On the mass-to-light ratios of fossil groups: Are they simply dark clusters? by Eduardo Cypriano (University of Sao Paulo, Brazil)

Fossil groups are defined as X-ray luminous groups that exhibit a large (two magnitude) gap between the optical luminosities of the first and second ranked galaxies. They are believed to be amongst the earliest forming large-scale structures in the universe.

Here we present the deepest study to-date of a selection of 10 fossil group candidates, with ~40 spectroscopically confirmed members per group. We examine the scaling relations between the dynamical, optical and X-ray properties of these systems. Our analysis shows that the most commonly accepted paradigm for their formation - the cannibalism of bright galaxies by the central galaxy, due to the action of dynamical friction - can not explain all the properties of these systems. In particular we find fossil groups to exhibit a dramatic under-abundance of satellite galaxies, resulting in a significant under-luminosity for their dynamical mass (i.e. very high mass-to-light ratios). We show that this property alone can explain all the salient features of these systems, including the large magnitude gap between first and second ranked galaxies. Our results therefore may require a reformulation of the ideas about the formation and evolution of these important systems.

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