The existence of a clumpy, large-scale filamentary structure is at the basis of the current paradigm of cosmic formation and evolution of clusters. The star-formation history of galaxies falling into a cluster is altered as a result of the environmental stresses. We investigate the relation between substructure and properties of the galaxy population in a 30'x30' region centered on the massive merging cluster A2744 at z~0.3. Multi-object spectroscopy at low resolution and BVR photometry are used to detect the presence of substructure through a Dressler-Schectman analysis and the photometric redshift technique, respectively. Galaxies at the same photometric redshift of the cluster are divided into red and blue according to their distribution in the B-R vs. R colour-magnitude diagram. We identify two large-scale filaments associated with A2744. Along these filaments, the blue-to-red galaxy number ratio increases together with the cluster-centric distance but peaks slightly beyond the cluster virial radius. The filaments host a population of bright, large (i.e. more luminous than R* for the main body of the cluster and with angular sizes of 13-22 $h_{70}^{-1}$ kpc) blue galaxies that is hardly found among galaxies lying in a neighbouring low-density environment at the same redshift of the cluster. These results can be interpreted as a manifestation of galaxy harassment.
The relationship between the optical Halpha filaments and the X-ray emission in the core of the Perseus cluster
NGC 1275位于英仙座星系团Abell 426的中心，被一个壮观的球状阿尔法星云所包围。深钱德拉X射线成像显示，在软X射线中也可以检测到更亮的外部细丝。这可能是由于暗条中的冷气体与热的、致密的团内介质的传导和混合。我们在两个波段的灯丝的对应关系，并提请注意的关系，两个突出的弯曲的西北灯丝的外部，浮力无线电泡被视为一个洞的X射线图像。在孔的形状和细丝的分布上，与水中上升的大气泡的行为非常相似。如果这是一个正确的类比，那么流动是层流，而这个射电源周围的星团内气体不是湍流。我们得到了这种气体粘度的一个极限。
