4.3.2. Model outflow cone

The velocity field of the outflow cone in NGC 1365, shown in Fig. 21, was derived by Hjelm and Lindblad (1996) from a number of slit spectra containing the [O III] 4959, 5007 and [Ne III] 3869 lines. The authors fitted a model of an outflow double cone to the observations with the requirement that it should reproduce the velocities along the slits, giving the cone field of Fig. 21, as well as the distribution of the intensity of line emission as found along the slits and in the narrow band image from Fig. 16b. The geometry of the cone in the best fitting model is shown in Fig. 22. We see the cone extending from the nucleus out of the galactic plane with its symmetry axis closely aligned with the rotation axis of the galaxy. The counter cone is seen below the plane through absorbing dust. In this model the emission in the [O III] lines originates from three components:

1. The near half of the double cone, which is seen in front of the plane of the galaxy.

2. The symmetric counter cone seen absorbed through the plane. The absorption is given by a map of the extinction as derived from broad band observations at different wavelengths.

3. A component in the plane belonging to the disk and assumed to have the same relative distribution and the same velocities as the hydrogen lines.

Figure 21. The cone velocity field. The spectral slit positions are shown as straight lines, and the double lines indicate where double line profiles have been measured. Where double lines have not been seen the field is blended with the disk field. The zero point velocity is +1630 km s-1 and the contour interval is 20 km s-1. The units on the axes are arcseconds offset from the centre. From Hjelm & Lindblad (1996).

Figure 22. Model ionization double cone reaching out from the nucleus of NGC 1365. The straight line marks the line of nodes. The NW (upper right) side of the galaxy is the near one. The cone axis is closely aligned with the rotation axis of the galaxy and the counter-cone is seen absorbed through the galactic plane. From P.O. Lindblad et al. (1996b).

• The shape and orientation of the cone. The full opening angle of the cone comes out as 100° and the cone axis deviates by only 5° from the rotation axis of the galaxy at large as determined from the radio 21-cm line observations. The cone itself is rather hollow within a few degrees from the axis. With an inclination of the galaxy of 40° the orientation means that the line of sight to the nucleus falls inside the cone. If the opening angle of the cone is determined by an absorbing torus according to the unified picture of AGNs, the observer still will be able to see the Seyfert nucleus and also the broad line region as is actually the case in NGC 1365. Wilson and Tsvetanov (1994) find a possible trend for ionization cones to align with the rotation axis for late-type galaxies and argue that the reason may be that the inner gas disk is accreted from the galaxy-scale gas disk. As their diagram shows, NGC 1365 is an extreme case with very close alignment. This then might be due to the strong bar which is an efficient agent in transferring gas from the outer disk into the nuclear region.

  The position angle of the cone axis projected on the sky differs only 1° from that of the radio jet of Sandqvist et al. (1995) (see Section 4.4). Assuming that the cone axis and the jet are actually aligned in space we then will know the inclination of the jet to the line of sight. On the assumption that the absence of a counter-jet is exclusively due to relativistic beaming, Hjelm and Lindblad found that the jet velocity must be > 0.7c, where c is the velocity of light. On the other hand, the authors conclude that a nuclear disk with typical values as a temperature of 10⁴ K, electron density of 10⁴ cm^-3 and a column density of 10²³ atoms cm^-2 could give an optical thickness in the radio domain sufficient to block out the radiation from a counter-jet.

• The velocity pattern inside the cone. The radial outflow is accelerated. A constant acceleration with a velocity increasing proportionally with the distance r to the centre gives an acceptable fit, although an acceleration with the velocity increasing as r² gives a still somewhat better fit. The velocity is largest at the axis of the cone and decreases to zero at the edge. This is dictated by the need to get a sufficient amount of gas with low velocity.

Astrophysically meaningful line ratios, where the undisturbed cone contribution could be separated out from other components, could only be obtained from one region in the counter cone where also the H, [N II] 6583 and [S II] 6716, 6731 lines were seen split. The ratios [N II] 6583 / H and [S II] (6716 + 6731) / H from the cone, although very uncertain, fall in the realm of narrow line regions in AGNs in the diagnostic diagram of Veilleux and Osterbrock (1987). The line ratios from the disk component on the other hand indicate a large contribution of light from star forming regions.

The nuclear region of NGC 1365 was observed in the [O III] 5007 line and the neighbouring continuum with the HST/FOC by Kristen et al. (1997). As seen in Fig. 23 the HST image resolves the inner part of the outflow cone into numerous small clouds and a number of larger aggregates. Kristen et al. estimate the total [O III] luminosity of the half cone turned to us within the HST/FOC frame to be L_[OIII] 3.7 x 10⁴⁰ erg s^-1.

Figure 23. Deconvolved and continuum subtracted HST image of the nucleus (N) and the outflow cone in the light of [O III] 5007. The units on the axes are arcseconds form the centre. North is up and East to the left. From P.O. Lindblad et al. (1996b).

Similar cloud structures have previously been observed by the HST for the [O III] cones in the prototype Seyfert galaxies NGC 1068 (Macchetto et al. 1994) and NGC 4151 (Evans et al. 1993; Boksenberg et al. 1995).