PASJ: Publ. Astron. Soc. Japan 56, 723–741, 2004 October 25
c 2004. Astronomical Society of Japan.
Low-Ionization Emission-Line Regions
around the Nucleus of the Seyfert Galaxy NGC 1068
Tsuyoshi I SHIGAKI ,1∗ Tadashi H AYASHI,2∗ Hiroshi O HTANI,3∗† Minoru S ASAKI ,4∗ Hiroyuki M AEMURA,3‡
Shinobu O ZAKI,5∗ Takashi H ATTORI,3∗§ Hajime S UGAI ,3∗ and Motomi I SHII6∗
1
Department of Applied Physics, Graduate School of Engineering, Hokkaido University, Kita-ku, Sapporo 060-8628
[email protected]
2
Toyama Science Museum, Toyama, Toyama 934-8084
3
Department of Astrophysics, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502
4
Shimonoseki City University, Shimonoseki, Yamaguchi 751-8510
5
Nishiharima Astronomical Observatory, Sayo-cho, Hyogo 679-5313
6
Kurashiki Science Museum, Kurashiki, Okayama 712-8046
(Received 2004 March 31; accepted 2004 August 3)
Abstract
We present the results of tridimensional spectrophotometric observations of the central region of the Seyfert
galaxy NGC 1068 obtained by using Kyoto tridimensional spectrograph I. A brief description of the instrument is
presented. We have found the existence of low-ionized gas extending out of the ionization cone. It is characterized
by higher ([S II]λ6716 + λ6731)/Hα than within the cone. The ratio peaks at 5 –6 east and west of the nucleus and
reaches to 0.5–0.6. The ionization of the gas outside the cone by the scattered nuclear continuum is examined. We
argue that a substantial fraction of the gases outside the cone could be ionized by the scattered nuclear continuum,
including the diffuse arc structure seen in an HST image. In addition, we also attempt discussions according to the
kinematic and spectroscopic properties of two possible gaseous components derived by the line decomposition: a
narrow component that seems to follow the galactic rotation, and a broad one that may be deviated from it.
Key words: galaxies: individual (NGC 1068) — galaxies: Seyfert — instrumentation: spectrographs
1. Introduction
To study the extended emission-line regions around active
galactic nuclei (AGNs), including Seyfert galaxies, tridimensional spectroscopy, which provides spectral information with
two spatial dimensions (i.e., data cube), is effective. For such
purposes, we have developed the Kyoto tridimensional spectrograph I (Kyoto 3DI). This study has provided our first results
obtained by combining the data of the imaging Fabry–Perot
interferometry and integral-field spectroscopy, both of which
are the observing modes of Kyoto 3DI.
NGC 1068 is the brightest Seyfert 2 galaxy and has been
studied extensively. One of the results by these studies
is the discovery of broad permitted lines in the polarized
nuclear spectrum (Antonucci, Miller 1985). It is interpreted as
evidence that NGC 1068 harbors a hidden Seyfert 1 nucleus,
and that the emission from it cannot be observed directly,
but can be observed only in scattered polarized light. This
has led to a “unified model” of Seyfert galaxies: all Seyfert
galaxies basically have the same nucleus structure in which
the ionizing-continuum source and the broad-line region are
surrounded by a dust torus. If our line of sight is intercepted
∗
†
‡
§
Visiting Astronomer, Okayama Astrophysical Observatory of National
Astronomical Observatory.
Present address: 30-2 Hazama-cho, Shugakuin, Sakyo-ku, Kyoto 6068071.
Present address: Renesas LSI Design Corp., Itami, Hyogo 664-0851.
Present address: Okayama Astrophysical Observatory, Kamogata-cho,
Asakuchi-gun, Okayama 717-0232.
by the torus, we can recognize the galaxy as being a type 2
Seyfert.1
This idea is supported by the detection of an “ionization cone” in many Seyfert 2 galaxies, which is a cone-like
emission-line region extending to nearly the same direction
of the radio jet (Pogge 1989). The ionization cone is considered to be a region directly ionized by the nuclear continuum,
which escapes off a dust torus along its axis. An ionization
cone was also found in NGC 1068 (Pogge 1988), and has been
investigated in detail by Hubble Space Telescope (HST) observations (Evans et al. 1991; Macchetto et al. 1994). Imaging
polarimetry of the ionization cone of NGC 1068 with HST
revealed a centro-symmetric polarization pattern, which was
expected from scattering from a point source (Capetti et al.
1995). The center of symmetry of this pattern is considered to
represent the location of a hidden nucleus; this result strongly
supports the unified model of Seyfert galaxies. The ionization
cone is clearly recognized in high-ionization emission lines,
which are characteristic of a region photoionized by a featureless continuum from Seyfert nuclei. In particular, a study
on the morphology around the Seyfert nucleus concentrated
on the [O III]λ5007 (ionization energy: 35.1 eV) narrow-band
imaging, since it is usually the strongest line in the optical
region.
We have attempted to expand the morphological study
1
Recently, Tran (2003) postulated that there are two classes of Seyfert 2
galaxies: a class of Seyfert 2 galaxies with a hidden broad-line region
(HBLR), and that of Seyfert 2 galaxies intrinsically lack of a broad-line
region (non-HBLR or “pure” Seyfert 2).
724
T. Ishigaki et al.
We first briefly describe the outline of Kyoto 3DI in
section 2. In sections 3 and 4, the observations and the data
reductions are described. In section 5, we present the results
of the observations. These results are discussed in section 6,
and conclusions are given in section 7. In this study, we adopt
a distance to NGC 1068 of 14.4 Mpc (so that 1 corresponds
to 70 pc) and a systemic velocity of 1148 km s−1 (BlandHawthorn et al. 1997).
Table 1. Parameters of Kyoto 3DI.
Focal reducer
Focal length of
the collimator lens
Focal length of
the camera lens
300 mm
81 mm
CCD (2 × 2 binning)
2.
24 µm
512 × 512
Pixel size
Format
IFS mode
Enlarger
MLA lens size
MLA format
Focal ratio of the MLA lens
×15
3.26 mm × 3.26 mm
7 × 13 (7 × 11 for object
and 7 × 2 for sky)
F/8.6
Long-slit spectroscopy mode
Slit width
Slit length
[Vol. 56,
0.3 mm and 1.0 mm
45 mm
of a Seyfert nucleus into low-ionization emission lines,
such as [S II]λλ6716,6731 (ionization energy: 10.4 eV) and
[N II]λλ6548,6583 (14.5 eV). Previously, Bland-Hawthorn,
Sokolowski, and Cecil (1991) conducted a [N II] observation
of the galactic disk of NGC 1068, and revealed diffuse ionized
matter (DIM), which has a high [N II]/Hα ratio over the disk,
particularly, a high ratio near to the radio axis. Based on
their results, Sokolowski, Bland-Hawthorn, and Cecil (1991)
discussed the ionization of the gas away from the radio axis by
scattering of the nuclear continuum. The target of our study is
a more central region, and the purpose is to reveal the ionization states around the nucleus and details of the structure of the
emission-line regions and the radiation field in the context of a
unified model.
Kyoto Tridimensional Spectrograph I
In this section, we present an outline of the Kyoto tridimensional spectrograph I (Kyoto 3DI), which was used in this
study. A detailed description of the spectrograph was given in
Ohtani et al. (1998).
Kyoto 3DI has been developed for area spectroscopic observations of extended objects in the optical region. The spectrograph has been optimized for use at the f/18 Cassegrain
focus of the 188 cm telescope of the Okayama Astrophysical
Observatory (OAO). The spectrograph has four observing
modes, which are switched to one another by remote operation.
The modes are filter imaging, long-slit spectroscopy, imaging
Fabry–Perot interferometry, and integral-field spectroscopy
(IFS). The first two modes are conventional types. In figure 1,
the layout of the spectrograph is shown. In table 1, the
parameters of the optical elements and the detector are given.
The base of the optical system of the spectrograph is a
transmission-type focal reducer. The collimator is a custommade Petzvar system with a field lens. The focal length and
the focal ratio of the collimator are 300 mm and f/8.6, respectively. The camera is a Kowa Ultra High Speed Lens of focal
length 81 mm, resulting a reducing factor of 0.27. There is no
vignetting over a 60 mm circular input flat field. The CCD is
a Texas Instruments TC125. Its format is 1024 × 1024, but
we always use it as 512 × 512 to make 24 µm pixel by onchip-binning. The image size of a point source by the focal
reducer is less than the detector pixel size, 24 µm, over nearly
the whole field in a spectral range of 480–700 nm.
By inserting a filter near to the focal plane, or at the pupil
Table 2. Grisms and spectral bands.
Grism no.
