Longlsit Spectroscopy of the NLR of NGC 4151

In addition to the slitless spectroscopy, low dispersion longslit spectroscopy was also obtained along two slit postions through the NLR. Detailed results can be found in Nelson et al. ApJ, 531, 257, 2000 and Kraemer et al. ApJ, 531, 278, 2000. The location of the slits on the WFPC-2 narrow band [OIII] image is shown below (north is up and east to the left).

Spectra were taken using four low-dispersion gratings ranging from the ultraviolet to the near-infrared. The figure below shows the four spectra taken at PA 221 with the most important emission line features marked. The continuum and scattered light have been removed.

To show the complex velocity structure we can look at the spectral region around [OIII] 4959, 5007 and H-beta (right-hand image).The velocities are plotted as a function of distance from the nucleus in the left hand panel.

Two models of the velocity field are shown below. The model on the left is pure outflow with no lateral expansion away from the jet axis. This model would be expected if the gas were driven by a radial wind from the nucleus. On the right is a model which is purely expansion away from the jet axis as might be expected if the motion resulted from the expansion of jet plasma into the NLR. In both cases the conical shape is a result of the anisotropic illumination of the gas by the ionizing radiation field.