4. OPTICAL EMISSION-LINE DIAGNOSTICS

The excellent quantum efficiency of current CCDs combined with the high transparency and low emissivity of the Earth's atmosphere at optical wavelengths make optical spectroscopy the easiest way to identify AGNs. Table 3 lists the strongest diagnostic lines between 3000 Å and 1 µm. Classification schemes involving several line ratios which take full advantage of the physical distinction between the two types of objects and minimize the effects of reddening correction and errors in the flux calibration have proven very useful for the identification of galaxies as AGNs or starbursts (e.g., Phillips, Baldwin, & Terlevich 1981; Veilleux & Osterbrock 1987; Osterbrock, Tran, & Veilleux 1992; Dopita et al. 2000). Examples of emission-line diagrams are shown in Figure 2 for ultraluminous infrared galaxies from the 1-Jy sample (ULIGs; these are IRAS galaxies with infrared luminosities between 8 and 1000 µm larger than or equal to 10¹² L; Kim 1995). The results from this classification indicates that the fraction of Seyfert nuclei increases from ~ 5% at log[L_IR / L] = 10 - 11, to ~ 50% at log[L_IR / L] > 12.3 (Veilleux et al. 1995, Kim, Veilleux, & Sanders 1998; Veilleux, Kim, & Sanders 1999a).

Table 3. Optical Emission-Line Diagnostics

Low-to-Moderate Ionization Lines		High-Ionization Lines
Line	(eV)	Line	(eV)
[O II] 3727, 7325 Å	13.6	[Ne V] 3346, 3426 Å	97.1
[Ne III] 3869, 3968 Å	41.0	[Fe V] 3840, 3893, 4071 Å	54.8
[O III] 4363, 5007 Å	35.1	[Fe VII] 3588, 3760, 4071, 5721, 6087 Å	99.0
Fe II 4500, 5190, 5300 Å	7.9	He II 4686 Å	54.4
H 4861 Å, H 6563 Å	13.6	[Fe XIV] 5303 Å	344
He I 5876, 7065 Å	24.6	[Fe X] 6375 Å	235
[O I] 6300, 6363 Å	0.0	[Fe XI] 7892 Å	262
[N II] 5755, 6548, 6583 Å	14.5
[S II] 6716, 6731 Å	10.4
[S III] 6312, 9069, 9531 Å	23.3

Figure 2. Examples of optical line ratio diagrams used for the classification of ultraluminous infrared galaxies. From Veilleux et al. (1999a).