  In this work, we investigate the reliability of spectral synthesis methods in
the estimation of the mean stellar age and metallicity, addressing the question
of which signal-to-noise ratios (S/N) are needed to determine these quantities.
To address this problem we used simulated spectra containing stellar and
nebular emission, reproducing the evolution of a galaxy for a constant and
exponentially declining star formation law. The spectra have been degraded to
different S/N and analysed with three different spectral synthesis codes: FADO,
STARLIGHT, and STECKMAP assuming similar fitting set-ups and the same spectral
bases. 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 ages
of ~0.1 dex, while STECKMAP shows a higher value of ~0.2 dex. For S/N > 50 the
median differences in FADO are ~0.03 dex (~7%), a factor 3 and 4 lower than the
0.08 dex (~20%) and 0.11 dex (~30%) obtained from STARLIGHT and STECKMAP,
respectively. Our results indicate that phases of high specific star formation
rate (sSFR) in galaxies require analysis tools that do not neglect the nebular
continuum emission in the fitting process, since purely stellar models would
have strong problems in the estimation of star formation history, even in
presence 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 implies
a severe overestimation of the mass-to-light ratio and stellar mass, even in
the presence of a mild contribution from the nebular continuum. Our work
underlines the importance of a self-consistent treatment of nebular emission,
which is the only route towards a reliable determination of the assembly of any
high-sSFR galaxy.