Groove
(gr mm−1 )
Straight
(Å)
Spectral range
(Å)
300
600
600
5700
4500
6600
4800† –7000†
4800† –5400
5700–7000†
1
2
3
∗
†
Spectral resolution (Å)
Slit∗
IFS
30
14
10
15
7
5
For the 0.3 mm width slit.
Limited by image quality.
Table 3. Characteristics of etalons.
Etalon
Gap
(µm)
Wavelength region
(Å)
Peak transmission
Finess
at 6328 Å
Resolution
λ/δλ
ET50FS-952
ET50FS-953
3–8
55 ± 3
4000–7000
4000–7000
0.6
0.6
25
40
250–600
6800
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3D Spectrophotometric Observation of NGC 1068
725
Fig. 2. Preoptics of the integral-field spectroscopy mode.
A
dual-channel preoptics allows us to obtain the target object and sky
fields simultaneously. The fields of view and separation between object
and sky are for the case that the spectrograph is attached at the f/18
Cassegrain focus of the OAO 188 cm telescope.
Fig. 1. Optical layout of the Kyoto 3DI. a) Configuration for
the long-slit spectroscopy mode, the filter imaging mode, and the
imaging Fabry–Perot interferometer mode. b) Configuration for the
integral-field spectroscopy (IFS) mode.
plane of the focal reducer, the spectrograph can be used as
a filter imager. The image scale of 6. 1 mm−1 at the f/18
Cassegrain focus of the OAO 188 cm telescope is converted to
22. 6 mm−1 with f/4.9. The field of view is 4. 6 square, which
is limited by the active area of the CCD. The spatial resolution
is 0. 54 per pixel.
By setting a slit at the telescope focus and inserting a grism
in the collimated beam, the spectrograph works as a conventional slit spectrograph. We use two slits of widths 0.3 mm and
1.0 mm, which correspond, respectively, to 1. 8 and 6. 1 on the
sky at the OAO f/18 Cassegrain focus. The narrower slit is
used for objects, while the wider one is used for spectrophotometric standard stars when absolute flux calibration is required.
The effective length of the slits is 45 mm (4. 6), along which
the scaling is the same as in the imagery mode, i.e., 0. 54 per
pixel. In table 2, the parameters of available grisms are listed.
The spectral resolution in this table is for a 1. 8 slit.
In the imaging Fabry–Perot interferometer mode, an etalon
is inserted to the pupil position of the focal reducer, and an
order-sorting filter is set near to the telescope focus. The etalon
can be set with a tilt to the optical axis, to avoid ghost images,
which rise from reflections between the etalon surface and the
order-sorting filter, the window glass of the CCD, or the CCD
surface (Bland-Hawthorn 1995). We use two servo-stabilized
scanning Fabry–Perot etalons of the Queensgate Instruments
ET-50 series. Their parameters and properties are given in
table 3. One of the etalons is a low spectral resolution etalon
(“tunable filter”), which is used for monochromatic imaging
observations. The other one is a high spectral resolution etalon,
the resolving power of which is about 7000. It is used for
measurements of the velocity field. We can set in turn four
5 mm-aperture masks before the etalon for adjusting the parallelism of the etalon gap. These apertures are distributed so
that each aperture locates in a different quadrant of the pupil
from the others. The parallelism of the etalon is so adjusted
that any differences in central gap distance of the transmission
profile by a reference emission line between four apertures is
minimized.
Special care is paid to stabilize the temperature of the
etalons. The optical gaps of the etalons change due to a variation of the ambient temperature. Consequently, the wavelength
of the transmitted light varies. We conducted experiments
to examine the etalon characteristics. Both etalons showed
similar results, and the rate of the drift of the optical gap was
about −20 Å per 1 ◦C. This means that the rate of the drift of the
wavelength of transmitted light is −40/m Å per 1 ◦C, where m
is the order of interference. In order to avoid drift, the temperature of the space after the collimator is surrounded by thermal
insulators (figure 1a), and is controlled within ±0.5 ◦C during
the observations. Using the imaging Fabry–Perot interferometer mode, some astronomical results have been published to
date elsewhere (Ishigaki et al. 2000; Hattori et al. 2002, 2004).
An integral-field spectroscopic mode of the TIGER type
(Bacon et al. 1995) is adopted in Kyoto 3DI. In this mode,
the image at the telescope focus is magnified by 15 times
by a small lens behind the image. The enlarged image is
projected telecentrically on a microlens array (MLA), the
format of which is 7 × 13 lenses. In figure 2, this preoptics
is given schematically. We devised a dual-channel preoptics
that acquires a target field and a sky field far away from the
726
T. Ishigaki et al.
target simultaneously. 7 × 11 lenses are assigned to the target
field and 7 × 2 lenses are to the sky field. This allows us
to subtract the sky background accurately. At the Cassegrain
focus of the OAO 188 cm telescope, the separation between
the target field and the sky field is 3. 7. The FOV of the target
field is 9. 1 × 14. 3, and that of the sky field is 9. 1 × 2. 6. The
microlens array makes an array of micropupils. At the OAO
Cassegrain focus, a micropupil has a diameter of 147 µm , and
is imaged by the focal reducer described above on the CCD,
on which a micropupil is sampled by 1.7 pixels. The microlens
array is rotated around the axis of the central lens by 8.◦ 13 with
regard to the direction of the spectral dispersion to avoid any
overlap of the spectra. The available grisms are the same as
those of the slit spectroscopy mode (table 2).
The measured efficiency of the spectrograph, including the
telescope and atmosphere, is about 4–5%, depending on the
filters and/or grisms. It is somewhat lower (∼ 2%) in the
integral-field spectroscopy mode than in other modes, due to
an increasing number of the optical elements.
3.
[Vol. 56,
A log of the integral field spectroscopy is given in table 5.
Ten exposure frames were obtained in total. We also obtained
direct images of NGC 1068 with the imaging mode before
each exposure of the integral field spectroscopy mode. Direct
images were used to measure the relative offsets between
the positions of the aperture of each frame. The absolute
position was determined by comparing the positions of the
nucleus between the continuum image obtained with narrowband imaging and the image reconstructed from the frame
including the nucleus. The accuracy of the positions is smaller
than the size of a microlens, i.e., 1. 3, which is smaller than
the seeing size. The positions of the aperture of each frame
are shown in figure 3. One row of an microlens array for
the target field was not used so as to make a gap between the
target field and the sky field in order to avoid contamination.
Consequently, 7 × 10 lenses were assigned to the target field,
and the field of view was 9. 1 × 13. 0. The position angle
of the aperture was 94.◦ 4. The spectrum of the photometric
standard stars HD 161817, HD 192281, and HD 19445 were
also obtained for a flux calibration.
Observations
4.
3.1. Narrow-Band Imaging
Narrow-band images of NGC 1068 were obtained at the f/18
Cassegrain focus of the OAO 188 cm telescope. Kyoto 3DI was
used in the mode of an imaging Fabry–Perot interferometer
with a 5 µm gap Queensgate ET-50 etalon. The etalon provides
bandpasses of 17 Å at 6598 Å and 31 Å at 5016 Å. Hα and
[S II]λλ6716,6731 images were obtained in 1996 September,
and an [O III]λ5007 image was obtained in 1998 December. A
log of the observations is presented in table 4. In the observation during 1996 September, the photometric standard stars
BD + 17◦ 4708 and BD + 28◦ 4211 were observed to provide
the absolute flux calibration. Because the 1998 December data
were obtained under unstable weather conditions, we did not
observe any photometric standard stars.
3.2. Integral-Field Spectroscopy
Integral-field spectroscopy was carried out in 1999 October
at the f/18 Cassegrain focus of the OAO 188 cm telescope.
The dispersing element was a 600 lines mm−1 grism, providing
a dispersion of 3 Å per pixel and a spectral resolution of 5 Å.
The spectral range was 6300 Å–7000 Å, which contains Hα,
[N II]λλ6548,6583, and [S II]λλ6716,6731.
Reductions
4.1. Narrow-Band Imaging
The Fabry–Perot narrow-band images were reduced in
the standard manner of the reduction for narrow-band-filter
imaging by using the IRAF. The procedure includes bias
subtraction, dark subtraction, flat-fielding, cosmic-ray removal,
sky subtraction, and flux calibration. The measured flux
was corrected for any variations of the transmission of the
atmosphere using the brightness of a field star in the frame,
and that of a guide star monitored during the observation. The
former was used for making corrections between images with
the same wavelength, and the latter was for making corrections
between images with a different one. However, since it was too
cloudy on 1998 December 18 to correct the transmission of the
atmosphere, a flux calibration was not made to the frames in
the green spectral region.
