7.1. Contribution from Shocks
Violent gas motions associated with AGN-driven or starburst-driven outflows or galaxy mergers may cause shock waves with velocities of 100 - 500 km s-1 in the ISM of the host galaxies. The shocks may produce a strong flux of EUV and soft X-ray radiation which may be absorbed in the shock precursor H II region (e.g., Sutherland, Bicknell, & Dopita 1995). The combination of the low-ionization emission-line spectrum from the post-shock material and the high-ionization emission-line spectrum from the precursor H II region can reproduce many of the spectroscopic signatures of LINERs and narrow-line AGNs (e.g., Dopita & Sutherland 1995). Fortunately, there are important physical differences between shock ionization and photoionization by an AGN (e.g., Morse, Raymond, & Wilson 1996). First, the line ratios produced in photoionized objects should be independent of the gas kinematics, while they are expected to correlate with the kinematics of the shock-ionized material. This effect is seen in a few optically and infrared-selected LINERs (Veilleux et al. 1994, 1995). The ionizing ultraviolet and soft X-ray continuum in shock-ionized objects should be extended on the same scale as the shock structure, while it is expected to be a point source in the case of pure AGN photoionization. Finally, the electron temperature in shock ionized objects is expected to be considerably higher. Temperature-sensitive line ratios such as C III 1909 / 977 and N III 1750 / 991 in the ultraviolet and [O III] 5007 / 4363 and [N II] 5755 / 6583 in the optical range are the prime diagnostics of shock excitation. This method was used by Kriss et al. (1992) to deduce that shock excitation is likely to be important in the NLR of NGC 1068.
7.2. Aperture Effects
Circumnuclear starbursts often accompany AGNs (see, e.g., recent reviews by Veilleux 2000 and Gonzales Delgado 2001). The strength of the AGN signature is therefore a function of the size of the extraction aperture. This effect is particularly evident among infrared-selected galaxies where circumnuclear starbursts are nearly always present. Figure 5 shows the line ratios of luminous infrared galaxies as function of aperture size. The line ratios in some of these objects are seen to drift towards the H II region locus with increasing aperture size; large apertures dilute the AGN signature. Aperture effects will be particularly important in samples which cover a broad redshift range where a constant angular aperture corresponds to a wide range in linear scale. For a meaningful statistical analysis of the spectral classification one should use a fixed linear aperture for all objects in the sample (regardless of redshifts).
Figure 5. Aperture effects. Line ratios as a function of the size of the extraction aperture. The asterisks mark the nuclear values. The size of the extraction aperture increases (generally doubles) between each data point. From Veilleux et al. (1995).
7.3. Morphological Biases
Strong trends exist between the presence of an AGN, the mid-to-far infrared colors, and the host morphology. Objects with "warm" infrared colors (e.g., IRAS f25 / f60 > 0.2) often harbor an AGN at optical or near-infrared wavelengths or in polarized light (de Grijp et al. 1985; Veilleux et al. 1995, 1997b, 1999a, b; Heisler, Lumsden, & Bailey 1997). Infrared-selected samples are often biased towards or against the presence of AGNs (but this is not the case for the 1-Jy sample; Kim & Sanders 1998). The same thing can be said about galaxy morphology. ULIGs often show signs of galaxy interactions. Most ULIGs are involved in the merger of two relatively large galaxies. Optically-classified Seyferts (especially those of type 1) are generally found in advanced mergers, while H II galaxies and LINERs are found in all merger phases (Fig. 6; see also Veilleux 2001). This means that surveys which specifically look for compact objects will be biased against starburst galaxies and are not statistically reliable for spectral classification purposes.
Figure 6. Morphological biases among infrared-selected AGNs. The hosts of ultraluminous infrared galaxies which are optically classified as Seyferts generally are advanced mergers (morphological classes IVa, IVb, or V). From Veilleux et al. (2002, in prep.). See also Veilleux (2001).
7.4. Metallicity Effects
The line ratio diagnostics discussed in this review often are a sensitive function of the metal contents in the ionized gas (see, e.g., Ferland & Netzer 1983; Veilleux & Osterbrock 1987 for early papers describing the effects of metallicity). The metallicity is well known to be correlated positively with the mass of the host galaxies (e.g., Bender, Burnstein, & Faber 1993), although this result has only been proven at low redshifts. In the early universe, one would expect declining metal abundances with increasing redshifts. The redshift dependence of the relative abundances of the elements involved in the emission-line ratios is a complex function of the star formation history and chemical evolution (including the effects of gas accretion and outflows) of the host galaxy environment (see, e.g., Hamann & Ferland 1999 for a discussion of QSO hosts). The usefulness of emission-line diagnostics at high redshifts will directly depend on the availability of accurate metallicity measurements and diagnostic tools properly calibrated in terms of metallicity.