2.1.3. Diagnostic diagrams and composite nuclear spectra

shown empirically that several combinations of easily-measured emission lines can be used to separate emission line galaxies into three categories according to the principal excitation mechanism: nuclear H II regions or starbursts, Seyfert 2s and Liners (Fig. 2 shows typical spectra of a Seyfert 1, a Seyfert 2, a Liner and a starburst galaxy). They found that the three groups can be effectively segregated using plots of [N II] 6583 / H vs [O III] 5007 / H and of [O II]3727 / [O III]5007 vs [O III]5007 / H, [N II]6583 / H, or [O I]6300 / H.

Veilleux & Osterbrock (1987) have proposed a revised method of classification involving the line ratios 5007 / H, 6583 / H, [S II]6717,6731 / H and 6300 / H. These line ratios take full advantage of the physical distinctions between the various types of objects and minimize the effects of reddening correction and calibration errors. The 6583 / H, 6717,6731 / H and 6300 / H ratios are well correlated with each others ([199]), therefore all these diagrams should basically lead to the same classification.

Figure 2. Spectrum of a Seyfert 1 nucleus (IC 4329A), a Seyfert 2 nucleus (NGC 5135), a Liner (NGC 1052) and a high excitation starburst galaxy (IC 4870). These four spectra have been taken with the ESO 1.5m telescope.

The use of Veilleux & Osterbrock (1987) diagnostic diagrams generally yields an immediate classification of the nuclear emission line clouds; ``transition'' objects however exist which cannot be classified unambiguously from their line ratios ([438]). When observed with sufficient spectral resolution, such objects show different profiles for the lines, this being due to the superposition of several components that have different relative strengths and are kinematically and spatially distinct, usually a H II region and a Seyfert cloud; they have a ``composite'' spectrum ([298]; Gonçalves et al. 1999).

Fig. 3 shows the Veilleux & Osterbrock diagnostic diagrams for a large number of nuclear emission line galaxies with published line ratios, excluding objects which have been shown to have a composite spectrum; we have empirically drawn lines enclosing regions containing H II regions, Liners and Seyfert 2s.

Diagnostic diagrams using near-IR lines such as [O II]7320,7330 and [S III]9069,9531 also provide a powerful method in classifying emission line galaxies ([328]).

Several emission features in the mid-infrared (MIR) allow to distinguish between starbursts and AGNs. The 7.7µm ``polycyclic aromatic hydrocarbon'' (PAH) emission feature is strong in starbursts and weak in AGNs (Lutz et al. 1998; 1999). On the other hand, the high-excitation lines [O IV]25.9µm and [Ne V]14.3µm are strong relative to [Ne II]12.8µm in AGNs and weak in starbursts ([139]; [450]).

Figure 3. Diagnostic diagrams for all nuclear emission line galaxies with published line ratios, excluding objects with a composite spectrum. The regions containing H II regions, Seyfert 2s and Liners have been drawn empirically. A few Seyfert 2s with weak [N II] lines appear outside the region assigned to Seyfert 2s. A few other objects falling in ``intermediate'' regions deserve a more detailed study.