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We use 4 galaxies from the Calar Alto Legacy Integral Field Area (CALIFA)survey with clear signs of accretion onto supermassive black holes toinvestigate the relative contribution of star-formation and active galacticnucleus (AGN) activity to the line-emission of each galaxy as a function ofradius. The combination of star-formation and AGN activity produces curved"mixing sequences" on standard optical diagnostic diagrams, and the fraction ofemission due to AGN activity decreases smoothly with distance from the centreof the galaxy. We use the AGN activity profiles to calculate the size of theAGN narrow line regions, which have radii of ~ 6.3 kpc. We calculate thefractional contribution of the star-formation and the AGN activity to theglobal Halpha, [O II] $\lambda \lambda$ 3727,3729 and [O III] $\lambda$ 5007luminosities of each galaxy, and show that both ionization sources contributesignificantly to the emission in all three lines. We use weighted combinationsof stellar and AGN photoionization models to produce mixing models, which areconsistent with observations for 85 percent of spaxels across the galaxies inour sample. We also present a new diagnostic for starburst-AGN mixing whichrequires only the [O II], [O III] and Hbeta emission lines, and can thereforebe used to calculate AGN fractions up to z ~ 0.85 in the optical and z ~ 3.5 inthe near-infrared. We anticipate that this diagnostic will facilitate studiesof the properties of AGN ionizing radiation fields and the relative opticalcontribution of star-formation and AGN activity over cosmic time.

Starburst-AGN mixing: II. Optically-selected active galaxies