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 Space Telescope Imaging Spectrograph on the Hubble Space Telescope (HST). The spectra cover the [O III] lambdalambdalambda4959, 5007 emission lines at a spectral resolving power of lambda/Deltalambda approximate to 9000 and a spatial resolution of similar to0."1. We compare the slitless spectra with previous HST narrowband images to determine the velocity shifts and dispersions of the bright emission-line knots in the narrow-line regions (NLRs) of these Seyfert galaxies, which extend out to at least several hundred parsecs from their nuclei. Many knots are spatially resolved with sizes of tenths of arcseconds, corresponding to tens of parsecs, and yet they appear to move coherently with radial velocities between zero and +/- 1200 km s(-1) with respect to the systemic velocities of their host galaxies. The knots also show a broad range in velocity dispersion, ranging from similar to30 km s(-1) (the velocity resolution) to similar to1000 km s(-1) 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 optical continuum peak) and increasing to maximum blueshifts and redshifts within similar to1" of the nucleus, followed by a decline to the systemic velocity. However, there are large local variations around this pattern, and in one case (NGC 7212), the radial velocities are nearly chaotic. The emission-line knots also follow a general trend of decreasing velocity dispersion with increasing distance from the nucleus. In the Seyfert 2 galaxies, the presence of blueshifts and redshifts on either side of the nucleus indicates that rotation alone cannot explain the observed radial velocities. The most straightforward interpretation is that radial outflow plays an important role in the NLR kinematics. Each of the Seyfert galaxies in this sample ( with the exception of Mrk 3) shows a bright, compact (FWHM less than or equal to 0."5) [O III] knot at the position of its optical nucleus. These nuclear emission-line knots have radial velocity centroids near zero, but they typically have the highest velocity dispersions. Their similar properties suggest that they may be a common, distinct component of the NLR.