D. J. Axon
European Space Agency, Space Telescope Science Institute, 3700 San
Martin Drive, Baltimore, MD 21218, USA
A. Capetti
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
MD 21218, USA
F. D. Macchetto
European Space Agency, Space Telescope Science Institute, 3700 San
Martin Drive, Baltimore, MD 21218, USA
W. B. Sparks
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
MD 21218, USA
On Leave from The Nuffield Radio Astronomy Laboratory, University of Manchester, Jodrell Bank, Macclesfield Cheshire, SK11 9DL, UK.
Keywords: Active Galactic Nuclei,Seyfert Galaxies,Narrow Line Region
Largely due to the elegance of photoionization models in explaining the general form of the ionization structure and luminosity of the lines, the standard model of the NLR, as a turbulent zone of outflowing material photoionized by the nucleus, has remained nearly unchallenged for over a decade.
Figure: The [OIII] emission line profiles of the NLR of NGC 5929. Shown
as an insert is the radio structure. Notice that there are high velocity
profile component associated with each of the radio lobes.
The NLR line profiles in most nearby Seyfert galaxies
with linear radio sources show high velocity > 500
components
which are spatially associated with their off-nuclear radio structure. A
good example is NGC 5929 (Whittle et. al 1986) which is shown in
Figure 1. This kinematic association suggests that the NLR originates from
the shock compression in the region of interaction between the ejected
radio material and the ambient gas in the surrounding galaxy. The implied
shock velocities are very high and one must allow for the finite cooling
time of the gas. This lead Taylor, Dyson & Axon (1991) to develop fast
bow shock models for the NLR (Figure 2) which included both the effects of
the cooling time and nuclear photoionization and which are able explain
rather well the observed kinematics of the NLR.
Using the Hubble Space Telescope (HST), we can now resolve the NLR of many
nearby AGN. Ignoring the spectroscopic evidence for strong interactions,
the interpretation of the NLR structure seen in the early HST emission
studies of NLR was forced into a pure nuclear photoionization framework (e.g.,
Evans et al. 1993), most probably due to the seductive appeal of the
idea of ``radiation cones'' which have widely been invoked to explain the
much larger scale ENLR (Unger et al. 1987) emission line structures.
Figure: A sketch showing the basic assumptions of the fast bow shock
model of the NLR. The bowshock is formed around the advancing radio ejecta,
which for the case shown is moving away from the Nucleus which is situated
on the right hand side of the figure.
The extensive HST imaging and ground based spectroscopic results described here however require a fundamental revision in this picture of the physics of the NLR, driving us away from nuclear photoionization models, and providing dramatic proof that shocks do indeed play a dominant role, not only in forming, but ionizing the NLR.
Figure: A montage showing the 
emission line structures seen in
4 Seyfert galaxies which have been studied with HST. The 
images
are shown in grey scale, with superimposed the radio maps as contours.The
top two panels show examples of Seyferts with radio lobes, with on the
left left Mrk78 and on the right Mrk 573. The bottom two panels show
examples of Seyferts with jet-like radio morphologies, on the left Mrk 3
and on the right Mrk 348
Extensive HST emission line imaging unambiguously demonstrates that the interaction between the radio ejecta and the ISM is fundamental driver of the NLR. First, the NLR are often misaligned with, and much narrower than, the ENLR and to fit them into a radiation cone picture, very wide and partially filled cones would be needed since the photons cannot go around corners! Moreover, the morphology of the emission lines gas is directly related to that of the the radio emission (Figure 3). In Seyferts with radio jets (e.g., Mrk 3, Mrk 348 & Mrk 6) the emission line structure also appears jet-like and is spatially coincident with the jet itself (Capetti et al. 1995,1996). Azimuthal shocks created by the expansion of hot material laterally, around the jet axis, creates an expanding and cooling emission line halo around the jet.
Figure: The [OIII] emission line structure of NGC3393
In contrast, the radio lobes (e.g., Mrk 573, Mrk 78, NGC 3393,IRAS 0421+045 & IRAS 1105-035) are surrounded by bow-shock-like emission line structures (Capetti et al. 1995,1996), produced by the sweeping-up of gas by the expanding radio lobes (Figures 3 & 4).
