arXiv subjects

Arihant Tiwari

Publications and source records attributed to Arihant Tiwari.

2 recordsLinked to original sources

arXiv2026

Anomalous narrow line Seyfert I galaxies from SDSS DR17

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.

astro-ph.GA↗

arXiv2024

Spectroscopic Quasar Anomaly Detection (SQuAD) I: Rest-Frame UV Spectra from SDSS DR16

We present the results of applying anomaly detection algorithms to a quasar spectroscopic sub-sample from the SDSS DR16 Quasar Catalog, covering the redshift range 1.88 < z < 2.47. 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 with and without them, comparing both datasets for a clearer identification of other anomalous quasar types. We identified 1,888 anomalous quasars, categorized into 10 broad 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, super-solar metallicity for Excess Si IV emitters, and sub-solar 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. Further, 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. The detected anomalies are presented as a value-added catalog.

astro-ph.GA↗