Optical classification of southern warm infrared galaxies

In this paper, we present high-resolution optical spectra and optical class ifications from our large sample of 285 warm infrared galaxies 10(8) < L-IR < 10(12.5) L-.. We have classified these galaxies using new theoretical li nes on the standard optical diagnostic diagrams. We use a theoretical extre me mixing line between the starburst and AGN regions to classify LINER gala xies and we define a theoretical boundary separating AGNs from starbursts. We find that many galaxies previously classified as LINERs appear to lie on a mixing sequence between starburst and AGN type galaxies. These are likel y to be of a composite nature with their excitation being a combination of photoionization due to hot stars plus either ionization by a power-law radi ation field associated with an AGN or shock excitation where the shock may result from such processes as cooling flows, superwind activity, or an accr etion disk around an AGN. We compare our theory-based classification scheme with the previous semiempirical scheme of Veilleux & Osterbrock. We find t hat our classification method results in 6% ambiguity in classifications be tween the different diagnostic diagrams compared with 16% ambiguity using t he traditional Veilleux & Osterbrock method. We find that 70% of the galaxi es in our sample are classified optically as starburst, 17% are Seyfert 2, 4% are Seyfert 1, and 0.4% are LINERs. A further 2% of our sample are certa inly composite galaxies. A fraction (20%) of the Seyfert galaxies, 3% of th e starburst galaxies, and 71% of the ambiguous galaxies are possibly compos ite in nature (11% of the total sample).