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In this work, we investigate the reliability of spectral synthesis methods inthe estimation of the mean stellar age and metallicity, addressing the questionof which signal-to-noise ratios (S/N) are needed to determine these quantities.To address this problem we used simulated spectra containing stellar andnebular emission, reproducing the evolution of a galaxy for a constant andexponentially declining star formation law. The spectra have been degraded todifferent S/N and analysed with three different spectral synthesis codes: FADO,STARLIGHT, and STECKMAP assuming similar fitting set-ups and the same spectralbases. For S/N > 5 all tools considered show a large diversity in the results.FADO and STARLIGHT find median differences in light-weighted mean stellar agesof ~0.1 dex, while STECKMAP shows a higher value of ~0.2 dex. For S/N > 50 themedian differences in FADO are ~0.03 dex (~7%), a factor 3 and 4 lower than the0.08 dex (~20%) and 0.11 dex (~30%) obtained from STARLIGHT and STECKMAP,respectively. Our results indicate that phases of high specific star formationrate (sSFR) in galaxies require analysis tools that do not neglect the nebularcontinuum emission in the fitting process, since purely stellar models wouldhave strong problems in the estimation of star formation history, even inpresence of high S/N spectra.  The median values of these differences are of the order of 7% (FADO), 20%(STARLIGHT), and 30% (STECKMAP) for light-weighted quantities, and 20% (FADO),60% (STARLIGHT), and 20% (STECKMAP) for mass-weighted quantities. That impliesa severe overestimation of the mass-to-light ratio and stellar mass, even inthe presence of a mild contribution from the nebular continuum. Our workunderlines the importance of a self-consistent treatment of nebular emission,which is the only route towards a reliable determination of the assembly of anyhigh-sSFR galaxy.

Self-consistent population spectral synthesis with FADO: mean stellar metallicity of galaxies in spectral synthesis methods