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Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

Progress Towards Understanding the Physics of the Narrow Line Region of Seyfert Galaxies

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.

 

Abstract:

HST emission line imagery of the Narrow Line Region (NLR) of Seyfert galaxies, has for the first time resolved their structure and has resulted in considerable advances in our understanding of the physical processes involved in the formation of the NLR. It has become clear that the interaction between the ejected radio plasma and the interstellar medium plays a fundamental role in determining the structure of the NLR. In Seyfert galaxies whose radio morphology is lobe-like bow shock like emission line arcs invariably appear at the working surface on the advancing edge of the lobes. In contrast, Seyferts with jet-like radio morphologies also show linear (jet-like) NLR structures, which are formed as cooling cocoons of hot material expand azimuthally around the jet. A fundamental change in our basic picture of the NLR is required to explain the variations of the ionization parameter with radius, which remains much flatter than expected on the basis of the measured density gradients, and implies that local ionization from hot gas created in the shock interaction is a major contributor to ionizing photon budget.

Keywords: Active Galactic Nuclei,Seyfert Galaxies,Narrow Line Region

Introduction

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

The Physical Structure of the NLR Revealed---HST Imaging

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 Ratios

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.

Local Ionization from Fast Shocks

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.

Conclusions

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.

Acknowledgments:

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.

References:

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

Capetti, A., Axon, D. J., Macchetto, F. D., Sparks, W., & Boksenberg, A. 1995a, ApJ, 446,1155

Capetti, A., Axon, D. J., Kukula, M., Macchetto, F. D., Pedlar, A., Sparks, W., & Boksenberg, A. 1995b, ApJ, 454, L85

Evans, I. et al. 1993, ApJ, 417, 82

Kriss, G. E. et al. 1992, ApJ, 394, L37

Robinson, A. et al. 1994, A&A, 291, 473

Sutherland, R. S., Bicknell, G. V., & Dopita, M. A. 1993, ApJ, 414, 506

Taylor, D., Dyson, J., & Axon, D. J. 1992, MNRAS, 255, 35

Whittle, M. J. 1986, MNRAS, 222, 125

Unger, S. et al. 1987, MNRAS, 228, 67

D. J. Axon, A. Capetti, F. D. Macchetto, and W. B. SparksAxon et al.Progress Towards Understanding the Physics of the NLR in Seyferts



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