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Activity driven by mass accretion onto supermassive black holes differs in many ways from star-formation activity. The thermal and non-thermal processes associated with the accretion disk and its surroundings (e.g., corona) are at the origin of the "hard" ionizing continuum detected in quasars and AGNs (e.g., Krolik 1999). Material in the vicinity of the nucleus will bear the imprint of this strong radiation field. The deep gravitational potential at the center of these galaxies allows the presence of high-density (gtapprox 109 cm-3), high-velocity (gtapprox 2000 km s-1) gas clouds in the inner parsec of quasars and AGNs. This so-called broad-line region or BLR is a powerful diagnostic of nuclear activity in galaxies. The main signatures of the BLRs are broad recombination lines which are unaffected by the effects of collisional de-excitation at high densities. Two general methods have been used in the past to detect BLRs in galaxies: direct spectroscopy and spectropolarimetry. This last method relies on the presence of dust or electrons ("mirrors") to scatter the BLR signature towards the line of sight (e.g., Antonucci 1993). Direct spectroscopy searches for the presence of the broad recombination lines at wavelengths where the effects of dust extinction are reduced. As shown in Table 1 for representative Galactic extinction (see, e.g., Cardelli, Clayton, & Mathis 1989; Draine & Lee 1984; Draine 1989; Lutz et al. 1996; Lutz 1999), great increase in sensitivity can in principle be obtained by observing at longer wavelengths.

Table 1. Galactic Dust Extinction and Column Densities

lambda tau(lambda) / tau(Halpha) NH(cm-2) @ tau(lambda) = 1

Lyalpha 1216 Å 2.0 - 4.5 0.5 - 1.0 × 1021
V band 5500 Å 1.2 1.7 × 1021
Halpha 6563 Å 1.0 2.2 × 1021
J band 1.25 µm 1/3 6.1 × 1021
H band 1.65 µm 1/4.5 9.8 × 1021
K band 2.2 µm 1/7 1.6 × 1022
L band 3.4 µm 1/15 3.4 × 1022
M band 5.0 µm 1/30 6.4 × 1022
N band 10 µm 1/15 3.2 × 1022
12 µm 1/30 6.2 × 1022
25 µm 1/60 1.3 × 1023
60 µm 1/400 8.6 × 1023
100 µm 1/700 1.5 × 1024

In highly obscured objects with NH gtapprox 1024 cm-2, direct detection of the BLRs becomes very difficult and one has to rely on spectropolarimetry to search for the presence of a BLR. The obscuring screen may not be opaque in all directions, however. The ionizing radiation field may be able to escape in certain directions and ionize the surrounding material on scales beyond the obscuring material. Distributed in the shallower portion of the gravitational potential (~ 0.1 - 1 kpc), this "narrow-line region" or NLR is another excellent probe of nuclear activity. The ionizing spectra of all but the hottest O stars cut off near the He II edge (54.4 eV; Dopita et al. 1995). In contrast, the ionizing spectrum of AGNs contains a relatively large fraction of high-energy photons (e.g., Elvis et al. 1994). Optically thick gas clouds ionized by the hard continuum of AGNs will present a stratified ionization structure with (1) a highly ionized inner face (closest to the AGN), (2) a large partially zone with characteristic fraction of ionized hydrogen H+/H ~ 0.2 - 0.4 produced by the deposition of keV X-rays (recall that the absorption cross sections of H0, He0, and all other ions decrease rapidly with increasing energy; Osterbrock 1989), and (3) a neutral zone facing away from the AGN. The fast free electrons in the partly ionized zone will have a positive effect on the strengths of low-ionization lines produced by collisional effects, while the highly ionized conditions in the inner face will favor the production of emission lines from ions with high ionization potentials (e.g., Ferland & Netzer 1983; Ferland & Osterbrock 1986, 1987; Binette, Wilson, & Storchi-Bergmann 1996).

Based on these physical principles, one should choose narrow emission line diagnostics following ten basic rules or "Commandments" (a reminder of the 1700th anniversary of the adoption of Christianity as a national religion in Armenia):

  1. Thou shalt use lines which emphasize the differences between H II regions and AGNs; i.e., use high-ionization lines or low-ionization lines produced in the partially ionized zone.
  2. Thou shalt use strong lines which are easy to measure in typical spectra.
  3. Thou shalt avoid lines which are badly blended with other emission or absorption line features.
  4. Thou shalt use lines with small wavelength separation to minimize sensitivity to reddening.
  5. Thou shalt use line ratios from the same elements or involving hydrogen recombination lines to eliminate or reduce abundance dependence.
  6. Thou shalt avoid lines from Mg, Si, Ca, Fe - depleted onto dust grains.
  7. Thou shalt use lines easily accessible to current UV/optical/IR detectors.
  8. Thou shalt avoid lines affected by strong stellar absorption features.
  9. Thou shalt avoid lines affected by strong atmospheric features.
  10. Thou shalt use lines at long wavelengths to reduce the effects of dust extinction.

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