Starburst-AGN mixing: II. Optically-selected active galaxies
  We use 4 galaxies from the Calar Alto Legacy Integral Field Area (CALIFA)
survey with clear signs of accretion onto supermassive black holes to
investigate the relative contribution of star-formation and active galactic
nucleus (AGN) activity to the line-emission of each galaxy as a function of
radius. The combination of star-formation and AGN activity produces curved
"mixing sequences" on standard optical diagnostic diagrams, and the fraction of
emission due to AGN activity decreases smoothly with distance from the centre
of the galaxy. We use the AGN activity profiles to calculate the size of the
AGN narrow line regions, which have radii of ~ 6.3 kpc. We calculate the
fractional contribution of the star-formation and the AGN activity to the
global Halpha, [O II] $\lambda \lambda$ 3727,3729 and [O III] $\lambda$ 5007
luminosities of each galaxy, and show that both ionization sources contribute
significantly to the emission in all three lines. We use weighted combinations
of stellar and AGN photoionization models to produce mixing models, which are
consistent with observations for 85 percent of spaxels across the galaxies in
our sample. We also present a new diagnostic for starburst-AGN mixing which
requires only the [O II], [O III] and Hbeta emission lines, and can therefore
be used to calculate AGN fractions up to z ~ 0.85 in the optical and z ~ 3.5 in
the near-infrared. We anticipate that this diagnostic will facilitate studies
of the properties of AGN ionizing radiation fields and the relative optical
contribution of star-formation and AGN activity over cosmic time.
