S.J. Wagner and M. Dietrich
Landessternwarte, Königstuhl, 69117 Heidelberg, Germany
partly based on data collected at the European Southern Observatory partly based on data retrieved from the HST archive supported by the Deutsche Forschunggemeinschaft through SFB 328
35 pc). The NLR of NGC 1068 has long been
known to be unusually broad and highly structured. An identification
of the individual peaks with the knots resolved by HST allows the
derivation of the velocity distributions in the NLR of this
prototypical object. This is feasible with ground-based spectroscopy
combined with HST imaging.
We have taken 2D-spectra of NGC 1068 with high spectral resolution
(
). The slit covers the inner 6 arcsec of the
center of NGC 1068. Due to the high resolution the blend of clouds is
decomposed in velocity space. The deblended components can be located
in the l-v diagrams with an accuracy of 0 along one spatial
coordinate. Orthogonal slit orientations allow the precise location of
knots and the identification with the clouds resolved in the HST
narrow band images.
We derive the velocities of the 15 most luminous clouds within the
central 2 arcsec. The components cover a velocity range of 
1000
.
The widths of the individual clouds are an order of
magnitude larger than the thermal line width (
). No single regular velocity field (rotation, inflow,
outflow) fits the distribution of components in phase space completely.
We find evidence for two different kinematic systems and high degree of
turbulence and very high internal dispersions within individual
clouds.
Keywords: NLR, Seyfert Galaxies, NGC 1068, Outflows
NGC 1068 (3C 71), one of the best studied Seyfert galaxies is among
the nearest AGN. Lying at a distance of 
23 Mpc, 1''
corresponds to 110 pc. Subarcsec resolution studies hence allow
spatially resolved studies of the NLR. Although being regarded for a
long time as the prototypical Seyfert 2 object, the discovery of
polarized emission from a broad-line region suggests the presence of a
hidden Seyfert 1 nucleus, which can be observed in scattered light. The
position of the hidden nucleus has been determined from polarimetric HST
observations (Capetti et al. 1995). The light from the nucleus is
blocked along our line of sight, but nuclear radiation may escape more
freely along other directions. Interferometric observations show a
non-relativistic radio outflow (Wilson & Ulvestad 1987). The
Narrow Line Region is extended along the same direction (Walker 1968)
and shows indication of clumpy structure (Anderson 1971). If it is
photoionized by a nuclear continuum this morphology suggests the NLR
to be a radiation bounded ionization `cone' which owes its shape to an
obscuring torus. Alternatively, the emission may partly be caused by
interactions of the circumnuclear matter and the outflowing plasma.
In the former case, the widths of emission-lines reflect the strength of the gravitational potential at the luminosity-weighted radius where the corresponding line is emitted. In such a picture, non-gravitational forces are not assumed to contribute substantially to the line-widths of the NLR. This is independent of the details of the velocity field (chaotic motion or rotation) but contrasts those models which attribute the line-shapes to inflow or outflow dominated scenarios.
Emission-line widths of forbidden transitions are often found to be
comparable to the widths of absorption features of the integrated
light of the stellar population of the bulge of the host galaxy
(Nelson & Whittle 1995). There are a few notable exception to this
trend. Several AGN show forbidden lines whose widths exceed
1000
. The largest widths in an AGN have been recorded in
NGC 1068 (FWHM 1200
). This extreme value does not seem
to be related to the presence of a hidden Seyfert 1 nucleus (with
other sources of comparable polarization characteristics having normal
line widths of forbidden transitions).
The possibility to resolve the NLR spatially now permits investigations of the velocity field on parsec scales. This enables us to address the question whether the large FWHM is due to large-scale velocity gradients, turbulence or due to scattering. High resolution imaging with HST has demonstrated that the NLR is composed of a large set of clouds with angular separations much smaller than the limited resolution of ground based telescopes. Measuring their bulk velocities can be used to discriminate between rotation and outflow which in turn can be used to derive masses or orientation and acceleration mechanisms. Line widths can be used to determine turbulence and study length scales and shock formation.
