Scaling relations in fossil galaxy groups

Using Chandra X-ray observations and optical imaging and spectroscopy of a flux-limited sample of 5 fossil groups, supplemented by additional systems from the literature, we provide the first detailed study of the scaling properties of fossils compared to normal groups and clusters. Fossil groups are dominated by a single giant elliptical galaxy at the centre of an extended bright X-ray halo. In general, all the fossils we study show regular and symmetric X-ray emission, indicating an absence of recent major group mergers. We study the scaling relations involving total gravitational mass, X-ray temperature, X-ray luminosity, group velocity dispersion and the optical luminosity of the fossil groups. We confirm that, for a given optical luminosity of the group, fossils are more X-ray luminous than non-fossil groups. Fossils, however, fall comfortably on the conventional LX −TX relation of galaxy groups and clusters, suggesting that their X-ray luminosity and their gas temperature are both boosted, arguably, as a result of their early formation. This is supported by other scaling relations including the LX − σ and TX − σ relations in which fossils show higher X-ray luminosity and temperature for a given group velocity dispersion. We find that mass concentration in fossils is higher than in non-fossil groups and clusters. In addition, the MX − TX relation suggests that fossils are hotter, for a given total gravitational mass, both consistent with an early formation epoch for fossils. We show that the mass-to-light ratio in fossils is rather high but not exceptional, compared to galaxy groups and clusters. The entropy of the gas in low mass fossils appears to be systematically lower than that in normal groups, which may explain why the properties of fossils are more consistent with an extension of cluster properties. We discuss possible reasons for this difference in fossil properties and conclude that the cuspy potential raises the luminosity and temperature of the IGM in fossils. However, this works in conjunction with lower gas entropy, which may arise from less effective preheating of the gas.