å, for works considering a pseudo-continuum, which is more commonly used in the literature. These findings were corroborated when considering the results from the application of FADO in pure-stellar mode. Considering the observed redshift evolution of EW(Hα), galaxies in the stellar mass range between M_*=10^7--10^11 M_⊙ will reach, on average, this threshold in the z∼2--6 interval and the optical nebular contribution cannot be neglected. Our results highlight the importance of taking into account both stellar and nebular continuum when analysing the optical spectra of star-forming galaxies. In particular, this is a fundamental aspect for galaxies with a rest-frame EW(Hα)≳500 Å (or the scaled value of 375Å for pseudo-continuum measures). At low redshifts, this mostly impacts extreme emission line galaxies, while at higher redshifts it becomes a dominant aspect given the higher star-forming activity in the younger Universe. In light of current JWST observations and future instruments designed for high-redshift observations, such as MOONS, this reveals a critical issue that ought to be taken into consideration.