Self-consistent population spectral synthesis with FADO - I. The
  importance of nebular emission in modelling star-forming galaxies
  Spectral population synthesis (PS) is a fundamental tool in extragalactic
research that aims to decipher the assembly history of galaxies from their SED.
However, until recently all PS codes were restricted to purely stellar fits,
neglecting the essential contribution of nebular emission (NE). With the advent
of FADO, the now possible self-consistent modelling of stellar and NE opens new
routes to the exploration of galaxy SFHs. The main goal of this study is to
quantitatively explore the accuracy to which FADO can recover physical and
evolutionary properties of galaxies and compare its output with that from
purely stellar PS codes. With this in mind, FADO and STARLIGHT were applied to
synthetic SEDs that track the spectral evolution of stars and gas in
extinction-free mock galaxies that form their stellar mass ($M_\star$)
according to different parametric SFHs. Spectral fits were computed for two
different set-ups that approximate the spectral range of SDSS and CALIFA data.
Our analysis indicates that FADO can recover the key physical and evolutionary
properties of galaxies, such as $M_\star$ and mass- and light-weighted mean age
and metallicity, with an accuracy better than 0.2 dex. This is the case even in
phases of strongly elevated sSFR and thus with considerable NE contamination.
As for STARLIGHT, our analysis documents a moderately good agreement with
theoretical values only for evolutionary phases for which NE drops to low
levels. Indeed, fits with STARLIGHT during phases of high sSFR severely
overestimate both $M_\star$ and the mass-weighted stellar age, whereas strongly
underestimate the light-weighted age and metallicity. The insights from this
study suggest that the neglect of nebular continuum emission in STARLIGHT and
similar purely stellar PS codes could systematically impact $M_\star$ and SFH
estimates for star-forming galaxies.
