The braided jets in the spiral galaxy NGC 4258

We have used the Hawaii Imaging Fabry-Perot Interferometer to synthesize 44,000 Hα and [N II] emission-line profiles at velocity resolution 68 km s-1 FWHM across the disk of the nearby Seyfert/LINER galaxy NGC 4258, including the prominent "four-branched jet" that extends to 10 and 5 kpc radii in the radio continuum and optical emission lines, respectively. A long-slit spectrum, which includes the emission lines Hα, [N II] λλ6548, 6583 and [S II] λλ6717, 6731, has also been obtained along the jets. These data are used for a comprehensive kinematic study of the galaxy. The disk velocity field is best fitted with a model that incorporates elliptical, bar-forced streaming motions, as suggested by the H I and CO kinematics. Velocity dispersions along the SE jet average 80 km s-1, compared to 40 km s-1 in the H II regions. This broadening arises from a helical, braided structure of three intertwined plasma streams, as inferred from both the spatial and kinematic structure of the jet. The emission-line profiles split into two distinct velocity systems with separations 300 ± 30 km s-1 within 1.7 kpc of the nucleus along the midaxis of the braid pattern. Gaussian decomposition of the line profiles show that the internal velocity dispersion of each helical strand is ≈100 km s-1. We observe two cycles of the triple-braid pattern, with wavelength ≈1.4 kpc and side-to-side amplitude ≈400 pc. If the braided streams represent ballistic motion of gas ejected from orbiting objects, the outflow velocity is ≈2000 km s-1. The masses and separations of the compact objects would be ≈3 × 106 M⊙ and ≈6 pc (0″.2), respectively. However, the emission-line velocity field of the SE jet suggests that the gas moves along the helices, perhaps as a result of fluid instabilities at the interface between the jet and the interstellar medium or of motion along magnetic flux tubes. We show that the NNW jet follows a clear channel between molecular cloud complexes, and exhibits several emission-line "hot spots" and bends where it interacts with the surrounding molecular gas. High-velocity gas in the SE and NW radio "plateaus" probably results from earlier ejections of gas from the nucleus in these directions, with the jet nozzle having precessed to its present orientation. We find that the jets have total ionized mass 1.5 × 106 M⊙ (1 cm-3/ne) and [N II] λ6583/Hα ratios consistent with shock excita-tion. An Einstein HRI image reveals X-ray emission from the nucleus and SE jet. We show that the jet's X-ray emission may be interpreted as thermal bremsstrahlung from hot, shocked jet gas