6.2. The Southeast Jet
As shown in Figs 5a and c, the SE jet comprises at least three helical strands, which tightly wrap around each other within d 2 kpc of the nucleus, and then open out into a prominent three-pronged structure at the end of the emission-line jet (d 3 kpc). The kinematic data from both the Fabry-Perot interferometer (Fig. 5b) and the long slit spectrum (Fig. 6) show a regular velocity "wrapping" from blue to red with increasing nuclear distance at small distances (d < 1.3 kpc). Although the helical spatial structure is less distinct at these small radii, it seems clear that this kinematic wrapping is intimately associated with the helical structure. The individual kinematic features apparent in Fig. 6 range from -400 to +200 km s-1 with respect to systemic velocity and are responsible for the general line broadening along the jet (Fig. 4d).
Figure 5. (from Cecil, Wilson & Tully 1992). (a) An image of the SE jet of NGC 4258 formed by summing 3 monochromatic H images that straddle systemic velocity with total velocity width 100 km s-1. The figure has been rotated from the cardinal orientation so that p.a. 150° is vertical and runs from bottom to top. Vertical ticks are at 18 arc sec (620 pc) intervals and horizontal ticks at 8.5 arc sec (300 pc) intervals. The nucleus is indicated by the white cross. The flux distribution is seen to be composed of distinct helical strands. (b) Simulations of long-slit spectra along and parallel to the SE jet in p.a. 150° (the jet axis). SE is at top and NW at bottom. Each panel covers 715 km s-1 horizontally and the same spatial extent vertically as (a). The "slit width" is 2.5 arc sec, and each of the six panels is labeled by the midpoint (towards p.a. 240°) in arc secs relative to the mid-axis of the SE jet. Lowest velocities are left in each panel, and ticks along the horizontal axis are at 175 km s-1 intervals, with systemic velocity at the middle tick. (c) The difference map: panel (a) minus panel (a) smoothed with the uniform circular kernel of 12 arc sec diameter (shown). This processing accentuates the fine scale helical strands.
CWT attempted to distinguish between the two extreme kinds of motion that can lead to such helical spatial and velocity structure: a) pure ballistic outflow of the gaseous elements, with the sources of the helical strands oscillating or orbiting about one another ("garden hose model"), and b) real helical motion, in which the gaseous elements are forced to follow spiral trajectories. By comparing idealized models of such kinematics with the observations, CWT favored a significant helical component of motion over pure ballistic outflow. In detail, however, the interpretation is complicated by the observationally limited spatial and spectral resolutions of individual strands.
The physical origin of the helical structures is unclear. Ballistic ejection from a pair of orbiting, compact sources requires masses 106 M and a separation of 6 pc 0.2 arc sec (CWT). These objects would presumably be black holes, with the binary having formed through the merging of two galactic nuclei (Begelman, Blandford & Rees 1980). One problem with this picture is the absence of evidence in the large-scale dynamics of NGC 4258 for a recent merger. Another is that the three helical strands observed are not readily explained unless one invokes a triple system, which tends to be unstable (e.g., Saslaw, Valtonen & Aarseth 1974). Lastly, the kinematic evidence for helical motion is at odds with this picture. A second possibility is that the helices represent magnetic flux tubes from a central accretion disk, which is assumed to be threaded with a poloidal magnetic field (e.g., Blandford 1990). The problem here is that the helix cycle time inferred from the observations is 106 yrs, which exceeds any plausible dynamical time of an inner accretion disk. The foot points of the magnetic flux tubes would have to be anchored in a large-scale ( 100 pc) torus, but it is unclear how the jet could be powered from this radius. Nevertheless, the possibility that the helices represent magnetic flux tubes is exciting and can, in principle, be tested through high resolution radio polarization and intensity maps of the synchrotron emission of the jet. A third possible explanation of the helical structure would invoke fluid instabilities on the boundary layer between a light (?) jet (cf, Martin et al. 1989) and the surrounding interstellar medium. The observed recession velocity of the jet follows the rising rotation curve of NGC 4258 to the turnover point at d 3 kpc, at which location the helices begin to trail with respect to galactic rotation and "open out". These results indicate that the jet lies close to or in the galaxy disk and interacts strongly with the interstellar medium. Interface effects are thus expected to be important and it is possible that the observed jet structure is a result of periodic entrainment of material as the jet traverses denser regions of the galaxy disk. Although the nature of the intertwined, helical structures of the SE jet of NGC 4258 is unknown, high resolution radio, optical and X-ray observations, currently planned or in progress, should serve to improve our understanding of the physical processes at work.
Figure 6. (from Cecil, Wilson & Tully 1992). (a) The long slit spectrum of NGC 4258, including the H + [NII]6548, 6583 complex and [SII]6717, 6731 doublet at 0.76 arc sec increments along 150° p.a., after continuum subtraction. The broadest lines are at the nucleus. SE is at the top, NW at the bottom, and ticks are every 375 pc (11 arc sec). Note the repetitive pattern along the slit to the SE of the nucleus. (b) Fit to all line profiles simultaneously, assuming two Gaussian subsystems in each line at each location SE of the nucleus. The repetitive pattern along the slit is well described by this parameterization.