Probing the Kinematics of the Narrow-Line Region in Seyfert Galaxies with Slitless Spectroscopy: Observational Results

We present slitless spectra of 10 Seyfert galaxies observed with the SpaceTelescope Imaging Spectrograph on the Hubble Space Telescope. The spectra coverthe [OIII] 4959, 5007 emission lines at a spectral resolving power of ~9000 anda spatial resolution of 0.1". We compare the slitless spectra with previous HSTnarrow-band images to determine the velocity shifts and dispersions of thebright emission-line knots in the narrow-line regions (NLRs) of these Seyferts.Many knots are spatially resolved with sizes of tenths of arcsecs,corresponding to tens of pcs, and yet they appear to move coherently withradial velocities between zero and +/- 1200 km/s with respect to the systemicvelocities of their hostgalaxies. The knots also show a broad range in velocitydispersion, ranging from ~30 km/s (the velocity resolution) to ~1000 km/s FWHM.Most of the Seyfert galaxies in this sample show an organized flow pattern,with radial velocities near zero at the nucleus (defined by the opticalcontinuum peak) and increasing to maximum blueshifts and redshifts within ~1''of the nucleus, followed by a decline to the systemic velocity. Theemission-line knots also follow a general trend of decreasing velocitydispersion with increasing distance. In the Seyfert 2 galaxies, the presence ofblueshifts and redshifts on either side of the nucleus indicates that rotationalone cannot explain the observed radial velocities, and that radial outflowplays an important role. Each of the Seyfert galaxies in this sample (with theexception of Mrk 3) shows a bright, compact (FWHM < 0.5") [O III] knot at theposition of its optical nucleus. These nuclear emission-line knots haveradial-velocity centroids near zero, but they typically have the highestvelocity dispersions.Comment: 28 pages, 5 figures (on 9 pages), accepted for A