Ca II and O I as precision probes of the broad-line region in AGN

Ca II and O I emission lines in active galactic nuclei (AGN) have been used as powerful diagnostic probes of the broad-line region (BLR) for the past 40 years.In particular, line ratio diagnostics have been used to constrain the physical conditions in the low-ionization phase of the BLR, while line width measurements have provided rough constraints on the location of the emission region relative to the Balmer-emitting phase of the BLR.However, due to limited observational capabilities, detailed line-profile studies of these linesdirectly linking Ca II and O I with structural BLR models-have only become possible very recently.Here, we present an overview of our most recent study that has enabled investigations of Ca II and O I emission lines in unprecedented detail, directly linking Ca II triplet emission profiles to the kinematics of a rotating disk.We focus on our results for NGC 4593, but we also provide an outlook on how Ca II and O I open up a new observational window to probe the low-ionization part of the BLR in general.