To derive pure emission-line images of NGC 1068, the
off-band continuum image was subtracted from each on-band
image. Spatial alignments of the on-band images and offband images were accomplished using a field star. The differences in the seeing size from frame to frame were corrected
by degrading the images with a narrower PSF (point-spread
Table 4. Log of the narrow-band imaging observation of NGC 1068.
Wavelength
(Å)
Date
Exposure time
Order sorting filter
(center/FWHM, Å)
Seeing
( )
Remark
5026
5119
6520
6588
6680
6742
6756
6820
1998 Dec 18
1998 Dec 18
1996 Sep 18, 22, 23
1996 Sep 18, 22, 23
1996 Sep 18, 22, 23
1996 Sep 18, 22, 23
1996 Sep 18, 22, 23
1996 Sep 22, 23
300 s × 1
300 s × 3
300 s × 7, 240 s × 3
300 s × 5, 240 s × 2
300 s × 7, 240 s × 2
300 s × 7, 240 s × 1
300 s × 5, 240 s × 1
300 s × 5, 240 s × 1
5020/210
5115/210
6595/310
6595/310
6685/310
6685/310
6685/310
6830/320
2.0
2.1–2.2
1.5–2.2
1.5–2.3
1.5–2.4
1.5–2.2
1.5–2.3
1.7–2.3
[O III]λ5007
off-band
off-band
Hα
off-band
[S II]λ6716
[S II]λ6731
off-band
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3D Spectrophotometric Observation of NGC 1068
727
Table 5. Log of the integral field spectroscopy of NGC 1068.∗
No. Frame Position of the aperture† Exposure time
1 nucleus
nucleus
600 s
1800 s
2 north 6. 8 to north, 2. 2 to east
3 south 1 5. 2 to south, 0. 7 to east
1800 s
4 south 2 5. 2 to south, 0. 7 to east
1800 s
5 east 1 0. 9 to south, 9. 8 to east
1800 s
6 east 2 0. 2 to south, 9. 2 to east
1800 s
7 east 3 0. 2 to north, 9. 2 to east
1800 s
1800 s
8 west 1 1. 9 to north, 6. 2 to west
9 west 2 1. 2 to north, 6. 2 to west
1800 s
10 west 3 3. 0 to north, 8. 1 to west
1800 s
∗
†
Date : 1999 October 9, seeing : ∼ 2 .
Position of the center of the aperture from the nucleus.
function) to match their PSFs to the broadest PSF, 2. 4, by
convolving them with a Gaussian profile. With regard to
the [O III] image, which was not flux-calibrated, continuum
subtraction was made by matching the count of a field star in
an off-band image to that in an on-band image.
The Hα image was not corrected for the contamination
of [N II]λλ6548,6583. The transmission of the Hα band
was about 20% at the wavelength of [N II]λ6548 and 10%
at [N II]λ6583. To estimate the contribution of [N II] lines
to the Hα image, we multiplied the spectra of the integralfield spectroscopy by the transmission profile of the Fabry–
Perot interferometer, and found that the contamination of [N II]
lines to the Hα image was 10–40%. The [S II]λ6716 + λ6731
image was made by summing the image obtained through the
narrow band centered at the wavelength of [S II]λ6716 and
one at the wavelength of [S II]λ6731. The crosstalk between
both lines was not corrected. The effect of crosstalk was also
estimated to be 10–20% by using integral-field spectroscopic
data. As shown in subsection 5.1, ([S II]λ6716 + λ6731)/Hα
ratios obtained from the narrow-band images are in good agreement with those from the integral-field spectroscopy; these
effects do not affect our results.
4.2. Integral-Field Spectroscopy
Integral-field spectroscopic data were also reduced by using
the IRAF. After bias subtraction, the spectra in each frame
were extracted by summing 3 pixels perpendicular to the
dispersion. Extracted spectra were reduced by the following
procedures: flat-fielding, wavelength-calibration, and fluxcalibration. Proceeded spectra were stacked so as to make
a data cube. Crosstalk between adjacent spectra was not
corrected, but when it appeared to be appreciable, we took the
effect into account (e.g., see figure 11).
The PSF of the integral-field spectroscopic mode was
estimated from the data of the standard stars. The FWHM
is ∼ 2 and the radial profile falls to ≤ 1% of the peak
at a distance of 4 from the center. It appears to have a
somewhat broader wing than the Gaussian function with the
same FWHM. This may be due to image blurring by the
preoptics of the integral-field spectroscopic mode. To avoid
the effect of irradiation of the nucleus, we did not include
Fig. 3. Positions of the aperture of the integral-field spectroscopy.
The numbers stand for those in table 5. The background image is the
Hα image obtained by narrow-band imaging. North is up and east is
left.
the nucleus into the aperture, as shown in figure 3, except the
exposure for the nucleus (No.1 in table 5). Therefore, the effect
of irradiation of the nucleus is less than the above estimation of
the PSF.
To make reconstructed emission-line images, the continuum
level of the spectrum at each position was subtracted by
linear fitting. Then, the profile of emission lines, Hα,
[N II]λλ6548,6563, and [S II]λλ6716,6731 were fitted by
Gaussian functions to measure the intensity of each line.
5.
Results
5.1. Morphology of the Low-Ionization Emission-Line
Region
In this subsection, the distribution of the low-ionized gases
emitting the low-ionization emission lines [S II] and [N II]
is described based on emission-line images and line-ratio
maps obtained by narrow-band imaging and integral-field
spectroscopy.
The continuum and emission-line images obtained by
narrow-band imaging are shown in figure 4. The most prominent feature in the Hα image (figure 4b) is the well-known starforming arms (or ring) with a diameter of about 20 –30 . The
elongation of the emission-line region to northeast from the
nucleus,2 seen in the [O III]λ5007 image (figure 4e), is due to
the “ionization cone”. A close comparison between figures 4b
and 4c shows that the elongation to the northeast is more
evident in the Hα image than in the [S II]λ6716 + λ6731 image,
and that the distribution of the surface brightness of [S II] is
2
Here, we regard the location of the continuum peak as the nucleus.
Capetti, Macchetto, and Lattanzi (1997a) determined the location of the
hidden nucleus 0. 3 offset toward the south from the optical peak.
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T. Ishigaki et al.
[Vol. 56,
Fig. 4. Narrow-band images of the central region of NGC 1068. a) Average of continuum images of 6520 Å and 6680 Å, b) Hα image,
c) [S II]λ6716 + λ6731 image, d) continuum image of 5119 Å, and e) [O III] image. The gray-scale indicates that darker regions have brighter surface
brightness. The cross marks correspond to the peak of the continuum image (a). North is up and east is left.
more symmetrical with regard to the nucleus. In figure 5, the
relative intensity profiles of Hα and [S II] along the east-west
direction are plotted. They definitely show that the profile of
[S II] is more extended than Hα to the east-west direction, i.e.,
a direction different by more than 45◦ from that of the axis of
the ionization cone.
The reconstructed emission-line images by integral-field
spectroscopy are shown in figure 6. To derive these images,
each line profile was fitted by a single Gaussian (cf. subsection 5.2). The [N II]λ6583 image also seems to have a distribution more extended to the east-west direction than Hα, as well
as the [S II] image.
In order to demonstrate the extension to east and west
of the distribution of the low-ionization emission-line gases,
the [S II]/Hα line-ratio map was made from the narrow-band
imaging data. The map is shown in figure 7a. In figure 7b,
the contour plots of the [O III] surface brightness of figure 4e is
superposed on it. It is evident that the [S II]/Hα ratio has two
peaks at 5 –6 east and west of the nucleus. As can be seen
in figure 7b, the distribution of the [S II]/Hα ratio shows anticorrelation with that of the [O III] surface brightness. In other
words, it means that the [S II]/Hα ratio is higher outside the
ionization cone defined by [O III] than inside the cone. The
ratios around the two peaks are 0.5–0.6, and about 2-times
larger than the ratios inside the cone. The difference is sufficiently larger than the uncertainty due to the effects of the transmission profile of the Fabry–Perot interferometer mentioned in
Fig. 5. Intensity profiles of Hα (dashed line) and [S II] (solid line)
along the east-west direction. They are normalized by the peak intensity.
No. 5]
3D Spectrophotometric Observation of NGC 1068
729
Fig. 6. Reconstructed emission-line images of the central region of NGC 1068 from integral-field spectroscopy, which are surrounded by thick lines.
a) Hα, b) [N II]λ6583, and c) [S II]λ6716 + λ6731. Background images are all Hα images shown in figure 4b. The gray-scale indicates that darker regions
have brighter surface brightness. The cross marks indicate the peak of continuum light. North is up and east is left.
