The active jet in NGC 4258 and its associated shocks

We present images and spectra of the active jet and anomalous arms on subpa rsec through kiloparsec scales in the LINER/Seyfert galaxy NGC 4258 (M106). New VLBA and multiconfiguration VLA images show that, on 0.3-300 pc scales , the jet in projection aligns with (1) the spin axis of the underlying acc retion disk and (2) two radio hot spots 24 " S (840 pc) and 49 " N (1.7 kpc ) from the nucleus. Hubble Space Telescope WFPC2. [N II] lambda 6583 + H al pha images locate interactions between the jet and the interstellar medium (ISM). The most prominent is a pair of emission line arcs whose apices face away from the galaxy nucleus and envelop the leading edges of the radio ho t spots. Ground-based (WHT) spectra with 2 Angstrom resolution confirm that the gaseous kinematics and excitation of both arcs have the spatio-kinemat ic structure expected for jet working surfaces with a shock velocity 350 +/ - 100 km s(-1). The north shock is oblique and may lie in a nuclear ionizat ion cone. The south shock shows a detached, putative Mach disk. Models sugg est that the S shock is a bow shock around a jet whose progress toward us t hrough the galaxy ISM has stalled. This is notable because the inferred out flow axis is misaligned by similar to 65 degrees (in three dimensions) with the spin axis of the accretion disk. Our emission line ratios and profiles diagnose the physical properties of the shocks, possible Mach disk, and th ence the jets. The shocks lie at one end of a swath of kinematically distur bed gas that reaches back to the previously recognized spiral "anomalous ar ms," suggesting that they are linked dynamically by precession of the centr al engine; although claimed elsewhere to be bar shocks, the anomalous arms are probably a fossil record of changing jet activity in NGC 4258. Our resu lts imply that the jet has recently moved a long way out of the plane of th e galaxy. A deep Taurus Tunable Filter Her image shows that discrete strand s in the anomalous arms persist to galactocentric radii of at least 4' (>8 kpc), indicating an ongoing ISM interaction.