Italiano: This image shows the fossil galaxy group RX J1416.4+2315. The blue haze indicates hot, X-ray emitting gas at a temperature of up to 50 million degrees, heated by shock heating during gravitational collapse. The hot gas halo is three million light years across. Gravitation attraction by dark matter in galaxy groups pulls the galaxies towards the centre. In a fossil galaxy all of the original constituent galaxies have merged into one giant elliptical galaxy at the centre; only about twelve examples of these systems have been identified. This elliptical galaxy is enclosed in a dark matter halo which gravitationally attracts the hot gas seen in blue the image. The group has a luminosty of 500 thousand million times that of the Sun. The investigators found that the global X-ray distribution shows a relaxed profile, with the exception of a small central region. This indicates that there has been no recent merging activity between groups or large galaxies, and hence and early epoch of formation for the central galaxy. The mechanism by which a fossil group may form is dynamical friction - where orbital energy is lost from large galaxies to surrounding dark matter. The giant elliptical galaxy in the centre of RX J1416.4+2315 is aligned with the the dark matter distribution (shown by the distribution of the X-ray emitting gas). This indicates that the galaxy was formed in anisotropic collapse, for example along a filament. EPIC-pn and MOS data were used in the study; the pn camera in extended full frame mode and the MOS cameras in full frame mode with medium filters. After cleaning, there were 8 and 4 ks of good time for the MOS and pn cameras, respectively.