Implications of CA II emission for physical conditions in the broad-line region of active galactic nuclei

Abstract

This paper summarizes observations of Ca II emission from AGN and discusses their implications for conditions in the emitting gas. Observations of Ca II K and H, the infrared triplet, and the forbidden lines near 7300 A are compiled to give a 'standard' set of line ratios for AGN with relatively strong Fe II emission. A series of photoionization calculations is presented which examines the conditions needed to produce the observed emission. Special attention is paid to the inclusion of heating due to free-free and H(-) absorption processes which couple the near-infrared to millimeter continuum with the emitting gas. It is found that, regardless of density or ionization parameters, thin clouds cannot produce the observed Ca II emission. The semiforbidden C III 1909 A/C IV 1549 A intensity ratio is not a good indicator of the ionization parameter for this continuum. Clouds with large column densities and ionization parameters largely reproduce the observed carbon, calcium, and hydrogen spectrum.