Counter-dispersed slitless-spectroscopy technique: planetary nebula
  velocities in the halo of NGC 1399
  Using a counter-dispersed slitless spectroscopy technique, we detect and
measure the line-of-sight velocities of 187 planetary nebulae (PNe) around one
of the nearest cD galaxies, NGC 1399, with FORS1 on the VLT. We describe the
method for identifying and classifying the emission-line sources and the
procedure for computing their J2000 coordinates and velocities. The number of
PN detections and the errors in the velocity measurements (37 km/s indicate
that this technique is comparable to other methods, such as that described by
Teodorescu et al. (2005). We present the spatial distribution of the PNe and a
basic analysis of their velocities. The PN two-dimensional velocity field shows
marginal rotation consistent with other studies. We also find a low-velocity
substructure in the halo and a flatter velocity-dispersion profile compared to
previous observations that extends to ~400 arcsec. The detection of a
low-velocity subcomponent underscores the importance of discrete velocity
tracers for the detection of un-mixed components. The new velocity-dispersion
profile is in good agreement with revised velocity dispersions for the red
globular clusters in NGC 1399, using the data of Schuberth et al. (2009). The
outer parts of this profile are consistent with one of the dynamical models of
Kronawitter et al. (2000), which corresponds to a circular velocity of ~340
km/s and a rescaled B-band mass-to-light ratio of ~20 at 7' radius. These
measurements trace the kinematics of the outer halo and disentangle the
heterogenous populations in the Fornax Cluster core. The new data set the stage
for a revised dynamical model of the outer halo of NGC 1399.