Fig. 7. [S II]/Hα ratio map from the narrow-band imaging data. a) Ratio map only and b) [O III] contour plots are superposed. The dashed line in (b)
shows the position of the diffuse arc structure (Capetti et al. 1997b). North is up and east is left.
subsection 4.1, and the ratios are in good agreement with those
from integral-field spectroscopy, as shown below. The position
angle of the two peaks is ∼ 90◦ . Interestingly, it is similar
to that of the distribution of the CO line emission around the
nucleus obtained by Helfer and Blitz (1995). It should be noted
that the east portion with high [S II]/Hα ratios coincides with
the diffuse arc structure (a dashed curve of figure 7b) seen in
the HST/WFPC2 Hα + [N II] image (Capetti et al. 1997b).
Line-ratio maps obtained by integral-field spectroscopy are
shown in figure 8. The [S II]/Hα map shown in figure 8b
confirms the result of narrow-band imaging: the [S II]/Hα ratio
has two distinct peaks at 5 –6 east and west of the nucleus. In
figure 8a, the [N II]/Hα map is also given. The [N II]/Hα ratio
is also higher in the east and west regions of the nucleus than in
the northeast region, but it is not so distinct as in the [S II]/Hα
map.
5.2. Emission-Line Ratio
In this subsection, the emission-line ratios around the
nucleus are quantitatively described. The [N II]λ6583/Hα
and ([S II]λ6716 + λ6731)/Hα ratios from the integral-field
spectroscopic data are plotted as a function of the radial
distance from the nucleus in figure 9. Here, we discriminated the ratios outside the cone from those inside the cone.
The ionization-cone morphology of NGC 1068 has been determined by various studies. Bergeron, Petitjean, and Durret
(1989) detected the high excitation line Ne V in a cone with
an opening angle of 80◦ . Cecil, Bland, and Tully (1990) and
Crenshaw and Kraemer (2000) investigated the kinematics of
the biconical outflow, and found that the emission-line gas
is distributed in a bicone with an opening angle of ∼ 80◦ .
According to these studies, we adopt a cone morphology with
an opening angle ∼ 80◦ and a cone-axis PA ∼ 30◦ ; the latter
730
T. Ishigaki et al.
[Vol. 56,
Fig. 8. Line-ratio maps from integral-field spectroscopy. a) [N II]λ6583/Hα, b) ([S II]λ6716 + λ6731)/Hα, and c) [S II]λ6716/[S II]λ6731. The map
shown in (d) is the same as the map in (b), where the circles show the regions used for the averaged spectra (figure 10), and the dashed lines indicate the
boundary of the ionization cone. Background images are all Hα images shown in figure 4b. North is up and east is left.
coincides with the radio axis (Wilson, Ulvestad 1983). The
boundary of the cone is indicated by dashed lines in figure 8d.
The line ratios in the southwest part inside the ionization cone
are not plotted, because it is considered to suffer from appreciable extinction due to the galaxy disk through which this part
is seen (Cecil et al. 1990; Crenshaw, Kraemer 2000).
To derive the line intensities, we fitted the line profiles with
Gaussian functions. The spectra around the nucleus often
indicate the presence of asymmetrical line profiles, which
have been interpreted as being evidence of radial motions,
such as the biconical outflow (Cecil et al. 1990; Crenshaw,
Kraemer 2000). However, severe line blends between Hα
and [N II]λλ6548,6583 or [S II]λ6716 and [S II]λ6731, as well
as low signal-to-noise ratios of the data make it difficult to
decompose the profiles based on a model of multicomponent
Gaussians. Therefore, in figure 9, the ratios based on a model
of a single Gaussian component are plotted, although it can not
adequately fit the asymmetrical line profiles. For the average
spectra shown in figure 10, we attempted a decomposition
based on a model of two Gaussian components. The total ratios
No. 5]
3D Spectrophotometric Observation of NGC 1068
731
Fig. 9. Emission-line ratios as a function of the distance from the nucleus. a) [N II]λ6583/Hα and b) ([S II]λ6716 + λ6731)/Hα. The line ratios were
derived by line decompositions based on a model of a single Gaussian. The open circles and cross points indicate the ratios inside the cone and outside
the cone, respectively.
by a model of two Gaussian components are different from the
ratios by a model of a single Gaussian; the difference is about
15%.
As shown in figure 9a, [N II]/Hα is almost constant between
3 and 7 from the nucleus. Beyond 8 , the ratio outside
the cone is affected by the contribution from the star-forming
arms. The average ratios between 3 and 7 inside and outside
the cone are 1.5 and 1.6 with standard deviations of 0.1 and
0.2, respectively. It seems that the ratio outside the cone is
slightly higher on average than that inside the cone. However,
taking account of the uncertainty due to the inadequacy of
the model of a single Gaussian mentioned above, the difference in the ratios between inside and outside of the cone is
not definite. Figure 9b shows that [S II]/Hα inside the cone is
also constant. The average between 3 and 7 is 0.30 with
a standard deviation of 0.05. The ratio outside the cone is
peaked around a radius of 5 –6 , and reaches up to 0.5–0.6,
which is about 1.5–2 times higher than that inside the cone.
The average between 3 and 7 is 0.46 with a standard deviation of 0.08. This difference in [S II]/Hα is significantly larger
than the uncertainty.
We made average continuum-subtracted spectra of better
signal-to-noise ratios for typical regions inside and outside the
ionization cone (figure 10). These regions have a diameter of
4 , and are designated in figure 8d: the east and west regions
outside the cone and the northeast region inside the cone. The
spectra also show that the east and west regions have higher
[S II]/Hα ratios than the northeast region. The emission-line
ratios in the northeast region, derived by a model of a single
Gaussian, are [N II]/Hα = 1.43, [S II]/Hα = 0.30. The ratios in
the east region are [N II]/Hα = 1.76, [S II]/Hα = 0.54 and those
in the west region are [N II]/Hα = 1.77, [S II]/Hα = 0.47.
As mentioned above, a model of a single Gaussian cannot
necessarily fit the line profiles adequately. Therefore, we
attempted to decompose the line profiles of the averaged
spectra, which are improved in signal-to-noise ratios, based
on a model of two Gaussian components: a narrow component and a broad one. Since the blend of [S II] lines is still too
severe to obtain a unique solution, the line center and the line
width derived from the Hα and [N II] fitting are used as fixed
parameters to decompose the [S II] lines; in other words, the
fitted parameters are amplitudes of [S II]λ6716 and [S II]λ6731
Table 6. Results of line decomposition.
∗
Component
Radial velocity
(km s−1 )
FWHM∗
(km s−1 )
[N II]/Hα
[S II]/Hα
[S II]λ6716/λ6731
NE narrow
NE broad
E narrow
E broad
W narrow
W broad
1179 ± 1
1226 ± 10
1091 ± 5
1184 ± 7
1237 ± 11
+ 30
1078−80
146 ± 6
1120 ± 50
207 ± 24
706 ± 50
249 ± 40
+ 300
897−130
1.45 ± 0.02
1.82 ± 0.08
1.41 ± 0.07
2.33 ± 0.20
+ 0.54
1.48−0.24
+ 0.91
2.93−0.39
0.45 ± 0.02
0.32 ± 0.03
0.51 ± 0.08
0.62 ± 0.13
0.57 ± 0.08
0.63 ± 0.20
1.24 ± 0.08
0.86 ± 0.12
1.20 ± 0.30
1.13 ± 0.33
1.11 ± 0.23
0.81 ± 0.39
Corrected for the instrumental line width.
732
T. Ishigaki et al.
[Vol. 56,
Fig. 10. Continuum-subtracted spectra averaged within the regions around the nucleus: a) northeast region, b) east region, and c) west region. The
positions and sizes of each regions are indicated in figure 8d. The units of the vertical axes are arbitrary.
of each component, only. The results of the fitting based on
a model of two Gaussian components are shown in figure 11,
and the parameters are summarized in table 6. To estimate the
errors of the results of the fitting, we artificially added expected
noises to the observed spectra, and repeatedly conducted the
fitting. In addition to random noise, a systemic noise associated with the procedure of the continuum subtraction was
added. The uncertainty of the continuum subtraction particularly affects the results of the broad components. The standard
deviations are presented as errors in table 6. The conspicuous
result of the decomposition is that the broad components of the
east and west regions show much higher [N II]/Hα ratios (2–3)
than the narrow components.
Arribas, Mediavilla, and Garcı́a-Lorenzo (1996) also
performed bidimensional spectroscopy for a region 10
(east-west) × 7 (north-south) around the nucleus. They
decomposed the line profiles with narrow, intermediate, and
broad components. Our narrow component may correspond
to their narrow component and our broad component to their
intermediate and broad components. Their result that the
narrow components show lower [N II]/Hα ratios than the intermediate and broad components is consistent with our result.
