Anomalous narrow line Seyfert I galaxies from SDSS DR17

Abstract

We present an analysis of 22,656 narrow-line Seyfert 1 galaxies (NLSy1s) from the Sloan Digital Sky Survey (SDSS) DR17 (0.1 ≤ z ≤ 0.9), identifying a sample of spectroscopically anomalous sources. These anomalies were detected via the Spectroscopic Quasar Anomaly Detection (SQuAD) algorithm, which employed principal component analysis and hierarchical k-means clustering. Various physical diagnostic analyses were performed such as color excess (E(B − V)) calculations, Wide-field Infrared Survey Explorer color analysis, probing [O III] equivalent width as an inclination indicator, the Baldwin─Phillips─Terlevich (BPT) diagram, and eigenvector 1 diagram. We detected 620 anomalous NLSy1s classified into two groups. The first is 246 red NLSy1s, exhibiting host-galaxy-dominated spectra with a low-luminosity active galactic nucleus (AGN) core revealed by the emission line widths. The other set consists of 374 blue NLSy1s, which are highly luminous galaxies with enhanced AGN activity, bluer continua as compared to a typical NLSy1, and have stronger Fe II emission. Finally, we identify a third group of 257 outliers, identified as intermediate Seyferts, which are a class of Seyfert galaxies identified by composite emission profiles and extremely strong emission lines paired with virtually no continuum. These sources also exhibit rare and high ionization emission lines unseen in any other NLSy1 spectra (e.g., [Ne v] λ3345, Ne v λ3426, Ne III λ3869, etc.). We conclude that the differentiating factor between red and blue NLSy1s is not dust obscuration or orientation effects, but intrinsic distinction in AGN activity. The resulting sample is presented as a value-added catalog.