NGC 5506 Unmasked as a Narrow Line Seyfert 1: A Direct View of the Broad Line Region using Near-IR Spectroscopy

TL;DR

This study provides definitive evidence that NGC 5506 is a Narrow Line Seyfert 1 galaxy with an obscured broad line region, offering new insights into its classification and properties through near-infrared spectroscopy.

Contribution

The paper demonstrates that NGC 5506 is an optically-obscured NLSy1, revealing its broad line region via near-IR spectroscopy, which was previously unconfirmed.

Findings

NGC 5506 hosts a broad line region detectable in near-IR.

It is the first confirmed case of an optically-obscured NLSy1.

NGC 5506 exhibits both a type 1 AGN and a water vapor megamaser.

Abstract

This letter presents incontrovertible evidence that NGC5506 is a Narrow Line Seyfert 1 (NLSy1). Our new 0.9-1.4 micron spectrum of its nucleus clearly shows the permitted OI 1.1287 micron line (with full width at half maximum <2000 km/s) and the `1 micron FeII lines'. These lines can only originate in the optically-thick broad line region (BLR) and, among Seyfert nuclei the latter series of lines are seen only in NLSy1s. The obscuration to the BLR, derived from a rough estimate of the OI 1.1287 micron / OI 8446 Angstrom ratio and from the reddening of the near-IR Paschen lines, is A_v > 5. Together, these results make NGC5506 the first identified case of an optically-obscured NLSy1. This new classification helps explain its radio to X-ray properties, which until now were considered highly anomalous. However, interesting new concerns are raised: e.g., NGC5506 is unusual in hosting both a `type 1' AGN and a nuclear water vapor megamaser. As the brightest known NLSy1, NGC5506 is highly suitable for study at wavebands less affected by obscuration.