The ground-based spectroscopic data were acquired with the CASPEC
echelle spectrograph of the European Southern Observatory attached to
the 3.6m telescope of ESO during two runs. In November 1993 data were
taken in the red wavelength regime, including H 
and [NII], in
December 1994, we could use the blue sensitive cross-disperser to get
data of [OIII] and H 
. The spectral resolution was measured to
be 15
, the seeing varied throughout both campaigns with an
average effective spatial FWHM of 1. Details of the reduction
procedure are described elsewhere. Following standard reduction, the
individual orders were rectified, maintaining the full spatial
information and rebinned into velocity with respect to to a systemic
velocity of 1145
(Allen et al. 1971). The slit was
oriented along PA 32^o and 122^o (along and perpendicular to the
radio feature and the ENLR). Three sets of data in each of the two
position angles were taken in both wavelength ranges. The effective
exposure on NGC 1068 amounted to 12 hours.
The HST images were taken from the archive (frame x24e0101r). We used the F501N filter post-COSTAR observations with FOC (f/96) by Macchetto et al. (1994) which have a scale of 0/pix and a resolution of 0.
The velocity field of NGC 1068 was constructed by identifying all
individual components in each of the long slit spectrograms. The
integrated flux, velocity, and the position along the slit
of each knot was determined by fitting elliptical Gaussians to the
profiles. The identification was checked by treating all three frames
taken with the same slit orientation separately. The uncertainty of
the model fitting was determined by comparing the parameters derived
for each of the knots in the three spectrograms. After locating
individual knots in the HST images, we cross-identified the knots by
requiring identical spatial coordinates along the slit and comparable
intensities in the long-slit spectrograms and the [OIII] image. While
the spatial coordinate is a unique but insufficient criterion, the
fluxes (or flux ratios) cannot be expected to be identical. The F501N
filter has a FWHM of 7.4 nm and contains a significant fraction of
continuum emission. The equivalent width of the spatially integrated
[OIII] line within the 6 
1.5 arcsec slit is about 43 nm, but
may vary for individual knots. In a few cases, this unknown
contamination of continuum emission prohibits a cross identification
of knots with identical locations along the slit. In these cases,
we used the spectrograms taken with a PA of 122^o, perpendicular to
the long axis to break the degeneracy. This led to a unique location
of the 15 best defined clumps that can be seen the central 6 
2 arcsec of the HST image in the longslit spectrograms. The comparison
of the FOC image and the CASPEC spectrograms is shown in
Figure 1. Peak velocities and line widths of each of the
clouds were determined to derive the velocity field and the internal
kinematic state of the clouds. The luminosity function of these clouds
is fairly steep with more then 90 % of the line flux being emitted by
four components.
Figure: Left panel: [OIII] FOC image (F501N) of the NLR of NGC 1068
within a centered aperture of 1.5 
6 arcsec (equivalent to the
CASPEC slit used to obtain the long-slit spectrogram on the right. All
individual components visible in the direct image can be identified
(and separated) in the velocity map. This allows a complete mapping of
the velocity field of the clouds seen in the HST image.
Within the central 2 (275 pc), a large-scale gradient of
V 
1350
can be observed. While the range of
velocities of the large-scale gradient is not centered on the systemic
velocity (-850
to +500
), the systemic
velocity is observed at the `hidden' nucleus. Both of these results
suggest that emission is due to outflow rather than to
rotation. Since the linear bulk motion is oriented along the radio
structure, it is tempting to identify it with the outflow. The blue
shift towards the north-east then indicates that the outflow on this
side is approaching the observer. The string of five of the brighter
clouds which form a ridge at PA 10^o follow a common trend in
velocity space with much less scatter than the overall field.
