The Resolved Narrow‐Line Region in NGC 4151

Abstract

We present slitless spectra of the narrow-line region (NLR) in NGC 4151 from the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope and investigate the kinematics and physical conditions of the emission-line clouds in this region. Using medium resolution (~0.5 A) slitless spectra at two roll angles and narrow-band undispersed images, we have mapped the NLR velocity field from 1.2 kpc to within 13 pc (H0 = 75 km s-1 Mpc-1) of the nucleus. The inner biconical cloud distribution exhibits recessional velocities relative to the nucleus to the NE and approaching velocities to the SW of the nucleus. We find evidence for at least two kinematic components in the NLR. One kinematic component is characterized by low velocities and low velocity dispersions (LVLVD clouds: v < 400 km s-1, and Δv < 130 km s-1). This population extends through the NLR, and their observed kinematics may be gravitationally associated with the host galaxy. Another component is characterized by high velocities and high velocity dispersions (HVHVD clouds: 400 < v 1700 km s-1, Δv ≥ 130 km s-1). This set of clouds is located within 11 (~70 pc) of the nucleus and has radial velocities that are too high to be gravitational in origin but show no strong correlation between velocity or velocity dispersion and the position of the radio knots. Outflow scenarios will be discussed as the driving mechanism for these HVHVD clouds. We also find clouds characterized by low velocities and high velocity dispersions (LVHVD clouds: v < 400 km s-1, Δv ≥ 130 km s-1). These clouds are located within 32 (~200 pc) of the nucleus. It is not clear if the LVHVD clouds are HVHVD clouds whose low velocities are the result of projection effects. Within 32 (~200 pc) of the nucleus, the [O III]/Hβ ratio declines roughly linearly for both the high-velocity-dispersion (HVD) and LVLVD clouds. Since the ionization parameter is proportional to r-2n-1, it appears that the density, n, must decrease as ~r-1 for the clouds within the inner ~32. At distances further from the nucleus, the [O III]/Hβ ratio is roughly constant.