Broad Uv Emission Lines In Type-1 Active Galactic Nuclei: A Note On Spectral Diagnostics And The Excitation Mechanism

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 10(-4.5)-10(+1.0), hydrogen density n(H) similar to 10(7)-10(14) cm(-3), metallicity Z between 0.01 and 100 Z circle dot, and column density in the range 10(21)-10(23) cm(-2). The focus is on the most prominent UV emission lines observed in quasar spectra, namely Nv lambda 1240, Siiv lambda 1397, Oiv]lambda 1402, Civ lambda 1549, Heii lambda 1640, Aliii lambda 1860, Siiii]lambda 1892, and Ciii]lambda 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.