N. M. Nagar1 - E. Oliva1,2 - A. Marconi1 - R. Maiolino1
1 -
INAF, Arcetri Observatory, Largo E. Fermi 5, Florence 50125, Italy
2 -
INAF, Telescopio Nazionale Galileo, Aptdo de Correos, 565, 38700 Santa Cruz de La Palma, Canary Islands,
Spain
Received 12 May 2002 / Accepted 13 July 2002
Abstract
This letter presents incontrovertible evidence that NGC 5506 is a
Narrow Line Seyfert 1 (NLSy1).
Our new 0.9-1.4 m spectrum of its nucleus clearly shows the
permitted O I
m 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
O I
m/O I
8446 ratio
and from the reddening of the near-IR Paschen lines, is
.
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 isunusual 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.
Key words: line: formation - line: identification - galaxies: active - galaxies: individual: NGC 5506 - galaxies: Seyfert - galaxies: infrared
The Seyfert nucleus of NGC 5506 has resisted a clear type classification
within Seyfert galaxies, and there is a long standing debate on whether it
is an intermediate type 1 (broad H
directly visible) or type 2
(broad H
not directly visible) Seyfert.
The presence of "broad'' Pa
has been reported
by Blanco et al. (1990), Rix et al. (1990), and Ruiz et al. (1994), but
Goodrich et al. (1994) found that the "narrow'' line emission profiles
become broader at longer wavelengths and suggested that the
"broad'' Pa
was the strong, highly reddened wings of this profile.
Based on data available at that time Goodrich et al. (1994) interpreted
the broadening of emission lines with wavelength as due to obscuration
of the inner parts of the narrow line region. Morris & Ward (1985) reported
a marginal detection of O I
8446, characteristic of Seyfert 1s, and
suggested the presence of high-density optically thick gas.
At odds with other type 2 objects, the nucleus of NGC 5506 is dominated by
a bright compact core at all near-IR wavelengths and 60% of the J-band
(1.25
m) flux in its central few arcsec is non-stellar in origin
(Oliva et al. 1999; Alonso-Herrero et al. 2001).
In the hard X-ray it is one of the most luminous and brightest Seyferts
in the local universe (
;
Mushotzky 1982)
and its obscuring column (
cm-2; Bassani et al. 1999)
is intermediate between typical values for Seyfert 1s and 2s.
Nuclear water vapor masers, a property highly correlated with
a type 2 spectral classification (Braatz et al. 1996),
have been detected towards its nucleus (Braatz et al. 1994).
The host galaxy causes additional complications. The galaxy disk is close to
edge on (
), and dust in the galaxy disk is responsible
for some or all of the nuclear reddening (Veilleux et al. 1997; Imanishi 2000).
NGC 5506 is therefore variously treated as a type 1.9 or type 2 Seyfert
in the literature and in either case is usually an outlier among the
members of its class.
In this letter we report on near-IR spectroscopy of NGC 5506, which
unequivocally identifies it as a Narrow Line Seyfert 1 (NLSy1).
In a NLSy1 the broad line region (BLR) is directly visible with the BLR
emission lines having widths typically 2000 km s-1, significantly
narrower than those in classical Seyfert 1s. NLSy1s show several anomalous
properties, most notably in the X-ray
(for a nice overview of these see Véron-Cetty et al. 2001) and explanations for
these include accretion rates close to the Eddington rate (implying lower
black hole masses than other Seyferts) or a view to the AGN along its axis.
Data were obtained on 17 January 2001 using NICS, the near-IR
imager and spectrograph on the 3.5 meter Telescopio Nazionale Galileo (TNG).
We used the low-resolution IJ grism and a slit width of
0
75, yielding a dispersion of 5.7 Å/pix (resolution 16 Å)
over the wavelength range 0.89-1.46
m,
and a scale of 0.25
/pixel along the slit.
The slit was centered on the near-IR peak and rotated
to position angle 140
North of East.
Total integration time on source was 20 min, consisting of four 5 min
exposures in the standard "ABBA'' position raster. The observations
were immediately followed by observations of the star HIP 69160
using the same instrument setup.
The sky was not photometric and the seeing about 1
.
Data were reduced using standard tasks within IRAF. The spectrum of
HIP 69160 (stellar type G5V) was corrected for its intrinsic spectral
shape and then used to correct the atmospheric absorption lines in the
spectrum of NGC 5506. Absolute flux calibration was derived by previous
observations of photometric standard stars and should be accurate
to about 30%. Finally, the spectrum was Doppler corrected using a
recessional velocity of 1815 km s-1 as derived from H I observations
(de Vaucouleurs et al. 1991). The final resolution was 16 Å over the full
wavelength range.
![]() |
Figure 1:
NICS/TNG spectrum of NGC 5506 in an
0
![]() ![]() ![]() |
Open with DEXTER |
Line | Obs. | Eg. | ![]() |
Comments |
flux | Dered. | ![]() |
||
(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![]() |
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![]() |
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![]() |
100 | 100 | 0.26 | case-B = 100 |
1.3206 | 6 | ... | 0.25 | |
Pa![]() |
84 | 280 | 10-15 erg/cm2/s |
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
m
line and the detection of the "1 micron Fe II lines''.
The O I
m line, along with O I
8446, is produced by Ly
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
m,
m,
m, and
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
m, is blended with the very strong
He I
m line and the fourth, Fe II
m,
is only marginally detected as it is in a region of atmospheric absorption.
A broad emission feature at 1.07
m, just blue-ward of
He I
m, is also present. This feature is also seen in
all six NLSy1s listed above but has not been identified.
The Pa
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
53% of the flux.
The similarity between the O I (BLR only) and Pa
(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
(Fig. 2).
These results are different from those of Veilleux et al. (1997) who found that both
Pa
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
(
km s-1 from data taken by R. Maiolino in 1995)
and Br
(
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
m and Pa
profiles trace the bulk of the BLR rather
than only the outer less obscured part. If this is true, the profile of the H
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,
Pa
,
and Pa
fluxes
are consistent with those expected when viewing case-B recombination
through an extinction of
5 (Cols. 3 to 5 of Table 1).
That is, for
the case-B assumption and an error of
13% in the Pa
/Pa
ratio,
our data are consistent with extinction
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 (
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
m/O I
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
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
m/O I
8446 flux ratio,
which translates into a relative reddening
A0.8446 - A1.1287 >1. Adopting a standard reddening curve, this gives A
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![]() ![]() ![]() |
Open with DEXTER |
The currently used classification for NLSy1s
(from Pogge 2000) is:
1. narrow permitted lines only slightly broader than forbidden lines;
2. FWHM(H)
< 2000 km s-1;
3. [O III]/H
< 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
line profiles likely sample the bulk of the BLR.
Thus, with O I (from the BLR only) and Pa
(from BLR and NLR) line widths
<2000 km s-1, NGC 5506 directly satisfies the first two conditions.
The observed [O III]/H
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
flux would likely change the unextincted nuclear
[O III]/H
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/
.
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.