We have modeled a large sample of infrared starburst galaxies using both the PEGASE v2.0 and STARBURST99 codes to generate the spectral energy distribution (SED) of the young star clusters. PEGASE utilizes the Padova group tracks, while STARBURST99 uses the Geneva group tracks, allowing comparison between the two. We used our MAPPINGS III code to compute photoionization models that include a self-consistent treatment of dust physics and chemical depletion. We use the standard optical diagnostic diagrams as indicators of the hardness of the EUV radiation Ðeld in these galaxies. These diagnostic diagrams are most sensitive to the spectral index of the ionizing radiation Ðeld in the 1È4 ryd region. We Ðnd that warm infrared starburst galaxies contain a relatively hard EUV Ðeld in this region. The PEGASE ionizing stellar continuum is harder in the 1È4 ryd range than that of STARBURST99. As the spectrum in this regime is dominated by emission from Wolf-Rayet (W-R) stars, this discrepancy is most likely due to the di †erences in stellar atmosphere models used for the W-R stars. The PEGASE models use the Clegg & Middlemass planetary nebula nuclei (PNN) atmosphere models for the W-R stars, whereas the STARBURST99 models use the Schmutz, Leitherer, & Gruenwald W-R atmosphere models. We believe that the Schmutz et al. atmospheres are more applicable to the starburst galaxies in our sample ; however, they do not produce the hard EUV Ðeld in the 1È4 ryd region required by our observations. The inclusion of continuum metal blanketing in the models may be one solution. Supernova remnant (SNR) shock modeling shows that the contribution by mechanical energy from SNRs to the photoionization models is >20%. The models presented here are used to derive a new theoretical classiÐcation scheme for starbursts and active galactic nucleus (AGN) galaxies based on the optical diagnostic diagrams.

In this paper, we present high-resolution optical spectra and optical classiÐcations from our large sample of 285 warm infrared galaxiesWe have classiÐed these galaxies using new 108 \ L IR \ 1012.5 L _ . theoretical lines on the standard optical diagnostic diagrams. We use a theoretical extreme mixing line between the starburst and AGN regions to classify LINER galaxies and we deÐne a theoretical boundary separating AGNs from starbursts. We Ðnd that many galaxies previously classiÐed as LINERs appear to lie on a mixing sequence between starburst and AGN type galaxies. These are likely 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 radiation Ðeld associated with an AGN or shock excitation where the shock may result from such processes as cooling Ñows, superwind activity, or an accretion disk around an AGN. We compare our theory-based classiÐcation scheme with the previous semiempirical scheme of Veilleux & Osterbrock . We Ðnd that our classiÐcation method results in 6% ambiguity in classiÐcations between the di †erent diagnostic diagrams compared with 16% ambiguity using the traditional Veilleux & Osterbrock method. We Ðnd that 70% of the galaxies in our sample are classiÐed optically as starburst, 17% are Seyfert 2, 4% are Seyfert 1, and 0.4% are LINERs. A further 2% of our sample are certainly composite galaxies. A fraction (20%) of the Seyfert galaxies, 3% of the starburst galaxies, and 71% of the ambiguous galaxies are possibly composite in nature (11% of the total sample).

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