Aim

We present the results of applying anomaly detection algorithms to a quasar spectroscopic subsample from the SDSS DR16 quasar catalog, covering the redshift range of 1.88 $\leq$ z $\leq$ 2.47. A principal component analysis (PCA) was employed for the dimensionality reduction of the quasar spectra, followed by a 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 separate analyses on BAL and non-BAL quasars (a.k.a. QSOs), comparing both classes for a clearer identification of other anomalous quasar types. We identified 2066 anomalous quasars, categorized into 10 broadly defined groups. The anomalous groups include: C IV Peakers: quasars with extremely strong and narrow C IV emission lines; Excess Si IV emitters: quasars where the Si IV line is as strong as the C IV line; and Si IV deficient anomalies: which exhibit significantly weaker Si IV emission compared to typical quasars. The anomalous nature of these quasars is attributed to lower Eddington ratios for C IV Peakers, supersolar metallicity for Excess Si IV emitters, and subsolar metallicity for Si IV deficient anomalies. Additionally, we identified four groups of BAL anomalies: blue BALs, flat BALs, reddened BALs, and FeLoBALs, distinguished primarily by the strength of reddening in these sources. Furthermore, among the non-BAL quasars, we identified three types of reddened anomaly groups classified as heavily reddened, moderately reddened, and plateau-shaped spectrum quasars, each exhibiting varying degrees of reddening. We present the detected anomalies as an accompanying value-added catalog.

Catalog

Conclusion

Applying hierarchical k-means clustering in a 20-dimensional PCA eigenvector hyperspace representing quasar spectra. We have presented five broad categories of quasar anomalies divided into 10 homogeneous groups:

1. C iv Peakers: A total of 299 quasars with an extremely strong, yet narrow (median $\sim2000$km s$^{-1}$) C iv emission line.

2. Excess Si iv emitters: Quasars in this group exhibit an excessively high Si iv  to C iv  emission line flux ratio, nearly double the median value observed for all quasars in @Wu_2022 catalog. A total of 227 such quasars were found.

3. Si iv deficient anomalies: 328 quasars with disproportionately low Si iv emission with Si iv to C iv flux ratio being one third of the median ratio for all quasars in @Wu_2022 catalog.

4. BAL anomalies: A total of 871 quasars were identified as anomalies with BAL profiles. These were further subdivided into four subgroups as follows:

   • Blue BALs: A total of 103 HiBAL quasars with a strong negatively sloped (“blue”) continuum which is atypical of a BAL quasar.

   • Flat BALs: 371 BAL quasars with a relatively flat continuum. Among these, 80% are LoBAL quasars

   • Reddened BALs: A total of 276 BAL quasars with heavily reddened continuum, hence, with a strongly positive slope spectra.

   • FeLoBALs: These are a very rare class of BAL quasars with strong Fe absorptions and heavily reddened continua. There are a total of 121 such quasars detected.

5. Reddened anomalies: A total of 341 quasars were identified as reddened anomalous quasars, characterized by extreme reddening, as evident from their significantly red spectral slope. They were further subdivided into three subgroups primarily based on the degree of reddening, as follows:

   • Heavily reddened quasars: A total of 165 quasars were identified with steep, positively sloped spectra, attributed to heavy dust reddening.

   • Moderately reddened quasars: Dust reddened quasars with relatively flat or slightly convex shaped continuum. A total of 93 such quasars were found.

   • Plateu-shaped spectrum quasars: A total of 83 peculiar quasars were identified with a plateau-shaped spectrum, characterized by a flat continuum followed by a negatively sloped continuum. These quasars exhibit a nearly flat continuum between 1250 and 2000 Å, which then rapidly declines as the wavelength increases from 2000 to 3000 Å.

In our analysis, we initially applied the methodology to the entire dataset without distinguishing BAL quasars from non-BAL quasars. This approach aimed to demonstrate what one would obtain if all spectra were analyzed without prior classifications; that is, working with spectral data as they are obtained by the spectroscopic surveys. However, this resulted in groups that were not as pure, making interpretation more challenging. Ultimately, we found that preclassifying BAL quasars significantly simplified the process and led to much cleaner and more distinct groupings. This highlights the importance of BAL classification as a preliminary step when working with large spectroscopic datasets. We recommend that future studies employing similar techniques first isolate BAL quasars before applying clustering or statistical analyses to ensure clearer and more meaningful results.

This work has significantly expanded the number of sources in each anomalous quasar group. For example, we present a sample of 121 extreme FeLoBALs with strong, broad Fe ii  absorption, along with an additional 127 FeLoBALs from the red BAL anomalies, where the iron absorption lines are considerably narrower. We have developed an efficient method for identifying anomalous quasars and classifying them into distinct categories. The detected anomalies are presented in a value-added catalog, available at the SQuAD website[^1]. This approach has been successfully applied to the rest-frame UV spectra from the SDSS DR16 catalog and is currently being prepared for broader implementation. We applied the same algorithm to rest-frame optical spectra from SDSS DR16, covering around 75,000 quasars in the redshift range $0.1\leq z\leq1.1$, including key emission lines such as O iii, H$\beta$, and optical iron. This catalog will be crucial, as these emission lines, particularly in the context of Eigenvector 1, are tightly correlated with the accretion rate and orientation of quasars [see @shen2014diversity]. The catalog is currently under preparation.

With upcoming large-scale surveys such as Dark Energy Spectroscopic Instrument (DESI), 4-metre Multi-Object Spectroscopic Telescope (4MOST), and William Herschel Telescope (WHT) Enhanced Area Velocity Explorer (WEAVE), the number of quasar spectra will increase significantly, and our methodology will aid in identifying anomalous quasars in these vast datasets. Additionally, our approach can be adapted for photometric surveys, such as the Legacy Survey of Space and Time (LSST) survey, to find photometrically anomalous quasars. By extending the sample size of these anomalous quasars, this study enables the statistical analysis of these peculiar sources, contributing to a deeper understanding of AGNs and their diverse characteristics.