The first data set was taken as part of the Early Release Observation program with STIS and consisted of slitless spectroscopy of the NLR (see the STScI press release and Hutchings et al. ApJ, 492, L115, 1998 ). Further slitless spectra taken at a different roll angle allowed us to further analyze the kinematics and do some simple analysis concerning the physical state of the emission line gas (see Hutchings et al. AJ, 118, 210, 1999 and Kaiser et al. (ApJ, 528, 260, 2000). The slitless spectrum of NGC 4151 taken in the visible region (grating G430M) centered on the H-beta and [OIII] 4959, 5007 lines is shown below.
On the left is the WFPC-2 [OIII] image of NGC 4151 showing the biconical NLR structure. The image has been rotated to a position angle of 44.6° and rescaled to match the orientation and scale of the G430L slitlless spectrum. On the right is the slitless G430L spectrum of NGC 4151 centered on the [OIII] 5007 emission line.
Velocities of the emission line clouds can be measured by aligning the nuclei in the direct image and slitless spectrum. Displacements in the slitless spectrum should be entirely due to the Doppler shifting of the emission lines due to the velocity of the clouds. Taking cuts parallel to the dispersion at a given distance along the slit as shown below left allows the displacements to be determined. The results are shown below right where the color coding corresponds to velocity blue for approaching (about -800 km/s) and red for receding (+600 km/s).
The velocity image shows that the emission line gas just a few arcseconds below the nucleus is approaching and that just above the nucleus is receding. The material farther out is rotating in the disk of the galaxy. Another way to show this is simply to plot the cloud velocities versus distance from the nucleus as shown below.
