The unmixed kinematics and origins of diffuse stellar light in the core of the Hydra I cluster (Abell 1060)

Context. Diffuse intracluster light (ICL) and cD galaxy halos are believed to stem from galaxy evolution and disruption in clusters. Aims. The processes involved may be constrained by studying the dynamical state of the ICL and the galaxies in the cluster core. Here we present a kinematic study of diffuse light in the Hydra I (Abell 1060) cluster core, using planetary nebulas (PNs) as tracers. Methods. We used multi-slit imaging spectroscopy with FORS2 on VLT-UT1 to detect 56 PNs associated with diffuse light in the central 100 x 100 kpc(2) of the Hydra I cluster, at a distance of similar to 50 Mpc. We measured their [OIII] m(5007) magnitudes, sky positions, and line-of-sight velocity distribution (LOSVD), and compared them with the phase-space distribution of nearby galaxies. Results. The luminosity function of the detected PNs is consistent with that expected at a distance of similar to 50 Mpc. Their number density is similar to 4 times lower for the light seen than expected, and we discuss ram pressure stripping of the PNs by the hot intracluster medium as one of the possible explanations. The LOSVD histogram of the PNs is highly non-Gaussian and multipeaked: it is dominated by a broad central component with sigma similar to 500 km s(-1) at around the average velocity of the cluster, and shows two additional narrower peaks at 1800 km s(-1) and 5000 km s(-1). The main component is broadly consistent with the outward continuation of the intracluster halo of NGC 3311, which was earlier shown to have a velocity dispersion of similar to 470 km s(-1) at radii of >= 50 ''. Galaxies with velocities in this range are absent in the central 100 x 100 kpc(2) and may have been disrupted earlier to build this component. The PNs in the second peak in the LOSVD at 5000 km s(-1) are coincident spatially and in velocities with a group of dwarf galaxies in the MSIS field. They may trace the debris from the ongoing tidal disruption of these galaxies. Conclusions. Most of the diffuse light in the core of Abell 1060 is still not phase-mixed. The build-up of ICL and the dynamically hot cD halo around NGC 3311 are ongoing, through the accretion of material from galaxies falling into the cluster core and tidally interacting with its potential well.