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 0.9-1.4 mum spectrum of its nucleus clearly shows the permitted OI lambda1.1287 mum line (with full width at half maximum <2000 km s(-1)) 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 OIλ1.1287 μm/OIλ8446 ratio and from the reddening of the near-IR Paschen lines, is A(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.