Figure: Emission line ratio variations with radius along the major axis
of the NLR of NGC 3393. The top panel shows the variation of the 
flux with radius as a dotted line, together with the variation of
(solid squares). The positions of the two emission line
arcs are clearly identified. The middle panel show a similar plot for the
ratio, while the bottom panel shows the
variation of the density sensitive 
ratio (which is essentially
constant). Notice that there is little evidence for a change of these line
ratios with radius.
Emission line ratio maps (Capetti et al. 1995) show that, except at
the lobes, where dramatic decrease in ionization occurs,
the diagnostic line ratios (e.g., 
, 
,
) are essentially constant throughout NLR (c.f., decline in
ENLR with radius (e.g., Robinson et al. 1994,1995.)) We illustrate this
with the results obtained for NGC 3393 (Axon et al. (1996) which are
shown in Figures 4 & 5.
Figure 4 shows the emission line structure of the NLR ,which is very similar
to that of Mrk 573, with two prominent bow-shock created arcs located at
the radio lobes. Figure 5 shows the radial variations of the
and 
line ratios
and the electron density 
along the major axis of the NLR. A similar
behavior is also seen in the 
ratio.
The radial variations of the key line ratios which measure the Ionization
Parameter, U, do not behave as expected for a geometrically diluted
nuclear radiation field, but remain essentially flat. After correction for
density changes, this result implies that the Ionizing Photon
Luminosity, 
, is apparently increasing with radius.
This ionization structure in the NLR can be explained if we include the
combined effects of shocks and nuclear photoionization, if the shocks are
sufficiently fast (velocities > a few hundred 
) to be auto-ionizing.
In such fast shocks the bulk of the mechanical energy input is radiated in
the UV and soft X-ray bands in the cooling zone behind the shock. This hot
gas then provides a source of ``local'' ionization. Even though the
ionizing luminosity produced locally can be many orders of magnitude
smaller that of the nucleus it can have an important impact on the observed
ionization structure, because of its close proximity to the emission line
clouds, and can even dominate over the nucleus.
Sutherland et al. (1993) have argued that auto-ionizing shocks and
nuclear photoionization can be distinguished as the relative strengths of
the UV resonance and inter-combination lines (e.g., O[III]/ OIV] and
CII]/CIII]/CIV) produced by auto-ionizing shocks are substantially
higher than expected from nuclear photoionization.
Some credence to this idea is provided by the HUT observations of NGC 1068
(Kriss et al. 1994) who show such anomalous line ratios in NLR. In
general however both sources of ionization are expected to play a role and
in this situation the contribution from local ionization is now not so
clearly recognizable, as the crisp distinction between the photo-ionization
and the shock models is lost.
HST emission line imagery of the Narrow Line Region (NLR) of Seyfert galaxies
clearly shows that the interaction between the ISM and the ejected radio
plasma is instrumental in both forming the NLR and moulding its physical
structure. Seyfert galaxies with a lobe-like radio morphology have bow
shock shaped emission line regions while those with a jet-like radio
structure show linear (jet-like) NLR structures.
The radial variations of key
line ratios which measure the ionization parameter, U
do not behave as expected for a geometrically diluted nuclear radiation
field, but remain essentially flat. After correction for density changes,
this result implies that the Ionizing Photon Luminosity ,
is
apparently increasing with radius.
This ionization structure in the NLR can be explained if we include the
combined effects of shocks and nuclear photoionization. The shocks are
sufficiently fast to be auto-ionizing and this ``local'' ionization is
a major contributor to the ionizing photon budget, and may even dominate
over the nucleus is some cases.
We are grateful to A. Robinson for stimulating discussion on the ionization structure of the Narrow Line Region, and to Alan Pedlar for providing us with numerous Merlin radio maps.
Axon D. J., Robinson, A., Capetti, A., Macchetto, F. D. & Stirpe, G. 1996, in press
Capetti, A., Axon, D. J., Macchetto, F. D., Sparks, W., & Boksenberg, A. 1996, ApJ, in press
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D. J. Axon, A. Capetti, F. D. Macchetto, and W. B. SparksAxon et al.Progress Towards Understanding the Physics of the NLR in Seyferts