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When possible, ultraviolet diagnostic tools should be avoided because of their sensitivity to dust extinctions (see the Tenth Commandment and Table 1). However, investigators of the high-redshift universe often have very little choice but to study this region of the electromagnetic spectrum. The ultraviolet domain is potentially a rich source of diagnostic lines. The main emission lines are listed in Table 2. Among the most useful diagnostics to discriminate between AGNs and starbursts are the N V lambda1240 / He II lambda4686, N V lambda1240 / C IV lambda1549, N V lambda1240 / Lyalpha, and C IV lambda1549 / Lyalpha emission-line ratios. As shown in Figure 1, these ratios are sensitive functions of the shape of the ionizing continuum (harder spectra provide more heating per photoionization, therefore increasing the temperature). These line ratios have been used extensively in studies of high-z quasars (e.g., Hamann & Ferland 1999) and radio galaxies (e.g., Röttgering et al. 1997; Villar-Martin et al. 1996, 1999), and the analysis of low-z AGNs/LINERs (e.g., Ho et al. 1996; Barth et al. 1996, 1997; Maoz et al. 1998; Nicholson et al. 1998) and starburst galaxies (e.g., Robert, Leitherer, & Heckman 1993).

Table 2. Ultraviolet Emission-Line Diagnostics

Low-to-Moderate Ionization Lines High-Ionization Lines
Line chi(eV) Line chi(eV)

C III 977 Å 24.4 O VI 1032, 1038 Å 114
N III 991, 1750 Å 29.6 N V 1240 Å 77.4
Lybeta 1026 Å, Lyalpha 1216 Å 13.6 O IV] 1407 Å 54.9
Si IV 1394, 1403 Å 33.5 N IV] 1488 Å 47.4
O III] 1663 Å 35.1 C IV 1549 Å 47.9
N III] 1750 Å 29.6 He II 1085, 1640 Å 54.4
Si III 1895 Å 16.3
C III] 1909 Å 24.4
Fe II 2080, 2500, 3300 Å 7.9
[O III] 2322 Å 35.1
C II] 2326 Å 11.3
Si II 2336 Å 8.2
Mg II 2798 Å 7.6

Figure 1

Figure 1. Predicted UV line flux ratios, gas temperatures, and dimensionless equivalent widths in Lyalpha for clouds photoionized by different power-law spectra (nualpha). The UV-to-X-ray slopes of QSOs are roughly consistent with alpha approx - 1.5. From Hamann & Ferland (1999).

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