Dependence of Optical Active Galactic Nuclei Identification on Stellar Population Models

We have conducted a study to quantify the systematic differences resulting from using different stellar population models (SPM) in optical spectroscopic identification of type II active galactic nuclei (AGNs). We examined the different AGN detection fractions of 7069 nearby galaxies (z ≤ 0.09) with Sloan Digital sky Survey (SDSS) DR8 spectra when using the Bruzual & Charlot (BC03), Vazdekis et al. (MILES), and solar metallicity Maraston & Strömbäck (MS11_solar) SPM. The line fluxes obtained using BC03 and MS11_solar 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 MS11_solar 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 MS11_solar results in 22% "false negatives" when using the Kewley et al. 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 _{[O iii]5007} ≥ 10⁴⁰ erg s^-1 and increasing to ∼50% for L _{[O iii]5007} ≤ 10³⁸ 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.