6.3. The Northwest Jet
Unlike the SE jet, the NW jet shows no evidence for helical strands. Nevertheless, there is clear evidence for significant kinematic disturbance by the NW jet at both small and large nuclear distances (Figs 4c and d). The observed recession velocities along the NW jet tend to follow those of the disk rotation, but are systematically redshifted by 80 km s-1, on average, from those of the surrounding gas (see Fig. 18 of CWT). As for the SE jet, this is clear evidence for interaction between the jet and interstellar gas in the galaxy disk. Even more striking is the relationship between the inner part (d 1 kpc) of the NW jet and molecular cloud complexes (Fig. 7). The Fabry-Perot data show direct evidence for confinement of the jet by molecular gas, revealing a stream of high velocity gas between the CO-observed clouds. Furthermore, the jet seems to be deflected by the molecular gas at three, or possibly four, locations. The deflections are characterized by localized brightenings in H - knots a, b, c and d in Fig. 7. These knots each contain a few x 104 M (1 cm-3/ne) of ionized gas and may result from internal shocks (e.g., Icke 1991). The interaction with CO cloud nr. 2 (Fig. 7) may accelerate this molecular cloud to a velocity of several hundred km s-1 according to Plante et al. (1991). These jet deflections and accelerations of interstellar clouds are reminiscent of the similar processes occurring in powerful radio galaxies (Section 3).
Figure 7. (from Cecil, Wilson & Tully 1992). The interaction between the NNW jet of NGC 4258 and dense molecular cloud complexes. The nucleus is at the cross and the ticks are at 5 arc sec intervals. (a) A gray-scale plot of the recession velocity of H, darker shades representing higher velocities. The shades representing 475 and 600 km s-1 are indicated. (b) A gray-scale plot of the H flux, with the shade cycling from white (lowest flux) to black and back to white in the brightest knots. The black contours represent CO emission (from Plante et al. 1991) and the three CO clouds are numbered 1, 2, and 3 following the notation of Plante et al. The two white contours are drawn at H velocities of 550 and 625 km s-1. The four bright knots (labeled [a]-[d]) in the H emission along the jet boundary are plausibly interactions between the jet and the interstellar medium.