The Kinematics of the Intracluster Light in the Core of the Hydra I Cluster

In the current cosmological scenario clusters, that are the biggest virialized systems of the Universe, are formed through accretion of smaller objects from the cosmic web in which they are embedded. During these events the galaxies involved are modified due to interaction with other galaxies, the cluster hot X-ray emitting gas and the cluster potential well. Some of mechanisms acting in dense environments are tidal stripping, mergers and ram pressure stripping, just to name a few. These mechanisms involved in galaxy evolution are also responsible for the formation of what is called the intracluster light (ICL). ICL consists of stars in clusters that are not bound to any cluster member. Numerical simulations predict that the stars contributing to the ICL component are unbound by the processes that are involved in galaxy evolution and in the formation of the brightest cluster cD galaxies in particular. The aim of this work is to study the ICL component in the central core of Hydra I. Hydra I is a medium compact cluster in the Southern Hemisphere at a distance of 50 Mpc. The cluster, a relaxed system from X-ray observations, is dominated by two non interacting elliptical galaxies, NGC 3311 (cD) and NGC 3309 (E3). We are interested in the kinematic relation between the ICL component and the cD halo of NGC 3311 and in exploring what can be understood from the photometric and kinematic characteristics of the ICL about the evolutionary history of the cluster as a whole. We studied the core of Hydra I using three different kinds of data. With UT1-FORS2 longslit spectroscopy we investigated the absorption line kinematics of NGC 3311. With UT1-FORS2 multislit-imaging spectroscopy we probed the kinematics of the Planetary Nebulas (PNs) tracing the stellar light in the central 100 × 100 kpc2 of the cluster, around NGC 3311. Finally, with WFI-ESO2.2m telescope photometry we studied the light distribution around the cD galaxy and its substructures. The absorption line kinematics of NGC 3311 shows that the stellar halo of NGC 3311 is dominated by the central intracluster stars of the cluster. The transition from predominantly galaxybound stars to cluster stars occurs in the radial range from 4 to 12 kpc from the center of the galaxy. The diffuse light in the Hydra I cluster is still un-mixed. The PN line-of-sight velocity distribution (LOSVD) shows a multi-peaked structure and reveals the presence of subcomponents in the cluster core. From V-band photometric data we discovered an excess of light, with respect to a symmetric distribution, in the North-East part of the halo of NGC 3311. The excess has a velocity compatible with the velocity of the PNs contributing to a high-velocity peak in the PN LOSVD and a group of dwarf galaxies populating the central core of the Hydra I cluster. The excess of light could have formed from stars unbound from the small group of dwarf galaxies that were partially disrupted during a recent close passage through the dense cluster core and which are now behind NGC 3311. The stellar population and structural properties of one of the DWs, HCC 26, are consistent with those of other dwarf elliptical galaxies. We conclude that even if from X-rays Hydra I appears to be the prototype of a relaxed and dynamically evolved cluster, the build up of the diffuse light and halo of its central cD galaxy, NGC 3311, is still ongoing.