Diffuse intracluster light (ICL) has now been observed in nearby and in intermediate redshift clusters. Individual intracluster stars have been detected in the Virgo, Hydra, and Coma clusters () and the first color-magnitude diagram and velocity measurements have been obtained. Recent studies show that the ICL contains of the order of 10% and perhaps up to 30% of the mass in stars of the cluster, but in the cores of more dense and rich clusters like Coma, the local ICL fraction can be high as 40%-50%. What can we learn from the ICL about the formation of galaxy clusters and the evolution of cluster galaxies? How and when did the ICL form? What are the dominant processes: tidal stripping, disruption, mergers? What is the connection to the central brightest cluster galaxy? Cosmological N-body and hydrodynamical simulations are now beginning to make predictions for the kinematics and origin of the ICL. The evolution of baryonic substructures in dense environments is traced by the ICL which can thus be used to constrain those aspects of cosmological simulations that are most uncertain, such as the modeling of star formation and the mass distribution of the baryonic component in galaxies. The first conference dedicated to the ICL was the Joint Discussion 2 at the General Assembly of the IAU at Rio de Janeiro in August 2009.

A tale of two tails and an off-centered envelope: Diffuse light around the cD galaxy NGC 3311 in the Hydra I cluster

Arnaboldi  M.,  Ventimiglia  G.,  Iodice  E.,  Gerhard  O.,  Coccato  L.,  2012, A&A, 545, A37

Residual VLT/FORS1 V-band image after subtracting a photometric model for NGC 3311 and NGC 3309. The white ellipse indicates the extended halo off-centered towards the NE of the bright inner regions of NGC 3311 (black). The most noticeable substructure on top of the extended envelope NE of NGC 3311 is the tidal stream associated with the dwarf galaxy HCC 026. Its NW part is indicated on the image with the blue dashed polygon. At larger radii, and at fainter surface brightness levels, we see a thicker tail around HCC 007. The two small objects near the center of the NE polygon appear to be connected by a further small stream. NGC 007 has already lost ~half of its light, while NGC026 is mostly dissolved.

The formation of intracluster light and of the extended halos around brightest cluster galaxies is closely related to morphological transformation, tidal stripping, and disruption of galaxies in clusters. We analyze Ks- and V-band surface photometry as well as deep long-slit spectra, and establish a link between the structures in the light distribution, the absorption line kinematics, and the LOS velocity distributions of nearby galaxies and planetary nebulae (PNs). The central galaxy NGC 3311 is surrounded by an extended symmetric outer halo with n=10 and an additional, off-centered envelope ~ 50" to the North-East. Its luminosity L_V= 1.2x10^{10} +/- 6.0 x 10^8 L_sun corresponds to ~50 % of the luminosity of the symmetric halo in the same region. Based on measured PN velocities, at least part of the off-centered envelope consists of high-velocity accreted stars. We have also discovered two tidal streams in the cluster center, emerging from the dwarf galaxy HCC 026 and from the S0 galaxy HCC 007. The HCC 026 stream is redshifted by ~1200 km/s with respect to NGC 3311, similarly as 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 already mostly dissolved in the tidal field. The tidal stream around HCC 007 extends over at least 110 kpc. It 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_sun, 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 that for HCC 007 and HCC 026.

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