In addition to the line ratios, according to the cone
morphology mentioned above, the Hα flux outside the cone
No. 5]
3D Spectrophotometric Observation of NGC 1068
733
Fig. 11. Results of a decomposition based on a model of two Gaussian components for the spectra shown in figure 10. a,c,e) Hα and [N II] spectral
regions and b,d,f) [S II] regions. A thick line indicates the observed spectrum and thin lines indicate the result of the decomposition. The spurious feature,
indicated by the cross mark shown in (b), is due to the contamination of Hα and [N II] of the adjacent spectra on the detector.
was also measured based on the narrow-band imaging data.
The total Hα flux outside the cone between 3 to 8 from the
nucleus is ∼ 1 × 10−12 ergs s−1 cm−2 , which corresponds to a
Hα luminosity of ∼ 2 × 1040 ergs s−1 .
5.3. Electron Density
A [S II]λ6716/[S II]λ6731 ratio map was also made from
the integral-field spectroscopic data. It is shown in figure 8c.
Though it has a low signal-to-noise ratio, the trend that the ratio
is lower near the nucleus is seen. Plots of the ratio against
projected distances from the nucleus are shown in figure 12a.
Their average values in a bin of every 1 are also plotted with
bars of the standard deviations. Within a radius of 4 , almost
all points have ratios lower than 1. On the other hand, averaged
values are higher than 1 in regions over a radius of 4 . We also
compared the radial profiles of the [S II] ratio inside and outside
the cone. However, the signal-to-noise ratio of the data is too
low to discriminate between them.
The [S II] ratio is interpreted as a measure of the electron
density. According to Osterbrock (1989), we calculated the
electron density, Ne , for each average line ratio, assuming an
electron temperature of 104 K. From the results, we derived
734
T. Ishigaki et al.
[Vol. 56,
Fig. 12. Radial plots of [S II]λ6716/[S II]λ6731 ratio from the nucleus. (a) Observed values and averages of them in a bin of every 1 . The error bars
mean the standard deviations. (b) Predicted curves along with the averages of the observed values.
two formulae for the variation of Ne with the distance from the
nucleus:
Ne = 1.5 × 105 /r,
(1)
and
Ne = 5 × 107 /r 2 ,
(2)
−3
where Ne is an electron density in units of cm and r is the
radius from the nucleus in units of pc. These relations are
shown in figure 12b. Both lines could well explain the observed
values. The electron density is estimated to be ∼ 400 cm−3 at
r = 350 pc (5 from the nucleus).
The [S II] ratios of the individual components of the northeast, east, and west regions based on the model of two Gaussian
components, mentioned in subsection 5.2, were also obtained
(table 6). It implies that the broad components have lower [S II]
ratios, that is, larger densities than the narrow components.
However, the uncertainty of the decomposition is quite large,
particularly in the east and west regions. Therefore, we stop
any further discussion on the difference between the densities
of the individual components here.
5.4. Velocity Field
To investigate the kinematic properties of the emission-line
regions outside the ionization cone, we examined the radial
velocities and line widths of Hα and [S II]. In this subsection,
we first describe the properties of the radial velocity map, along
with the results of Arribas, Mediavilla, and Garcı́a-Lorenzo
(1996) and Garcı́a-Lorenzo, Mediavilla, and Arribas (1999).
Then, the kinematics of typical regions inside and outside the
cone is analyzed according to the decomposition of line profiles
based on a model of two Gaussian components, mentioned in
subsection 5.2.
In figure 13, we show a map of the Hα radial velocity based
on a model of a single Gaussian, which gives the integrated
kinematic properties along the line of sight. The map from
[S II] is not shown here, since it has almost the same appearance as that from Hα. In figure 14, plots of the radial velocities and FWHMs of Hα and [S II] derived by a model of a
single Gaussian along the east-west strip shown in figure 13
are presented. The FWHMs are corrected for the instrumental
line widths.
The radial velocity map shown in figure 13 confirms the
maps obtained by Arribas, Mediavilla, and Garcı́a-Lorenzo
(1996) and Garcı́a-Lorenzo, Mediavilla, and Arribas (1999).
At large radii beyond 5 , emission-line gases are blueshifted
with respect to the systemic velocity, 1148 km s−1 , in east
regions, and are redshifted in west regions. Their velocities can
be smoothly connected to the outer velocity field of emissionline gases derived by Kaneko et al. (1992). It has been interpreted as the galactic disk rotation with a kinematic major axis
of PA ∼ 90◦ (Kaneko et al. 1992; Helfer, Blitz 1995). On
the other hand, the velocity field near the nucleus is largely
deviated from the interpolation of the outer velocity field. As
shown in figure 14a, the radial velocities around 3 east are
larger than the systemic velocity, and those around 2 west are
smaller, which is opposite to what is expected from the outer
velocity field. The isovelocity contour is twisted in an S-shaped
structure in figure 13, which was previously noticed as an
“S-distortion” by Arribas, Mediavilla, and Garcı́a-Lorenzo
(1996).
Line profiles around the nucleus show asymmetry, as
mentioned in subsection 5.2. The averaged spectra shown in
figure 10 can be decomposed by a model of two Gaussian
components (figure 11). The results of the decomposition
are given in table 6, and the radial velocities and FWHMs
of each component in east and west regions are also plotted
in figure 14. As a result, the narrow component in the east
region is blueshifted, and that in the west region is redshifted.
Their radial velocities could be explained by an interpolation
No. 5]
3D Spectrophotometric Observation of NGC 1068
Fig. 13. Hα radial velocity map from the integral-field spectroscopic
data based on a model of a single Gaussian. The unit is km s−1 . The
white dashed line indicates the isovelocity line corresponding to the
systemic velocity, 1148 km s−1 . The thick black dashed lines and the
white circles are the same as in figure 8d, and show the boundary of the
ionization cone and the regions used for the averaged spectra, respectively. The strip indicated by thin dashed lines is the region where the
plots of the radial velocities and FWHMs are shown in figure 14.
of the outer velocity field, which means that they follow
the galactic rotation. On the other hand, the broad components are obviously deviated from the galactic rotation. This
means that the broad components are responsible for the
735
S-distortion shown in figure 13. The radial velocity of the
narrow component of the northeast region inside the cone,
1179 km s−1 , is slightly redshifted with respect to the systemic
velocity. This could be explained by the model of the galactic
rotation affected by the bar potential (Helfer, Blitz 1995)
mentioned below. Arribas, Mediavilla, and Garcı́a-Lorenzo
(1996) also decomposed the line profiles with four Gaussian
components. Their narrow component may correspond to our
narrow component. They also suggest that the narrow component follows the galactic rotation, which is consistent with our
result.
Helfer and Blitz (1995) investigated the kinematics of
molecular gases around the nucleus of NGC 1068. They
have detected a molecular bar along PA ∼ 63◦ , and found
that the kinematics of the molecular gases is responding to
the bar potential. They also examined the velocity deviation
from the model of rotation affected by the bar potential, and
found negative velocity residuals (∼ several × 10 km s−1 ) in
the nuclear regions that correspond to the east and west regions
in this study. It is worth noting that, although the trend of the
residuals is different from those of the optical emission-line
gases described above, the molecular gases in these regions
also show a complex kinematic structure.
6.
Discussion
6.1. Emission-Line Regions outside the Ionization Cone
As described in the previous section, observations of lowionization emission lines, [S II] and [N II], have revealed the
existence of the emission-line regions extending out of the
ionization cone. They are characterized by a higher [S II]/Hα
ratio and a lower [O III] surface brightness than the regions
inside the ionization cone. In particular, east and west regions
at 5 –6 (350 pc–420 pc) from the nucleus are remarkable: the
[S II]/Hα ratios are 0.5–0.6, which are about 2-times higher
Fig. 14. Plots of the radial velocities and FWHMs of Hα and [S II] along the strip shown in figure 13. (a) Radial velocities and b) FWHMs. The open
circles and filled circles represent the values of Hα and [S II] based on a model of a single Gaussian, respectively. The radial velocities and FWHMs of
the individual components of the averaged spectra derived by a model of two Gaussian components are also plotted with the symbols at lower left in (a).
736
T. Ishigaki et al.
than those inside the ionization cone. The kinematic properties
of these regions are largely deviated from those of the outer
regions, which could be explained by the galactic rotation.