In addition to the coherent large scale gradient, a large velocity
spread is observed at different separations from the nucleus. A few
components along a common line of sight (l.o.s.) are found which have
velocities that differ by up to 
v 
1700
.
While large spreads in velocities might be expected in case of
outflows within cones of very wide opening angles, we attribute the
difference to random motion (i.e., large scale turbulence). First,
the morphology of the NLR in general suggests an opening angle of less
than 90^o; second, the observed spread along a single
l.o.s. 
v > 
V, the large scale gradient. Furthermore,
l.o.s. integration through an axisymmetric cone does not explain
complexity of large scale spread.
In spite of the blending of the individual clouds in ground-based
images, the components can be separated well enough in the high
spectral resolution spectrograms to investigate the line profiles of
the individual knots. A few of the individual clouds have large
line-widths of up to FWHM 
300
. These features do
not show any substructure in the line profile. The corresponding
clouds in the HST image are not blended with any significant
contribution from other clouds. We, hence, rule out that the large line
widths are due to superpositions of two distinct components along the
line of sight. The clouds are very compact in the HST images with
diameters of about 100 mas, i.e., 11 pc, and separated from the hidden
nucleus. Their peak velocities are different from the systemic
velocity. Both of these characteristics clearly indicate that these
clouds do not lie in the central potential well. They are not confined
by the gravitational field of the nucleus. Irrespective of whether
these clouds are photoionized, shock heated or scatter nuclear line
emission, confinement by gas pressure of magnetic fields would require
very extreme conditions. We conclude that the individual clouds are
transitory. They dissolve on a time-scale of several 10
years.
A few of the components can be spatially resolved in the FOC image,
indicating substructure on the scale of a few parsecs. Pronounced
substructure within 0 is seen in clouds A, B, C, D, and F of
Evans et al. (1991). In some of these cases, substructure is seen in
the line profiles of these knots. The line widths of these compact
clumps reaches up to 600
, but the role of
l.o.s. integration of spatially separated cannot be estimated.
In spite of the overall trend in the large scale velocity field, many
clouds are found to show substantial differences in their velocity.
Small complexes of clouds spread over as much as 600
within about (30 pc)
. Individual clouds of about 10 pc diameter
show line widths of up to 300 
. We find evidence for
turbulent motion with high velocities on all scales investigated in
this study. The large velocities with respect to adjacent clouds and
intercloud matter induce strong shocks which are likely to contribute
significantly to the ionization of the NLR plasma. Although there does
not seem to be a close relation between bright knots of the radio
outflow and bright, line-emitting clouds, the former propagates
through the turbulent NLR. Shock induced excitation has been shown to
be an important contributor in the more powerful jets of radio sources
(e.g., Sutherland et al. 1993), but is likely to play an important
role in NGC 1068 as well. Quantitative analyses require additional
information on the temperature and density structure within the NLR.
Such information is currently derived by determining diagnostic
line ratios for individual clouds.
We have combined the high spatial resolution of post-COSTAR FOC
observations with high spectral resolution of ground-based longslit
spectroscopy to determine the velocity field of the clumpy Narrow Line
Region of NGC 1068. We find that the unusually large width of the
entire forbidden line region is due to the superposition of the individual
clouds. We find that a large fraction of the clouds cluster around a
linear trend in the l-v diagram derived from the longslit
spectrograms. This regular velocity field crosses the location of the
hidden nucleus at the systemic velocity. The asymmetry of the maximum
velocities and the large systemic velocities suggest that this regular
velocity field is not caused by rotation. We favor an outflow
hypothesis. Several individual clouds differ from this general
velocity field by up to 1700
. Some of the compact, unblended,
off-nuclear clouds exhibit large line widths. It is highly unlikely
that the widths are due to blending along the l.o.s.. Since the
clouds cannot be confined by gravity, we favor the hypothesis that
the clouds are shock heated and dissolve fast.
We are grateful to Dr. A. Jüttner, who carried out most of the observations.
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