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Up: Discussion
Previous: Morphology
The spectroscopy which initially discovered the entended ionized
component of MR 2251-178 also provided the first kinematical information on
the nebula ([Bergeron et al. 1983]). Those observations revealed a normal
rotation curve along each of three position angles, with peak
velocities of 
km s-1. Suprisingly, the extended nebula
appears to be rotating in an opposite sense to the inner (EELR)
regions of the galaxy (e.g., [Nørgaard-Nielsen et al. 1986]). The only published
two-dimensional kinematics of the system are narrow-field H
Fabry-Perot observations of the EELR ([Mulder & Valentijn 1992]).
Comparison of our H
imagery at each of the two distinct etalon
positions reveals a broad velocity gradient across the quasar nebula.
The maximum gradient appears to be along a position angle of
,
with the eastern filaments generally less redshifted
than the western filaments.
The eastern filaments are clearly detected in both the ``blue'' and
``red'' images. Since an upper limit of 
km s-1 has been
measured for their line widths ([Bergeron et al. 1983]), the central velocity
of these filaments must lie within the overlap region of our two
velocity regimes. Accounting for its distance from the optical axis,
the ionized gas east of the quasar must be near the systemic velocity
of the quasar.
The western filaments are detected only in the ``red'' image and
appear to be redshifted from the systemic velocity by a value of
km s-1. This correlates well with the imaging observations of
Macchetto et al. (1990), whose broad (
Å) H
filter barely detected the western filaments out to a velocity of
km s-1 relative to the quasar. Furthermore, these authors
more readily detected the western filaments in their [OIII] imagery,
which encompassed a redder range of velocities.
We therefore confirm that the velocity structure of the extended ionized gas component does not appear to follow the rotation curve of the quasar ([Nørgaard-Nielsen et al. 1986]; [Mulder & Valentijn 1992]). The broad velocity range of our images and the limitation of two distinct velocity samples restricts us from analyzing more precise velocity variations, e.g., amongst individual knots and filaments.