Impact of an AGN featureless continuum on estimation of stellar
  population properties
  The effect of the featureless power-law (PL) continuum of an active galactic
nucleus (AGN) on the estimation of physical properties of galaxies with optical
population spectral synthesis (PSS) remains largely unknown. With this in mind,
we fit synthetic galaxy spectra representing a wide range of galaxy star
formation histories (SFHs) and including distinct PL contributions of the form
$F_{\nu} \propto \nu^{-\alpha}$ with the PSS code STARLIGHT to study to which
extent various inferred quantities (e.g. stellar mass, mean age, and mean
metallicity) match the input. The synthetic spectral energy distributions
(SEDs) computed with our evolutionary spectral synthesis code include an AGN PL
component with $0.5 \leq \alpha \leq 2$ and a fractional contribution $0.2 \leq
x_{\mathrm{AGN}} \leq 0.8$ to the monochromatic flux at 4020 \AA. At the
empirical AGN detection threshold $x_{\mathrm{AGN}}\simeq 0.26$ that we
previously inferred in a pilot study on this subject, our results show that the
neglect of a PL component in spectral fitting can lead to an overestimation by
$\sim$2 dex in stellar mass and by up to $\sim$1 and $\sim$4 dex in the light-
and mass-weighted mean stellar age, respectively, whereas the light- and
mass-weighted mean stellar metallicity are underestimated by up to $\sim$0.3
and $\sim$0.6 dex, respectively. Other fitting set-ups including either a
single PL or multiple PLs in the base reveal, on average, much lower
unsystematic uncertainties of the order of those typically found when fitting
purely stellar SEDs with stellar templates, however, reaching locally up to
$\sim$1, 3 and 0.4 dex in mass, age and metallicity, respectively. Our results
underscore the importance of an accurate modelling of the AGN spectral
contribution in PSS fits as a minimum requirement for the recovery of the
physical and evolutionary properties of stellar populations in active galaxies.