The symmetrical distribution of [S II]/Hα with regard to
the nucleus implies that the emission-line regions outside the
cone may be related to the nuclear activities, i.e., the Seyfert
nucleus and/or possible nuclear star-formation. Since the
[S II]/Hα and [N II]/Hα ratios in the regions outside the cone
are too high for normal star-forming regions, we consider
the possibility of ionization by Seyfert nucleus activity. As
mentioned in the introduction, according to the unified model
of Seyfert galaxies, the ionizing continuum from the nucleus
could be strongly absorbed by a dust torus to the direction
out of the ionization cone. Polarimetric observations, however,
have revealed that the ionizing continuum from the nucleus of
NGC 1068 escapes out of the cone by scattering. Sokolowski,
Bland-Hawthorn, and Cecil (1991) discussed the ionization
of diffuse ionized matter distributed over the galactic disk
by the scattered nuclear continuum. In the next subsection,
we attempt to explain the observed properties of the lowionization regions outside the cone based on the ionization by
the scattered nuclear continuum.
In addition to the above arguments, the line decomposition
into a narrow component and a broad component shows that
these regions could contain two different gaseous components.
While the broad component is deviated from the trend of the
galactic rotation, the narrow component seems to follow it.
The [N II]/Hα ratios of the broad components in the east and
west regions outside the cone are 2–3, and higher than those of
the narrow components (∼ 1.5). Although the present decomposition of the line profiles is ad hoc, i.e., it depends on the
spectral dispersion, the signal-to-noise ratio, the selection of
the regions, and so on, these components might correspond
to actual physical components to some extent. Therefore, we
will attempt discussions on the spectroscopic properties of each
component when it is possible.
6.2. Ionization by the Scattered Nuclear Continuum
6.2.1. Photon budget
To examine the possibility of ionization by the scattered
continuum, we first discuss the energetics. As described in
subsection 5.2, the Hα luminosity from the regions outside the
cone is ∼ 2 × 1040 ergs s−1 . In the Case B nebula (Osterbrock
1989), the number of emitted Hα photons is about 45% of
the number of the incident ionizing photons (cf. section 5.8
in Osterbrock 1989). This means that the required rate of the
ionizing photons for these regions is about 1052 photons s−1 .
To estimate the rate of the available scattered ionizing
photons, we assume a condition similar to that proposed by
Sokolowski, Bland-Hawthorn, and Cecil (1991). Here, we
take account of the scattered continuum from the central region
(1 –2 from the nucleus) only. In fact, Antonucci, Hurt,
and Miller (1994) estimate the spatial extent of the scattering
region to be on a scale of ∼ 1 . The composite optical
through X-ray continuum from the central scattering region
of NGC 1068 was given by Pier et al. (1994). From their
composite continuum (figure 3 in Pier et al. 1994), the rate of
the ionizing photons between 13.6 eV and 10 keV is estimated
to be 2.6 × 1052 photons s−1 . If we assume the isotropic
[Vol. 56,
scattered radiation field from the central scattering region, the
fraction of the scattered energy for the regions outside the cone
to the total scattered energy is (1–∆Ω/4π ), where ∆Ω is the
solid angle of the ionization cone. Since the opening angle
of the ionization cone is about 80◦ , ∆Ω is ∼ π . This means
that the number of scattered ionizing photons out of the cone is
about 2.0 × 1052 photons s−1 .
Consequently, although it is a quite rough estimation, the
rate of the available scattered photons could be comparable to
that required for the observed Hα luminosity outside the cone.
We thus conclude that the contribution of the scattered energy
cannot be ignored. Strictly speaking, the available scattered
energy for the ionization of the gases outside the cone may
actually be lower than the above value, because the covering
factor of these gases must be less than unity. In this case, an
additional ionizing source is required to explain the observed
Hα luminosity (e.g., star-forming regions discussed in subsection 6.3).
6.2.2. Ionization properties
Next, the ionization properties of the gases outside the cone
are discussed. In the case of photoionization, the ionization
properties mainly depend on the gas density and the shape and
intensity of the ionizing continuum. According to a spectropolarimetric observation by Antonucci, Hurt, and Miller (1994),
the polarization degree of the nucleus of NGC 1068 is constant
with wavelength. This means that the continuum emitted by
the nucleus is scattered by electrons, and that the spectral shape
of the scattered ionizing continuum is the same as that of the
intrinsic continuum. Consequently, the difference between the
ionizing continua inside and outside the cone is only intensity.
We computed the emission-line ratios by using CLOUDY
(Ferland et al. 1998). We considered the cases of the powerlaw continuum fν ∝ ν α with α = −1.4 and −1.7. We
also considered a continuum proposed by Pier et al. (1994)
and one proposed by Kraemer and Crenshaw (2000), which
are based on the UV and X-ray observations of the nuclear
continuum. With regard to [N II]/Hα and [S II]/Hα, however,
the continuum proposed by Pier et al. (1994) gives almost the
same results as the power-law continuum with α = −1.7, and
that proposed by Kraemer and Crenshaw (2000) does as the
power-law continuum with α = −1.4. Therefore, we show only
the results of models with a power-law continuum. A hydrogen
density of 400cm−3 is adopted, which is estimated as the value
at r = 350 pc (∼ 5 ) based on the [S II] ratio in subsection 5.3.
Once the shape of the ionizing continuum and the gas density
are specified, the ionization model of the gas cloud is parameterized by the ionization parameter, U , which is expressed as
U=
QH
,
4π r 2 cNH
(3)
where QH means the rate of the ionizing photons emitted by
the central source, r is the radius, and NH is the hydrogen
density. If the gases outside the cone are ionized by the
scattered continuum, and have the same density as the gases
inside the cone, the emission-line ratios inside and outside the
cone are expected to follow the same sequence of the ionization parameter. In this subsection, we discriminate between the
ionization parameters inside and outside the cone by subscripts,
“in” and “out”, respectively. For the ionization parameters of
No. 5]
3D Spectrophotometric Observation of NGC 1068
737
Fig. 15. [N II]λ6583/Hα vs. ([S II]λ6716 + λ6731)/Hα diagram. a) Line ratios inside and outside the ionization cone are plotted with the bars of
standard deviations. The line ratios are derived by a single Gaussian model (figure 9). The ratios predicted by the models are shown for the case that
the gas with a hydrogen density of 400 cm−3 is photoionized by a power-law continuum (fν ∝ ν α ) with α = −1.4 (solid lines) and α = −1.7 (dashed
lines). The ionization parameter varies from 10−4 at the upper right to 10−2 at the lower left. The step between the small circles is a factor of 100.25 .
The abundances of the gas are assumed to be solar (left lines), a 3-times solar nitrogen abundance (center lines), and a 4-times solar nitrogen abundance
(right lines). b) Line ratios derived by a model of two Gaussian components are plotted. Regions and components are indicated by the symbols at lower
right. Bars mean the errors due to the fitting process. The model lines are the same as in (a).
the narrow component and the broad one, subscripts, “n” and
“b”, are used, respectively.
The observed emission-line ratios based on a single
Gaussian model can be compared with the computed line
ratios. The computed line ratios are shown in figure 15a along
with the observed ratios. The observed trend that [S II]/Hα
outside the cone is higher than inside the cone could be
explained by the difference in the ionization parameter: as the
ionization parameter decreases, the [S II]/Hα ratio increases. It
is consistent with the hypothesis that the gases outside the cone
are ionized by the scattered continuum. On the other hand, the
[N II]/Hα ratios predicted by the solar abundance model are
much lower than the observed ratios. In the study of AGNs,
the discrepancy of [N II]/Hα between photoionization models
and observations has been often discussed. One of the possibilities to explain the discrepancy is a supersolar abundance
of nitrogen. For example, Kraemer, Ruiz, and Crenshaw
(1998) assumed a 3-times solar nitrogen abundance to explain
the relative intensities of various nitrogen emission lines. In
the present study, models with a 3–4 times solar nitrogen
abundance can well fit the observed line ratios, as shown in
figure 15a. The ionization parameter, Uin ∼ 10−2.5 –10−3.0 ,
could explain the line ratios for the regions inside the cone
and Uout ∼ 10−2.75 –10−3.25 for the regions outside the cone.
These results are summarized in table 7. Capetti, Axon, and
Macchetto (1997b) explored the ionization condition of the
extended emission-line regions by using the photoionization
code based on the emission-line images of HST/WFPC2. They
derived the ionization parameter, Uin ∼ 10−2.5 –10−2.8 , for the
northeast filament inside the cone based on [O III]/(Hα + [N II])
and [O III]/[O II]λ3727 ratios. Our result for the regions inside
the cone is consistent with theirs.
