NGC 5506 unmasked as a Narrow Line Seyfert 1:. A direct view of the broad line region using near-IR

This letter presents incontrovertible evidence that NGC 5506 is a Narrow Line Seyfert 1 (NLSy1). Our new 0.9–1.4  μ m spectrum of its nucleus clearly shows the permitted $\ion{O}{I}$ $\,\lambda 1.1287~\mu$m line (with full width at half maximum -1 ) and the “1 micron $\ion{Fe}{II}$ 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 $\ion{O}{I}$ $\,\lambda 1.1287~\mu$m/$\ion{O}{I}$ $\,\lambda$8446 ratio and from the reddening of the near-IR Paschen lines, is $A_{\rm V}\,>\,5$. 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.