4.3. High Excitation Outflow Cone

4.3.1. Line Splitting

In Fig. 16 the nuclear region of NGC 1365 was presented in three different ways: a. continuum subtracted H + [N II] 6548, 6583, b. continuum subtracted [O III] 5007, and c. broad band Johnson B. These three frames have been combined in Fig. 19 into a colour picture of the nuclear region of NGC 1365, where the blue component is derived from the Johnson B image, the green and red from the narrow-band continuum-subtracted [OIII] 5007 and H images respectively. There is a clear difference in the distributions of H and [O III]. Both H and [O III] are strong at the nucleus, while H is dominating in the circumnuclear hot spots and the H II regions along the northern edge of the strong dust lane entering the nuclear region from the bar. The [O III], on the other hand, shows a broad one-sided extension to the SE from the nucleus centered roughly along the minor axis of the galaxy.

Figure 19. Combined colour picture of the nuclear region of NGC 1365, where blue colour represents continuum B-band, red H, and yellow-green [O III] 5007 respectively. Danish/ESO 1.5-m telescope. The scale indicates offsets from the nucleus in arcseconds. The upper right side (NW) is the near one. From Jörsäter & Lindblad (1989); P.O. Lindblad et al. (1996b).

As mentioned already in Section 3.2, M.M. Phillips et al. (1983) pointed out this extension of the [O III] emission and also discovered that the [O III] lines were split into two components, where the stronger component gave a velocity field differing from that of H. One of possible models suggested by them implied that the double lines represented high excitation gas streaming out from the nucleus along the surface of a hollow cone aligned with the rotation axis of the galaxy.

Using a Boksenberg Image Photon Counting System on the ESO 3.6-m telescope, Jörsäter et al. (1984a) confirmed the peculiar velocities given by the [O III] lines and showed that the high excitation [Ne III] 3868 and He II 4686 also shared the peculiar velocities. However, from the few spectra available they could not propose a very plausible model.

With the TAURUS scanning Fabry-Perot interferometer on the Anglo-Australian telescope, Edmunds et al. (1988) confirmed the splitting of the [O III] 5007 line. They suggested that one of the components shared the velocity field of the H disk and that the other represented bipolar outflow from the nuclear region directed out from the galactic plane. From a number of slit spectra Jörsäter and Lindblad (1989) mapped the two velocity fields in the nuclear region. Also Storchi-Bergman and Bonatto (1991) mapped the nuclear [O III] distribution.

Considering that the NW side of the galaxy is the near one, the shape of the [O III] emission region as shown in Fig. 19 is suggestive of a cone-like structure with its apex at the nucleus and extending out of the plane being projected against the far side of the galaxy. The velocities connected with this structure then give an outward motion from the nucleus within this cone.

The splitting of the [O III] lines in the nuclear region of NGC 1365 is demonstrated in Fig. 20, where the spectrograph slit has been placed over the hot spot L1 (Fig. 16) in a position angle of 332°, such that the right side of Fig. 20 corresponds to the NW side of the galaxy. The negative velocities towards the NW and the positive towards SE, shared by [O III] and H, represent the disk velocity field, while the velocity components, positive towards NW and negative towards SE, only shown by [O III], represent the outflow cone.

Figure 20. Radial velocities along a slit passing over the hot spot L1 (see Fig. 16) with a position angle 332°. The distance scale has its zero point at L1 and is increasing in the direction PA = 332&176;. The zero point of velocity is +1630 km s-1. From P.O. Lindblad et al. (1996b).

1. A field given mainly by low excitation lines, i.e. the hydrogen Balmer lines and forbidden [N II], [S II], [O II] lines, and shown in Fig. 11. This field represents the rotational velocities in the disk.

2. A field given mainly by higher excitation lines, i.e. forbidden [O III] and [Ne III] lines which represents the outflow cone. This field, which contains also the disk component where the lines are not seen double, is shown in Fig. 21.

A comparison of Fig. 11 and Fig. 21 shows the obvious differences between the two fields.

High excitation emission regions, characterized by strong [O III] line radiation, and with asymmetric morphology are nowadays frequently observed around AGNs. The velocity fields of these emission regions often differ markedly from those of lower excitation regions. Their shapes sometimes suggest a cone or a double cone with its apex at the nucleus. Wilson and Tsvetanov (1994) present a list of 11 Seyfert galaxies with such ionization cones. It is generally believed that the ionizing radiation from the nucleus is confined to within this cone - in the frame of the unified picture - by an obscuring torus.

Analysis of the peculiar motions in such regions have generally led to the conclusion that they give evidence of outflow motion within the cone away from the active nucleus (e.g. Morris et al. 1985; Storchi-Bergman et al. 1992; Maiolino et al. 1994). If the cone has such an orientation and opening angle that it intersects with the main plane of the galaxy, it may however create what is called an extended narrow line region (ENLR), or more appropriately extended emission line region (EELR), confined to the plane, where the motions will agree with the motions of the galactic rotation (Unger et al. 1987).