Further, similar discussions to the above can be applied to
the line ratios derived by decomposition based on a model
of two Gaussian components. In figure 15b, the line ratios
of the individual components are plotted. With regard to
the broad components, 4-times solar nitrogen models can
well explain the observed line ratios. The broad component
of the northeast region could be explained by the model of
Uin,b ∼ 10−2.5 –10−3.0 . The broad components of the east and
west regions are plotted around Uout,b ∼ 10−3.0 –10−4.0 . On
the other hand, the narrow components have lower [N II]/Hα
ratios, and are plotted around the sequences of the models
with a 3-times solar nitrogen abundance. If these models are
applied to the narrow components, the ionization parameters
that could explain the observed ratio are Uin,n ∼ 10−2.75 –10−3.0
for the northeast region and Uout,n ∼ 10−3.0 –10−3.25 for the
east and west regions. These results are also summarized in
table 7. It appears that the narrow components outside the cone
have larger ionization parameters than the corresponding broad
components.
The above results suggest that the narrow and broad components may have different nitrogen abundances.
In this
case, however, the explanation for the difference in nitrogen
abundance is uncertain. One of the possible explanations is
that these components might locate at different positions along
the line of sight, and have different origins, and consequently
different nitrogen abundance, with each other (cf. subsection 6.5). Otherwise, if they spatially coexist, it implies that
the mixing might not yet work sufficiently to even out the local
abundance variations for some reason. While the possibility
that they have different nitrogen abundance cannot be ruled
738
T. Ishigaki et al.
out, other interpretations to explain the difference in [N II]/Hα
could also be possible. For example, the contribution from
shock heating could enhance the ratio. In fact, broader line
widths of the broad components imply that they have more
violent motion than the narrow components. If we adopt a
multicomponent model of varying density, such as the model
proposed by Kraemer and Crenshaw (2000), the difference in
[N II]/Hα might be explained by the difference in the fraction
of a tenuous component to a dense one.
From the viewpoint that the gases outside the cone are
ionized by the scattered continuum, the ionization parameter
outside the cone can also be evaluated from the flux of the
observed scattered continuum. The flux of the scattered
continuum was estimated by Pier et al. (1994), and QH could
be about 2.6 × 1052 photons s−1 , as mentioned in subsubsection 6.2.1. Since the spatial extent of the scattering region is
on a scale of ∼ 1 (Antonucci et al. 1994), the distance from
the scattering region to the regions outside the cone can be
approximated as the distance from the nucleus, r ∼ 5 (350 pc).
Therefore, the ionization parameter, Uout , is expected to be
∼ 10−3.8 , assuming NH ∼ 400 cm−3 in equation (3).
Next, we evaluate the expected ionization parameter inside
the cone. Since the intrinsic ionizing continuum cannot be
directly observed, it is difficult to evaluate the ionization
parameter inside the cone. However, many previous studies
had estimated frefl , the fraction of the flux of the scattered
continuum to the intrinsic flux from the nucleus, to be 0.002–
0.05 in a number of ways (see table 4 in Pier et al. 1994).
For example, Miller, Goodrich, and Mathews (1991) estimated
frefl ∼ 0.015 from the [O III] to broad Hβ ratio of the northeast knot, and the observed [O III] and broad Hβ luminosities.
If frefl ∼ 1–5%, the ionization parameter inside the cone, Uin ,
at ∼ 5 from the nucleus is expected to be ∼ 10−1.8 –10−2.5 .
In table 7, the expected ionization parameters evaluated
above are also presented along with those derived from the
observed line ratios. The ionization parameters derived from
the observed line ratios are consistent with the expected ones
within a factor of ∼ 3. We conclude that the ionization properties outside the cone could be explained basically by photoionization by the scattered continuum for a first approximation.
However, in more detail, the observed ionization parameter
of the regions outside the cone is generally somewhat larger
than the expected one. In the case based on a model of two
Gaussian components, it holds true particularly for the narrow
component. It is not clear whether the discrepancy is significant, since there are many uncertainties in this discussion. As a
possible explanation for the discrepancy, we discuss the effect
of the contribution from the star-forming activity in the next
subsection.
6.3. Effects of the Contribution from the Star-Forming
Activity
In addition to Seyfert activity, NGC 1068 is a galaxy with
intense star-forming activity. The Hα image (figure 4b) shows
that there are many features emitting Hα surrounding the
star-forming arms, which might also be star-forming regions.
The ionizing photons from the hot stars in these star-forming
regions as well, as the star-forming arms, might contribute
to the ionization of the gases around the nucleus. In this
[Vol. 56,
Table 7. Ionization parameters.
Observed log U ∗
Component
Expected log U †
Single Gaussian component
Inside the cone
Outside the cone
−2.5– − 3.0
−2.75– − 3.25
−1.8– − 2.5
−3.8
Two Gaussian components
NE narrow
NE broad
E & W narrow
E & W broad
∗
†
−2.75– − 3.0
−2.5– − 3.0
−3.0– − 3.25
−3.0– − 4.0
−1.8– − 2.5
−1.8– − 2.5
−3.8
−3.8
Estimated by comparing the observed line ratios with those predicted
by the photoionization models.
Expected by assuming that the scattered continuum has a flux obtained
by Pier et al. (1994) and frefl ∼ 1–5%. See text in detail.
subsection, the effect of the contribution is discussed.
In order to evaluate the contribution from the star-forming
region, we consider the composite ionizing continuum of a
power-law continuum and a stellar continuum. As a stellar
continuum, a continuum from a 40000 K star, which approximately corresponds to an O6 star, is adopted. The ionization
parameter of the power-law continuum is fixed at 10−3.8 , which
is expected from the flux of the scattered continuum for the
regions outside the cone (subsubsection 6.2.2). The number
of the ionizing photons, QH , of the stellar continuum is varied
from 0.1 to 10-times QH of the power-law continuum.
In figure 16, the line ratios calculated with CLOUDY are
plotted. The figure shows that as the contribution of the
stellar continuum increases, the [N II]/Hα and [S II]/Hα ratios
decrease. Since this trend is similar to that of the sequence
of the ionization parameter of the models in which a gas is
ionized only by a power-law continuum (figure 15), we can
not determine the contribution of the stellar continuum based
on only the [N II]/Hα and [S II]/Hα ratios. For example,
the line ratios outside the cone could be explained by the
model containing the stellar continuum comparable to the
power-law continuum (U ∼ 10−3.8 ) as shown in figure 16a.
On the other hand, the observed line ratios could also be
explained by a model with a power-law continuum (without
the stellar continuum) at U ∼ 10−2.75 –10−3.25 , as discussed in
subsubsection 6.2.2. In the case of the composite-continuum
model, an ionization parameter of a power-law continuum
as large as 10−2.75 –10−3.25 is not needed. Similarly, such
arguments as mentioned above hold for the narrow component outside the cone in the case based on a model of two
Gaussian components (figure 16b). Therefore, the contribution
of the stellar continuum might explain the discrepancy between
the observed ionization parameters outside the cone and the
expected one mentioned in subsubsection 6.2.2 (table 7).
Data on other emission lines are needed to verify the
model with the composite continuum. For example, the
[O III]/Hβ ratio strongly depends on the ionization parameter
in a model with only the power-law continuum: as the ionization parameter increases, the ratio sharply increases. On the
other hand, in a model with the composite continuum, the ratio
can not be enhanced very much (cf. subsection 6.4).
No. 5]
3D Spectrophotometric Observation of NGC 1068
739
Fig. 16. [N II]λ6583/Hα vs. ([S II]λ6716 + λ6731)/Hα diagram. The plotted line ratios are same as in figure 15: a) ratios derived by a single Gaussian
model and b) ratios based on a model of two Gaussian components. The lines indicate the ratios predicted by the models, in which a gas cloud with
a hydrogen density of 400 cm−3 is photoionized by the composite continuum of a power-law continuum and a stellar continuum. The cases of the
power-law continuum with α = −1.4 are indicated by solid lines and those with α = −1.7 are indicated by dashed lines. The ionization parameter for
the power-law continuum is fixed at 10−3.8 . As the stellar continuum, a continuum from a 40000 K star (O6 star) is assumed. The ionization parameter
for the stellar continuum varies from 10−4.8 at the upper right to 10−2.8 at the lower left. A step between the small circles is a factor of 100.5 . The
abundances of the gas are assumed as a 3-times solar nitrogen abundance (left lines) and a 4-times solar nitrogen abundance (right lines).
6.4. Diffuse Arc Structure outside the Ionization Cone
Capetti, Axon, and Macchetto (1997b) found the diffuse arc
structure outside the cone based on the HST/WFPC2 images
(see figure 7b). They derived the ionization parameter of
this structure, U ∼ 10−3.3 –10−3.6 , based on the [O III]/(Hα +
[N II]) and [O III]/[O II] ratios. Bruhweiler et al. (2001) also
analyzed the narrow-band images taken by the HST/WFPC2,
and obtained the [O III]/Hβ ratio map, which is insensitive to
extinction. They derived [O III]/Hβ ∼ 3 for the diffuse arc
structure. By comparing this ratio with our photoionization
models with the power-law continuum mentioned in subsubsection 6.2.2, the ionization parameter could be estimated as
∼ 10−3.25 . Interestingly, these are very similar to the ionization parameters estimated from the [N II]/Hα and [S II]/Hα
ratios outside the cone (table 7). This implies that the diffuse
arc structure might correspond to a part of the low ionization
regions in the present study and, moreover, it may be ionized
by the scattered nuclear continuum. Bruhweiler et al. (2001)
also mentioned photoionization by the scattered continuum as
the ionization mechanism for this structure.
