3 Results

Figure 1: NICS/TNG spectrum of NGC 5506 in an 0 $.\!\!^{\prime\prime}$75$~\times~$1 $.\!\!^{\prime\prime}$25 nuclear aperture. Most detected lines are labelled.

 

 

Table 1: Line fluxes in NGC 5506.

Line	Obs.	Eg.	$A_\lambda$/	Comments
	flux	Dered.	$A_{\rm V}$
(1)	(2)	(3)	(4)	(5)
O I 0.8447	<6:	<27:	0.55	Morris & Ward
[S III] 0.9069	39	105	0.48
9+Fe II 0.923	9	23	0.47	blend
0.9531	125	280	0.44	includes Pa8
0.9850	3	6	0.42
0.9913	1?	2?	0.41	marginal det.
Fe II 0.9997	6	12	0.41
Pa$\delta$ 1.0049	18	34	0.40	case-B = 34
He II 1.0123	8	15	0.40
1.033	12	21	0.38
Fe II 1.0501	6	10	0.37
?? 1.070	5	8	0.36	seen in NLSy1s
He I 1.083	74	110	0.35
Fe II 1.0863	8?	12	0.35	blend with He I
Pa$\gamma$ 1.0938	36	54	0.35	case-B = 55
Fe II 1.1126	1:	1:	0.35	marginal det.
O I 1.1287	14	19	0.33
1.1882	4	5	0.30
1.2567	20	21	0.27
Pa$\beta$ 1.2818	100	100	0.26	case-B = 100
1.3206	6	...	0.25
Pa$\beta$ flux	84	280		10-15 erg/cm2/s

Column (2) lists the flux measured in our 0 $.\!\!^{\prime\prime}$75$~\times~$1 $.\!\!^{\prime\prime}$25 nuclear aperture, relative to a Pa$\beta$ flux of 100, except for the O I 0.8446 line which was measured in a 1 $.\!\!^{\prime\prime}$5 $\times$ 10 $^{\prime\prime}$ nuclear aperture (Morris & Ward 1985). Entries marked with a "?'' or ":'' are uncertain. The absolute flux of the Pa$\beta$ line (accurate to $\pm$30%) in units of 10^-15 erg cm^-2 s^-1 is listed in the last line of the table. Column (3) lists the measured flux after dereddening by an illustrative extinction of $A_{\rm V}$ = 5 (see text), relative to a (dereddened) Pa$\beta$ flux of 100. Column (4) lists the $A_\lambda$/$A_{\rm V}$ values, derived from the standard extinction curve, used to calculate Col. (3). The expected relative fluxes for the first three lines in the Paschen series, assuming Case B recombination, are listed in Col. (5).

The final nuclear spectrum of NGC 5506 is shown in Fig. 1 with emission-line fluxes listed in Table 1. The main result of this paper is based on our clear detection of the permitted O I $~\lambda 1.1287~\mu$m line and the detection of the "1 micron Fe II lines''. The O I $~\lambda 1.1287~\mu$m line, along with O I$~\lambda$8446, is produced by Ly$\beta$ pumping in a Bowen fluorescence mechanism (Grandi 1980). The latter line was tentatively detected by Morris & Ward (1985). Both the above O I lines are produced only by high density optically-thick gas and are usually seen in Seyfert 1s but never in Seyfert 2s (e.g. Morris & Ward 1985).

The "1 micron Fe II lines'' at $\lambda0.9997~\mu$m, $\lambda1.0501~\mu$m, $\lambda1.0863~\mu$m, and $\lambda1.1126~\mu$m are posited to originate in BLR clouds. Theoretically, such Fe II lines and their related optical and UV counterparts are expected in only type 1 objects and to be strongest in NLSy1s (e.g. Collin & Joly 2000). Observationally, these lines have been previously detected in only six extragalactic objects, all NLSy1s: I Zwicky 1 (Rudy et al. 2000), Mrk 478 (Rudy et al. 2001), 1H 1934, Ark 564, Mrk 335, and Mrk 1044 (Rodríguez-Ardila et al. 2002). Detailed discussions on the origin of the lines can be found in these papers. Two of the four lines are clearly detected in our spectrum (Fig. 1). The third, Fe II  $\lambda1.0863~\mu$m, is blended with the very strong He I  $\lambda1.083~\mu$m line and the fourth, Fe II $~\lambda1.1126~\mu$m, is only marginally detected as it is in a region of atmospheric absorption. A broad emission feature at 1.07 $\mu$m, just blue-ward of He I $\lambda1.0630~\mu$m, is also present. This feature is also seen in all six NLSy1s listed above but has not been identified.

