AAS Meeting #193 - Austin, Texas, January 1999
Session 7. Nearby AGN II - Ionised Gas, Kinematics and Radiojets
Display, Wednesday, January 6, 1999, 9:20am-6:30pm, Exhibit Hall 1

## [7.10] Bow-shock Associated with the Active Jet in NGC 4258

G. Cecil (SOAR-NOAO/UNC-Chapel Hill), C. G. De Pree (Agnes Scott College), L. J. Greenhill, J. M. Moran (CfA), M. A. Dopita (Australian National University)

NGC 4258 (M106) is a nearby (7 Mpc, 0\farcs1 = 3.5 pc) active spiral galaxy, distinguished by long (5\arcmin\ projected extent), narrow (5\arcsec) radio jets. The jets are jointed, linear segments that are embedded in diffuse plateaux of radio and X-ray emission. They flow close to the galaxy gas disk, entraining and shocking the ISM; X-rays with a thermal spectrum are prominent from the hottest shocked gas, and optical line ratios are also consistent with shock excitation. The jets have been traced with VLBA back close to the 0.1-pc radius accretion disk that surrounds the nuclear black hole (3.5\times107~M\sun, dynamical estimate from ongoing maser monitoring.)

We discuss new H\alpha (658N -- 547M) WFPC2 images from HST. These reveal an emission-line bow" whose apex points away from the galaxy nucleus (25\arcsec\ distant.) This arc spans 5\arcsec\ across the jet (tracing precession of the jet in a gas recombination time if it is photoionized by dissipated mechanical energy, or the opening angle of the obscuring torus if it is photoionized directly by the AGN), is barely resolved in the WFC along the jet, and has a bright knot in its W half that coincides with the terminus of the linear radio jet in our ABC-configuration 20-cm VLA image. Our existing Fabry-Perot H\alpha and [O~III]\lambda5007 line-profile datacubes show kinematic disturbances near this putative bow shock. We use these data and the new WFPC2 images to derive kinematic and photometric parameters of this jet/ISM interaction. These data constrain the dynamical evolution of the active outflow. We also present simulations of an upcoming deep AXAF-grating spectrum to show how we will establish physical conditions of the hottest shocked gas.

Work supported by NASA GO grants (HST GO-6563, AXAF GO-TBD).