Context. The study of intracluster light (ICL) can help us to understand the mechanisms taking place in galaxy clusters, and to place constraints on the cluster formation history and physical properties. However, owing to the intrinsic faintness of ICL emission, most searches and detailed studies of ICL have been limited to redshifts z < 0.4. Aims. To help us extend our knowledge of ICL properties to higher redshifts and study the evolution of ICL with redshift, we search for ICL in a subsample of ten clusters detected by the ESO Distant Cluster Survey (EDisCS), at redshifts 0.4 < z < 0.8, that are also part of our DAFT/FADA Survey. Methods. We analyze the ICL by applying the OV WAV package, a wavelet-based technique, to deep HST ACS images in the F814W filter and to V-band VLT/FORS2 images of three clusters. Detection levels are assessed as a function of the diffuse light source surface brightness using simulations. Results. In the F814W filter images, we detect diffuse light sources in all the clusters, with typical sizes of a few tens of kpc (assuming that they are at the cluster redshifts). The ICL detected by stacking the ten F814W images shows an 8σ detection in the source center extending over a ∼50 × 50 kpc 2 area, with a total absolute magnitude of −21.6 in the F814W filter, equivalent to about two L * galaxies per cluster. We find a weak correlation between the total F814W absolute magnitude of the ICL and the cluster velocity dispersion and mass. There is no apparent correlation between the cluster mass-to-light ratio (M/L) and the amount of ICL, and no evidence of any preferential orientation in the ICL source distribution. We find no strong variation in the amount of ICL between z = 0 and z = 0.8. In addition, we find wavelet-detected compact objects (WDCOs) in the three clusters for which data in two bands are available; these objects are probably very faint compact galaxies that in some cases are members of the respective clusters and comparable to the faint dwarf galaxies of the Local Group. Conclusions. We show that the ICL is prevalent in clusters at least up to redshift z = 0.8. In the future, we propose to detect the ICL at even higher redshifts, to determine wether there is a particular stage of cluster evolution where it was stripped from galaxies and spread into the intracluster medium.

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Context. Large numbers of deep optical images will be available in the near future, allowing statistically significant studies of low surface brightness structures such as intracluster light (ICL) in galaxy clusters. The detection of these structures requires efficient algorithms dedicated to this task, which traditional methods find difficult to solve. Aims. We present our new detection algorithm with wavelets for intracluster light studies (DAWIS), which we developed and optimized for the detection of low surface brightness sources in images, in particular (but not limited to) ICL. Methods. DAWIS follows a multiresolution vision based on wavelet representation to detect sources. It is embedded in an iterative procedure called synthesis-by-analysis approach to restore the unmasked light distribution of these sources with very good quality. The algorithm is built so that sources can be classified based on criteria depending on the analysis goal. We present the case of ICL detection and the measurement of ICL fractions. We test the efficiency of DAWIS on 270 mock images of galaxy clusters with various ICL profiles and compare its efficiency to more traditional ICL detection methods such as the surface brightness threshold method. We also run DAWIS on a real galaxy cluster image, and compare the output to results obtained with previous multiscale analysis algorithms. Results. We find in simulations that DAWIS is on average able to separate galaxy light from ICL more efficiently, and to detect a greater quantity of ICL flux because of the way sky background noise is treated. We also show that the ICL fraction, a metric used on a regular basis to characterize ICL, is subject to several measurement biases on galaxies and ICL fluxes. In the real galaxy cluster image, DAWIS detects a faint and extended source with an absolute magnitude two orders brighter than previous multiscale methods.

Abstract. Galaxies in groups and clusters often experience strong tidal forces from its neighbors, expelling a significant amount of matter (stars and gas) to the surrounding environment, giving rise to 'tidal debris' features. We present the first experiments in a program of identification of substructures of extragalactic tidal debris elements in images from the HST archive, using wavelet-based multiresolution analysis. Tidal structures in two compact groups of galaxies, HCG 79 (Seyfert's Sextet) and HCG 92 (Stephan's Quintet) have been analyzed so far.Keywords. techniques: image processing, galaxies: clusters: individual (HCG 79, HCG 92) The multiresolution analysis approach consists in obtaining the representation of an image in multiscale space (with position and size as basis) and performing all relevant processing in this representation. We use a new multiresolution image processing system (Epitácio Pereira et al., in preparation) based on theà trous algorithm for the discrete wavelet transform (for details see, for example, Starck et al. (1998)). We perform multiscale reconstruction of the objects in the images, producing individual images for each of the main components of the tidal structures -both extended and point sources.We selected the tidal debris structures of HCG 79 and HCG 92 as the first targets of our project. From the debris in HCG 79 we reconstructed a main diffuse component and several overlapping smaller sources (sub-objects). On the diffuse component, we found two 'bridges' that connect the debris to the neighboring galaxy HCG79b. Some of the sub-objects are located along these bridges. In the images of HCG 92 we studied the 'new tail' (described by Sulentic et al. (2001)), and obtained individual images for the main sub-objects of the structure. Photometry and geometry of the detected components will be presented in a future paper, along with analyses of additional groups.