Examining the kinematics of narrow-line region gas in active galaxies using HST

A sample of 10 Seyfert galaxies were observed using the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST ). The spectra all show the [O III] λλ4959, 5007 doublet, and five show the Hβ line. The spatial resolution of STIS allows the study of individual knots of gas in the galaxies' narrow-line regions (NLRs), which extend out to a few hundred pc away from the active nucleus, which is powered by a putative supermassive black hole (SMBH).; Coupled with archival HST NLR observations, radial velocities and FWHMs of the individual knots of the emission-line gas are determined as functions of distance. Overall, the sample shows that radial motion away from the nucleus is the primary kinematic component of the NLR clouds. The FWHMs allow a clear distinction between the NLR region and the extended NLR ($\gtrsim$ few hundred pc) to be made for each of the sample objects.; The radial velocity data are used to fit kinematic models for the NLR gas. The models assume a biconical distribution for the clouds, consistent with observed NLR morphologies. Best-fit values are obtained for aspect bicone parameters using a two-step χ2 minimization procedure.; The model parameters for the overall sample clearly favor the following representative scenario: NLR clouds distributed within a partially filled bicone with the material undergoing radial acceleration away from the nucleus. At distances of ∼100 pc, a deceleration component is present which decreases the clouds' velocities at a constant rate to near-systemic values.; These model results are used to draw implications for dynamical models of the NLR gas. A dynamical model that expects to reproduce the features seen in the majority of sample objects must contain the following: a collimating agent that aligns the emission-line gas, a cloud creating mechanism that takes place closer to the nucleus than the NLR, organized flow of the NLR clouds over ∼100 pcs showing radial acceleration, and a deceleration component ∼100 pcs away. This is shown to satisfy some observations using a model invoking radiative acceleration of dusty clouds.