The Pa$\beta$ line has a broad pedestal (Fig. 2) and is best fit (after deconvolving the instrumental resolution) by two Gaussians with full width half maximum (FWHM) 500 km s^-1 and 1800 km s^-1; the broader line contains $\sim$53% of the flux. The similarity between the O I (BLR only) and Pa$\beta$ (BLR+NLR) line profiles (Fig. 2) suggests that this double Gaussian fit does not perfectly separate emission from the NLR and BLR, respectively. The [Fe II] line profile is only slightly resolved at our instrument resolution, but nevertheless appears different from that of Pa$\beta$ (Fig. 2). These results are different from those of Veilleux et al. (1997) who found that both Pa$\beta$ and [Fe II] have similar profiles, with their wings well fitted with a Lorenzian rather than a broad Gaussian component. We were unable to satisfactorily fit our line profiles following the functional form of their fit. It is likely that the line profiles are affected by significant variability. Given the FWHM of the broad components of Br$\gamma$ ( $1550 \pm 100$ km s^-1 from data taken by R. Maiolino in 1995) and Br$\alpha$ ( $1200 \pm 100$ km s^-1; Lutz et al. 2002), it appears that the width of the broad component of the near-IR permitted lines does not increase with wavelength, suggesting that the O I $~\lambda 1.1287~\mu$m and Pa$\beta$ profiles trace the bulk of the BLR rather than only the outer less obscured part. If this is true, the profile of the H$\alpha$ emission from the BLR should be similar to that of O I (Grandi 1980), though the latter line may be narrower in NLSy1s (Rodríguez-Ardila et al. 2002).

The observed Pa$\beta$, Pa$\gamma$, and Pa$\delta$ fluxes are consistent with those expected when viewing case-B recombination through an extinction of $A_{\rm V}~=$ 5 (Cols. 3 to 5 of Table 1). That is, for the case-B assumption and an error of $\sim$13% in the Pa$\beta$/Pa$\delta$ ratio, our data are consistent with extinction $A_{\rm V}~= 5~\pm$ 1 mag towards the Paschen lines. The true extinction to the broad line region is expected to be higher than this illustrative value as our aperture ($\sim$100 pc at the distance of NGC 5506) includes emission from the presumably less-extincted narrow line region.

A more direct measure of the extinction to the BLR comes from the O I $~\lambda 1.1287~\mu$m/O I$~\lambda$8446 ratio, which is reddening sensitive (intrinsic value = 1.34). In all the published spectra of NLSy1 the observations are compatible with zero relative extinction between the two lines (Rudy et al. 2000, 2001; Rodríguez-Ardila et al. 2002). The only available data on O I$~\lambda$8446 in NGC 5506 is the spectrum of Morris & Ward (1985, 1988) where this line is only marginally detected. Their spectrum is flux calibrated and partially overlaps with ours. From a comparison between both absolute O I fluxes and O I fluxes scaled to the [S III] lines we find a lower limit of 2 for the O I $~\lambda 1.1287~\mu$m/O I$~\lambda$8446 flux ratio, which translates into a relative reddening A_0.8446 - A_1.1287 >1. Adopting a standard reddening curve, this gives A $_{\rm V}~>$ 5. This result is uncertain due to potential variability, different aperture sizes, and non-photometric conditions. Simultaneous observations of both O I lines are therefore highly desirable.

Figure 2: Comparison of the velocity profiles of Pa$\beta$ (solid line), O I $~\lambda 1.1287~\mu$m (dotted line), and [Fe II] $~\lambda 1.2567~\mu$m (dash-dot line). The instrumental resolution (16 Å at all three lines) has not beensubtracted out.