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

This letter presents incontrovertible evidence that NGC 5506 is a Narrow Line Seyfert 1 (NLSy1). Our new 0.9–1.4μm spectrum of its nucleus clearly shows the permitted O Iλ1.1287μm line (with full width at half maximum < 2000 km s) and the ‘1 micron Fe II 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 O Iλ1.1287μm/O Iλ8446 ratio and from the reddening of the near-IR Paschen lines, is AV > 5. Together, these results make NGC 5506 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., NGC 5506 is unusual in hosting both a ‘type 1’ AGN and a nuclear water vapor megamaser. As the brightest known NLSy1, NGC 5506 is highly suitable for study at wavebands less affected by obscuration.