It should be noted that the [O III]/Hβ ratio (∼ 3) of the
diffuse arc structure means that the stellar continuum could not
dominate the ionization, since the contribution of the stellar
continuum can not enhance the ratio very much. If the diffuse
arc structure is ionized by only the scattered continuum, the
expected ionization parameter, U ∼ 10−3.8 , for the region
outside the cone evaluated in subsubsection 6.2.2 may be
underestimated, because [O III]/Hβ in this case is expected to
be lower than 1. In any cases, further spectroscopic investigations with higher spatial resolutions to obtain information on
the kinematics and other emission lines are needed to reveal
the nature of this structure.
6.5. Implication for the Structure of the Emission-Line
Regions around the Nucleus
In addition to the different nature of the kinematics of the
narrow and the broad components described in subsection 5.4,
these components are also different in the spectroscopic nature,
as discussed in subsections 6.2 and 6.3. From these, one might
infer the existence of two actual major components, mostly
corresponding to the two components of the line profile. These
components, if they exist, possibly have close relations with
the active nature of NGC 1068. In this context, the following
picture, although somewhat speculative, could be proposed
according to the results of the present observations.
Since the narrow components inside and outside the cone
follow the galactic rotation, as shown in subsection 5.4,
they may represent the gases in the galactic disk. Arribas,
Mediavilla, and Garcı́a-Lorenzo (1996) revealed that the intensity distribution of the narrow components is biconical and
consistent with the ionization cone. This means that the
ionizing continuum from the nucleus could dominate the
ionization of the narrow component inside the cone. On
the other hand, the narrow component outside the cone
might be ionized by the scattered continuum, as discussed
in subsection 6.2. The narrow component outside the cone,
however, has lower [N II]/Hα and [S II]/Hα ratios than the
ratios expected based on the model that they are ionized by
only the scattered continuum. One of possible explanations is
that they are affected by the star-forming regions in the galactic
disk, as discussed in subsection 6.3. Since the molecular clouds
740
T. Ishigaki et al.
detected in the concerned regions outside the cone also follow
the galactic rotation in general (Helfer, Blitz 1995), the narrow
components outside the cone might be associated with these
molecular clouds.
The appearance of the diffuse arc structure discussed in
subsection 6.4 is like the spiral arm. It may suggest that it is
the structure in the galactic disk. If this is the case, a relationship between the diffuse arc structure and the narrow component in the present study is implied. In the west region of the
nucleus, the faint spiral arms are also seen in the WFPC2 image
presented by Bruhweiler et al. (2001). These might also correspond to the narrow component in the west region.
As mentioned in subsection 5.4, the S-distortion seen in
the radial velocity map is due to the broad components.
Arribas, Mediavilla, and Garcı́a-Lorenzo (1996) interpret the
S-distortion as being a result of radial motions outside the
galaxy’s plane. Many kinematic studies on the non-rotational
motions inside the ionization cone have been carried out previously. For the innermost regions (≤ 3 ), two kinematic models
have been mainly proposed: a biconical outflow (Cecil et al.
1990; Crenshaw, Kraemer 2000) and an expansion outward
from the radio jet (Axon et al. 1998). Beyond a radius of
3 , a component near the systemic velocity and a component redshifted by ∼ 500 km s−1 have been detected (Pécontal
et al. 1997; Cecil et al. 2002). The broad component of the
northeast region in the present study might be connected to
the redshifted component. Since the redshifted components
spatially associate with a northeast radio lobe, Pécontal et al.
(1997) and Cecil et al. (2002), as well as Axon et al. (1998),
suggested that the redshifted components around a radius of
3 –4 within the ionization cone represent the gases pushed
into the galactic disk by a lateral expansion of the northeast
radio lobe.
A lateral expansion of the radio lobe could be a possible
mechanism responsible for the broad component outside the
ionization cone. The radio lobe might expand laterally in
all directions. Since the east region in this study is near the
boundary of the northeast cone, the broad component in this
region could represent gases around the boundary of the ionization cone pushed out of the cone by an expansion of the radio
lobe. The fact that the broad component in the east region has
a radial velocity near to the systemic one supports this picture,
because the direction of the expansion in this region is expected
to be nearly perpendicular to our line of sight. On the other
hand, the broad component in the west region has a velocity
blueshifted with respect to the systemic. This might represent the gases being pushed toward us by the southwest radio
lobe behind the galactic disk. The difference in the ionization property between the broad component in the northeast
region and those in the east and west regions could be explained
by the difference in the ionization parameter, as discussed in
subsubsection 6.2.2. The gases pushed out of the cone might
be illuminated by the scattered continuum, and observed as the
broad components in the east and west regions.
Finally, it should be noted that our photoionization model
is one of the simplest models, and is not a unique solution.
Recently, more complex models have been investigated based
on detailed slit spectroscopy, including the contribution of
matter-bounded clouds, attenuation of the ionizing continuum
[Vol. 56,
by X-ray and UV absorbers, and so on (e.g., Kraemer,
Crenshaw 2000). The discrepancy of [N II]/Hα between observations and photoionization models can also be explained
by including multicomponent models of varying density
(Kraemer, Crenshaw 2000), instead of the nonsolar abundance
models adopted in this study. Furthermore, shock heating
associated with radial outflow from the nucleus and/or the
expansion of the radio lobe could contribute to ionization of
the gases. Further tridimensional spectrophotometric investigations with a higher spatial resolution and a wider spectral
coverage will provide useful information to constrain the structure and radiation field outside of the ionization cone.
7.
Conclusions
We have developed a tridimensional spectrograph, Kyoto
tridimensional spectrograph I, which has four modes: imaging
Fabry–Perot interferometry, integral field spectroscopy, longslit spectroscopy, and filter imaging. A brief description of this
instrument is presented.
By using this instrument, tridimensional spectrophotometric
observations of NGC 1068 were conducted for the purpose
of investigating the properties of the low-ionization emissionline ([S II] and [N II]) regions around the nucleus. The results
reveal the existence of ionized gases extending out of the
ionization cone, which are characterized by about 1.5–2 times
higher [S II]/Hα ratios than inside the cone. In particular, the
[S II]/Hα ratio peaks at 5 –6 east and west of the nucleus
(∼ 0.5–0.6). The [N II]/Hα ratio shows a similar tendency,
but not so clearly. Ionization by scattered nuclear continuum
was discussed. The scattered nuclear continuum might have
a comparable energy (∼ 2 × 1052 photons s−1 ) to produce the
observed Hα flux. The ionization properties outside the cone
could be basically explained by ionization due to the scattered
continuum, while the contribution of the stellar continuum
from the star-forming regions could not be rejected by only
the [N II]/Hα and [S II]/Hα ratios. In addition, we suggest
that the diffuse arc structure seen in the HST image might be a
part of the low-ionization regions in the present study, and be
photoionized by the scattered nuclear continuum.
We have decomposed the line profiles of the averaged
spectra inside and outside the ionization cone into two components: a narrow component and a broad one. While the
narrow components seem to follow the galactic rotation, the
broad components largely deviate from it. The decomposition reveals the emission-line ratios of the individual components. The [N II]/Hα ratios of the broad components outside
the cone may be much higher than those of the narrow components. Although these results are largely dependent on the
decomposition, these two components might correspond to
the actual major gaseous components. In this context, we
suggest a picture, although somewhat speculative, to explain
the structure of the emission-line regions outside the cone.
The gases in the galactic disk, which are observed as the narrow
components, may be ionized by the scattered continuum, and
might be partly affected by the star-forming regions in the disk.
The broad component might be related to some non-rotational
motion driven by the nuclear activity, and could also be ionized
by the scattered continuum.
No. 5]
3D Spectrophotometric Observation of NGC 1068
We acknowledge and appreciate the contributions of
Dr. Kentaro Aoki for developing the instrument and during
test observations.
We are also grateful to Dr. Masaki
Sekiguchi and Dr. Junichi Noumaru for their help in developing the CCD system of Kyoto 3DI. We would like to
741
thank the staff of Okayama Astrophysical Observatory for
their kind help concerning the observations. We also thank
Dr. Ian Evans for his useful comments and suggestions as
the referee. We acknowledge partial financial support from
National Astronomical Observatory.
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