Context. The formation of intracluster light and extended halos around brightest cluster galaxies is closely related to morphological transformation, tidal stripping, and the disruption of galaxies in clusters. Aims. Here we look for observational evidence to characterize these processes, by studying the morphology and kinematics of the diffuse light in the core of the Hydra I cluster. Methods. With V-band surface photometry, we derive the structural parameters (Sersic index n, effective radius R e , b/a, and major axis position angle PA) of the two giant elliptical galaxies NGC 3311 and NGC 3309 in the cluster core. We construct a two-dimensional photometric model, and investigate the diffuse light structures in the residual image after subtracting the two-dimensional model. We also analyze deep long-slit spectra, and establish a link between the structures in the light distribution, the absorption-line kinematics, and the line-of-sight velocity distributions of nearby galaxies and planetary nebulae (PNs). Results. The central galaxy NGC 3311 is surrounded by an extended, symmetric outer halo with n = 10 and an additional, offcentered envelope whose centroid is shifted by about 50 to the north-east. Its luminosity L V = 1.2 × 10 10 (±6.0 × 10 8 ) L corresponds to ∼50% of the luminosity of the symmetric halo in the same region (∼15% of its entire luminosity). The velocity dispersion of the halo rises to cluster core values, ∼400−500 km s −1 , for R > 20 . On the basis of measured PN velocities, at least part of the offcentered envelope consists of high-velocity accreted stars. We also discover two tidal streams in the cluster center, emerging from the dwarf galaxy HCC 026 and the S0 galaxy HCC 007. The HCC 026 stream is redshifted by ∼1200 km s −1 with respect to NGC 3311 (V N3311 3800 km s −1 ), as for HCC 026 itself, a fraction of PNs in the off-centered envelope, and several other dwarf galaxies nearby. The stars in one of the HCC 026 tails are known to be consistent with the low-metallicity population of HCC 026, and our photometry shows that this galaxy is almost dissolved by the tidal field. The tidal stream around HCC 007 extends over at least ∼110 kpc, is fairly thick, and is brighter on the side of the asymmetric outer halo of NGC 3311, which it may join. Its luminosity is several 10 9 L , similar to the luminosity of the stripped-down galaxy HCC 007. The redshift of the stream is determined from a few PN velocities and is similar to those of both HCC 007 and HCC 026.Conclusions. An entire group of small galaxies is currently falling through the core of the Hydra I cluster; these galaxies have already been partially dissolved by the strong tidal field. Their light is being added to the outer halo and intracluster light around the cD galaxy NGC 3311. The Hydra I cluster provides a vivid example of the morphological transformation and tidal dissolution of galaxies in clusters.

Context. We investigate the stellar population and the origin of diffuse light around brightest cluster galaxies. Aims. We study the stellar population of the dynamically hot stellar halo of NGC 3311, the brightest galaxy in the Hydra I cluster, and that of photometric substructures in the diffuse light to constrain the origin of these components. Methods. We analyze absorption lines in medium-resolution, long-slit spectra in the wavelength range 4800-5800 Å obtained with FORS2 at the Very Large Telescope. We measure the equivalent width of Lick indices out to 20 kpc from the center of NGC 3311 and fit them with stellar population models that account for the [α/Fe] overabundance. Results. Stars in the dynamically hot halo of NGC 3311 are old (age > 13 Gyr), metal-poor ([Z/H] ∼ −0.35), and alpha-enhanced ([α/Fe] ∼ 0.48). Together with the high velocity dispersion, these measurements indicate that the stars in the halo were accreted from the outskirts of other early-type galaxies, with a possible contribution from dwarf galaxies. We identify a region in the halo of NGC 3311 associated with a photometric substructure where the stellar population is even more metal-poor ([Z/H] ∼ −0.73). In this region, our measurements are consistent with a composite stellar population superposed along the line of sight, consisting of stars from the dynamically hot halo of NGC 3311 and stars stripped from dwarf galaxies. The latter component contributes ≤28% to the local surface brightness. Conclusions. The build-up of diffuse light around NGC 3311 is on-going. Based on the observed stellar population properties, the dominant part of these stars may have come from the outskirts of bright early-type galaxies, while stars from stripped dwarf galaxies are presently being added.

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 × 100 kpc 2 of the Hydra I cluster, at a distance of ∼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 ∼50 Mpc. Their number density is ∼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 σ ∼ 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 ∼470 km s −1 at radii of > ∼ 50 . Galaxies with velocities in this range are absent in the central 100 × 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.

Context. An important open question is the relation between intracluster light and the halos of central galaxies in galaxy clusters. Aims. Here we report results from an on going project with the aim to characterize the dynamical state in the core of the Hydra I (Abell 1060) cluster around NGC 3311. Methods. We analyze deep long-slit absorption line spectra reaching out to ∼25 kpc in the halo of NGC 3311. Results. We find a very steep increase in the velocity dispersion profile from a central σ 0 = 150 km s −1 to σ out 450 km s −1 at R 12 kpc. Farther out, to ∼25 kpc, σ appears to be constant at this value, which is ∼60% of the velocity dispersion of the Hydra I galaxies. With its dynamically hot halo kinematics, NGC 3311 is unlike other normal early-type galaxies. Conclusions. These results and the large amount of dark matter inferred from X-rays around NGC 3311 suggest that the stellar halo of this galaxy is dominated by the central intracluster stars of the cluster, and that the transition from predominantly galaxy-bound stars to cluster stars occurs in the radial range 4 to 12 kpc from the center of NGC 3311. We comment on the wide range of halo kinematics observed in cluster central galaxies, depending on the evolutionary state of their host clusters.