  We have conducted a study to quantify the systematic differences resulting
from using different stellar population models in optical spectroscopic
identification of type II AGN. We examined the different AGN detection
fractions of 7069 nearby galaxies (z <= 0.09) with SDSS DR8 spectra when using
the Bruzual & Charlot (2003, BC03), Vazdekis et al. (2010, MILES), and solar
metallicity Maraston and Stromback (2011) (MS11solar) stellar population
models. The line fluxes obtained using BC03 and MS11solar are publicly
available from SDSS data releases. We find that the BC03 templates result in
systematically higher BPT line ratios and consequently higher AGN fractions and
the MS11solar templates result in systematically lower line ratios and AGN
fractions compared with the MILES templates. Using MILES as the standard, BC03
results in 25% "false positives" and MS11solar results in 22% "false negatives"
when using the Kewley et al. (2001a) boundary for AGN identification. The
fraction of galaxies whose AGN identification changes for different templates
is luminosity dependent, ranging from a few percent for L[OIII]5007 >= 10^40
erg s-1 and increasing to ~ 50% for L[OIII]5007 <= 10^38 erg s-1. These results
suggest that template choice should be accounted for when using and comparing
the AGN and emission line fluxes from different catalogs.

Astrophysics of Galaxies