Optical Fe II and near-infrared Ca II emission in active galaxies
  The CaFe Project involves the study of the properties of the low ionization
emission lines (LILs) pertaining to the broad-line region (BLR) in active
galaxies. These emission lines, especially the singly-ionized iron (Fe II) in
the optical and the corresponding singly-ionized calcium (Ca II) in the
near-infrared (NIR) are found to show a strong correlation in their emission
strengths, i.e., with respect to the broad H$\beta$ emission line, the latter
also belonging to the same category of LILs. We outline the progress made in
the past years that has developed our understanding of the location and the
efficient production of these emission lines. We have yet to realize the full
potential of Ca II emission and its connection to the black hole and the BLR
parameters which can be useful in - (1) the classification of Type-1 active
galactic nuclei (AGNs) in the context of the main sequence of quasars, (2) to
realize an updated radius-luminosity relation wherein the inclusion of the
strength of this emission line with respect to H$\beta$ can be an effective
tracer of the accretion rate of the AGN, and, (3) the close connection of Ca II
to Fe II can allow us to use the ratio of the two species to quantify the
chemical evolution in these active galaxies across cosmic time. In this paper,
we use our current sample and utilize a non-linear dimensionality reduction
technique - t-distributed Stochastic Neighbour Embedding (tSNE), to understand
the clustering in our dataset based on direct observables.
