On the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Clusters grow... Show moreOn the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Clusters grow through accretion from these large-scale filaments and by mergers with other clusters and groups. In a growing number of galaxy clusters, elongated Mpc-sized radio sources have been found1,2 . Also known as radio relics, these regions of diffuse radio emission are thought to trace relativistic electrons in the intracluster plasma accelerated by low-Mach-number shocks generated by cluster–cluster merger events 3 . A long-standing problem is how low-Mach-number shocks can accelerate electrons so efficiently to explain the observed radio relics. Here, we report the discovery of a direct connection between a radio relic and a radio galaxy in the merging galaxy cluster Abell 3411–3412 by combining radio, X-ray and optical observations. This discovery indicates that fossil relativistic electrons from active galactic nuclei are re-accelerated at cluster shocks. It also implies that radio galaxies play an important role in governing the non-thermal component of the intracluster medium in merging clusters. Show less
We present a multi-wavelength study of galaxy populations in the core of the massive, X-ray luminous cluster XMMU J2235 at z = 1.39, based on high quality VLT and HST photometry at optical and near... Show moreWe present a multi-wavelength study of galaxy populations in the core of the massive, X-ray luminous cluster XMMU J2235 at z = 1.39, based on high quality VLT and HST photometry at optical and near-infrared wavelengths. We derive luminosity functions in the z, H, and Ks bands, approximately corresponding to restframe U, R and z band. These show a faint-end slope consistent with being flat, and a characteristic magnitude M* close to passive evolution predictions of M* of local massive clusters, with a formation redshift z > 2. The color-magnitude and color-mass diagrams show evidence of a tight red sequence (intrinsic scatter less than or similar to 0.08) of massive galaxies already in place, with overall old stellar populations and generally early-type morphology. Beside the red colors, these massive (>6 x 10(10) M-circle dot) galaxies typically show early-type spectral features, and rest-frame far-UV emission consistent with very low star formation rates (SFR < 0.2 M-circle dot yr(-1)). Star forming spectroscopic members, with SFR of up to similar to 100 M-circle dot/yr, are all located at clustercentric distances greater than or similar to 250 kpc, with the central cluster region already appearing effectively quenched. Most part of the cluster galaxies more massive than 6 x 10(10) M-circle dot within the studied area do not appear to host significant levels of star formation. The high-mass end of the galaxy populations in the core of this cluster appears to be in a very advanced evolutionary stage, not only in terms of formation of the stellar populations, but also of the assembly of the stellar mass. The high-mass end of the galaxy stellar mass function is essentially already in place. The stellar mass fraction estimated within r(500) (similar to 1%, Kroupa IMF) is already similar to that of local massive clusters. On the other hand, surface brightness distribution modeling of the massive red sequence galaxies may suggest that their size is often smaller than expected based on the local stellar mass vs. size relation. An evolution of the stellar mass vs. size relation might imply that, in spite of the overall early assembly of these sources, their evolution is not complete, and processes like minor (and likely dry) merging might still shape the structural properties of these objects to resemble those of their local counterparts, without substantially affecting their stellar mass or host stellar populations. Nonetheless, a definite conclusion on the actual relevance of size evolution for the studied early-type sample is precluded by possible systematics and biases. Show less
