4 Discussion

The currently used classification for NLSy1s (from Pogge 2000) is: 1. narrow permitted lines only slightly broader than forbidden lines; 2. FWHM(H$\beta$) < 2000 km s^-1; 3. [O III]/H$\beta$ < 3, but exceptions allowed if there is also strong [Fe VII] and [Fe X] present, unlike what is seen in Seyfert 2s.

We have shown that the BLR emission is detected in the near-IR and that the O I and Pa$\beta$ line profiles likely sample the bulk of the BLR. Thus, with O I (from the BLR only) and Pa$\beta$ (from BLR and NLR) line widths <2000 km s^-1, NGC 5506 directly satisfies the first two conditions. The observed [O III]/H$\beta$ ratio is 7.5 at the nucleus and this ratio remains high over most of the extended emission-line region (Wilson et al. 1985). If the BLR is highly extincted as our results suggest then the BLR contribution to the H$\beta$ flux would likely change the unextincted nuclear [O III]/H$\beta$ ratio to <3, bringing NGC 5506 into agreement with the third condition for classification as a NLSy1. A high extinction to the BLR would also explain the lack of strong optical Fe II lines as usually seen in NLSy1s. NGC 5506 shares other properties unique to NLSy1s including the presence of the "1 micron Fe II lines'' as shown here, a high X-ray luminosity, steep X-ray slope, and fast X-ray variability (Lamer et al. 2000). NGC 5506 is now the brightest known NLSy1 and therefore most suited for studies in wavebands not affected by obscuration. An important issue raised is whether several other X-ray bright and highly variable "type 2'' Seyferts are, like NGC 5506, partially obscured NLSy1s.

Several properties of NGC 5506 still remain, or now become, anomalous. Mathur et al. (2001) find evidence that NLSy1s have preferentially lower black hole masses and are accreting at high values of L/ $L_{\rm Eddington}$. However, the high central velocity dispersion in NGC 5506 (180 km s^-1; Oliva et al. 1999) though somewhat uncertain, implies a relatively high black hole mass among Seyferts, if the scaling between velocity dispersion and black hole mass is valid among Seyferts (Wandel 2002). NGC 5506 is also unusual in being a type 1 AGN with a nuclear megamaser. Both the X-ray column (Risaliti et al. 2002) and narrow maser lines (Braatz et al. 1996) are variable, and it may be that the latter are produced during periods when the column to nucleus is temporarily higher.

Acknowledgements

We gratefully acknowledge Francesca Ghinassi's help during observations and the assistance of the NICS and TNG teams. NN thanks the Centro Galileo Galilei for hospitality during the writing of this paper. This work was partially supported by the Italian Ministry for University and Research (MURST) under grant Cofin00-02-36 and the Italian Space Agency (ASI) under grant 1/R/27/00.