Anomalous narrow line Seyfert I galaxies from SDSS DR17

Abstract

We present an analysis of 22,656 narrow-line Seyfert 1 galaxies (NLSy1s) from Sloan Digital Sky Survey (SDSS) DR17 ($0.1\leq z\leq 0.9$), 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 analysis were performed such as the color excess ($E_{(B-V)}$) calculations, Wide-field Infrared Survey Explorer (WISE) color analysis, probing O III, equivalent width as an inclination indicator, the BPT diagram and eigenvector 1 diagram. We detected 620 anomalous NLSy1s classified into two groups i.e. 246, Red NLSy1s, exhibiting host galaxy dominated spectra with a low luminosity active galactic nuclei (AGN) core revealed by the emission line widths. Another set of 374 Blue NLSy1s, strongly luminous galaxies with enhanced AGN activity, bluer continuum as compared to a typical NLSy1 and stronger Fe II emission. Finally, the third group of 257 outliers, identified as Intermediate Seyferts, 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]$\lambda3345$, Ne V $\lambda3426$, Ne III $\lambda3869$ etc). We conclude that the differentiating factor between red and blue NLSy1s is not dust obscuration or orientation effect, but intrinsic distinction in AGN activity. The resulting sample is presented as a value-added catalog.

Figures (14)

• Figure 1: Top Left: A 2D projection (PCA 1 versus PCA 2 coefficients) of the datasets. The dataset is divided into three clusters (cluster 1: brown, cluster 2: green, cluster 3: blue) within the 20-dimensional PCA hyperspace, using k-means clustering. Sources classified as anomalous after applying a $Q3 + 1.5 \times IQR$ threshold are shown as golden scatter points overlayed on top of the cluster members. The cluster centroids are marked by yellow crosses. Top Right: Continuum subtracted mean composite spectrum for each cluster to highlight the differences in emission line strength. Bottom: Mean composite spectrum for each cluster of the dataset. The color of each spectrum corresponds to the color of the cluster as shown in the left panel.
• Figure 2: Left: A 2D projection (principal component (PC1) versus PC2 coefficients) of the anomalous galaxies as divided into three groups (group 1: orange, group 2: blue, group 3: red), in the 20 dimensional PCA hyperspace, by the second k-means clustering applied only on the anomalous galaxies. The groups are characterized by further analysis (see Sect.\ref{['sec: Physical Diagnostics']}). Right: Mean composite spectrum of each anomaly group. The colors of the spectra correspond to the color of their group in the left panel. The black dashed spectrum is the mean composite of all the galaxies in the paliya2024narrow catalog. The red, blue and black spectra are arbitrarily normalized for better comparison.
• Figure 3: The redshift distribution of red (red), blue (blue) NLSy1s and Intermediate(orange) Seyferts. The median redshift for each class is marked by a dashed line.
• Figure 4: The density normalized distribution of parameters for the total paliya2024narrow dataset and the three anomalous groups. Top Left: The distribution of Eddington ratio on log scale. Top Right: Distribution of H$\beta$ derived black hole mass on log scale. Bottom Left: log scale distribution of bolometric luminosity and; Bottom Right: Distribution of Fe ii emission strength (R4750). In all the cases, the red NLSy1s lie at the lowest end and blue NLSy1s at the highest. Intermediate Seyferts are the galaxies with the lowest Fe ii emission strength in the complete dataset. The number annotation in each plot indicates the ratio of the respective value for blue to red NLSy1s.
• Figure 5: Absolute B band magnitude distribution for the red, blue NLSy1s and Intermediate Seyferts. The respective median is marked by dashed lines for both classes. The black dashed line marks a $M_B=-23$ which is considered to be a distinction limit between quasars and Seyfert galaxies. paliya2024narrow