Determining star-formation rates in Active Galactic Nuclei hosts via
  stellar population synthesis
  The effect of active galactic nuclei (AGN) feedback on the host galaxy, and
its role in quenching or enhancing star-formation, is still uncertain due to
the fact that usual star-formation rate (SFR) indicators -- emission-line
luminosities based on the assumption of photoionisation by young stars --
cannot be used for active galaxies as the ionising source is the AGN. We thus
investigate the use of SFR derived from the stellar population and its relation
with that derived from the gas for a sample of 170 AGN hosts and a matched
control sample of 291 galaxies. We compare the values of SFR densities obtained
via the Ha emission line (SFRg) for regions ionised by hot stars according to
diagnostic diagrams with those obtained from stellar population synthesis
(SFRstars) over the last 1 to 100~Myr. We find that the SFRstars over the last
20~Myrs closely reproduces the SFRg, although a better match is obtained via
the transformation: log(SFRstars) = (0.872+/-0.004) log(SFRg) -(0.075+/-0.006),
which is valid for both AGN hosts and non-active galaxies. We also compare the
reddening obtained via the gas Ha/Hb ratio with that derived via the full
spectral fitting in the stellar population synthesis. We find that the ratio
between the gas and stellar extinction is in the range 2.64 < A_Vgas/A_Vstar} <
2.85, in approximate agreement with previous results from the literature,
obtained for smaller samples. We interpret the difference as being due to the
fact that the reddening of the stars is dominated by that affecting the less
obscured underlying older population, while the reddening of the gas is larger
as it is associated to a younger stellar population buried deeper in the dust.
