Catalog of Anomalous Quasars from SDSS DR16

Principal component analysis (PCA) was employed for dimensionality reduction of the quasar spectra, followed by hierarchical K-Means clustering in a 20-dimensional PCA eigenvector hyperspace. To prevent broad absorption line (BAL) quasars from being identified as the primary anomaly group, we conducted the analysis separately on BAL and Non-BAL QSOs, comparing both categories for a clearer identification of other anomalous quasar type.

We identified 2066, anomalous quasars, categorized into 10 broad groups. The anomalous groups include CIV Peakers-quasars with extremely strong and narrow CIV emission lines; Excess SiIV emitters-quasars where the SiIV line is as strong as the CIV line; and SiIV deficient anomalies, which exhibit significantly weaker SiIV emission compared to typical quasars. The anomalous nature of these quasars is attributed to lower Eddington ratios for CIV Peakers, super-solar metallicity for Excess SiIV emitters, and sub-solar metallicity for Si iv deficient anomalies.