Counter-dispersed slitless-spectroscopy technique: Planetary nebula velocities in the halo of NGC 1399

Aims. 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. Methods. 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^-1) indicate that this technique is comparable to other methods. Results. 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″. The detection of a low-velocity subcomponent underscores the importance of discrete velocity tracers for the detection of un-mixed components. Conclusions. The new velocity-dispersion profile is in good agreement with revised velocity dispersions for the red globular clusters in NGC 1399. The outer parts of this profile are consistent with one of the dynamical models, which corresponds to a circular velocity of ≃340 s ^-1 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.