| Summary: | This paper reviews several basic emission properties of the UV emission lines observed in the spectra of quasars and type-1 active galactic nuclei, mainly as a function of the ionization parameter, metallicity, and density of the emitting gas. The analysis exploits a general-purpose 4D array of the photoionization simulations computed using the code CLOUDY, covering ionization parameter in the range <inline-formula><math display="inline"><semantics><msup><mn>10</mn><mrow><mo>−</mo><mn>4.5</mn></mrow></msup></semantics></math></inline-formula>–<inline-formula><math display="inline"><semantics><msup><mn>10</mn><mrow><mo>+</mo><mn>1.0</mn></mrow></msup></semantics></math></inline-formula>, hydrogen density <inline-formula><math display="inline"><semantics><msub><mi>n</mi><mi mathvariant="normal">H</mi></msub></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><mrow><mspace width="3.33333pt"></mspace><mo>∼</mo><msup><mn>10</mn><mn>7</mn></msup></mrow></semantics></math></inline-formula>–<inline-formula><math display="inline"><semantics><msup><mn>10</mn><mn>14</mn></msup></semantics></math></inline-formula> cm<sup>−3</sup>, metallicity <i>Z</i> between 0.01 and 100 <inline-formula><math display="inline"><semantics><msub><mi>Z</mi><mo>⊙</mo></msub></semantics></math></inline-formula>, and column density in the range <inline-formula><math display="inline"><semantics><msup><mn>10</mn><mn>21</mn></msup></semantics></math></inline-formula>–<inline-formula><math display="inline"><semantics><msup><mn>10</mn><mn>23</mn></msup></semantics></math></inline-formula> cm<sup>−2</sup>. The focus is on the most prominent UV emission lines observed in quasar spectra, namely N<span style="font-variant: small-caps;">v</span><inline-formula><math display="inline"><semantics><mi>λ</mi></semantics></math></inline-formula>1240, Si<span style="font-variant: small-caps;">iv</span><inline-formula><math display="inline"><semantics><mi>λ</mi></semantics></math></inline-formula>1397, O<span style="font-variant: small-caps;">iv</span><inline-formula><math display="inline"><semantics><mi>λ</mi></semantics></math></inline-formula>1402, <span style="font-variant: small-caps;">Civ</span><inline-formula><math display="inline"><semantics><mi>λ</mi></semantics></math></inline-formula>1549, He<span style="font-variant: small-caps;">ii</span><inline-formula><math display="inline"><semantics><mi>λ</mi></semantics></math></inline-formula>1640, Al<span style="font-variant: small-caps;">iii</span><inline-formula><math display="inline"><semantics><mi>λ</mi></semantics></math></inline-formula>1860, Si<span style="font-variant: small-caps;">iii</span><inline-formula><math display="inline"><semantics><mi>λ</mi></semantics></math></inline-formula>1892, and C<span style="font-variant: small-caps;">iii</span><inline-formula><math display="inline"><semantics><mi>λ</mi></semantics></math></inline-formula>1909, and on the physical conditions under which electron-ion impact excitation is predicted to be the dominant line producer. Photoionization simulations help constrain the physical interpretation and the domain of applicability of spectral diagnostics derived from measurements of emission line ratios, reputed to be important for estimating the ionization degree, density, and metallicity of the broad line emitting gas, as well as the relative intensity ratios of the doublet or multiplet components relevant for empirical spectral modeling.
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