We review the formation and evolution of fossil groups and clusters from both the theoretical and the observational points of view. In the optical band, these systems are dominated by the light of the central galaxy. They were interpreted as old systems that had enough time to merge all the M* galaxies within the central one. During the last two decades, many observational studies were performed to prove the old and relaxed state of fossil systems. The majority of these studies that spans a wide range of topics including halos global scaling relations, dynamical substructures, stellar populations, and galaxy luminosity functions seem to challenge this scenario. The general picture that can be obtained by reviewing all the observational works is that the fossil state could be transitional. Indeed, the formation of the large magnitude gap observed in fossil systems could be related to internal processes rather than an old formation.

中文翻译：

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星系化石群的性质

我们从理论和观察的角度回顾了化石群和团簇的形成和演化。在光学波段中，这些系统由中央星系的光线所控制。它们被解释为具有足够时间来合并中心一个星系中的所有M *星系的旧系统。在过去的二十年中，进行了许多观察研究以证明老旧的和放松的化石系统的状态。这些研究涵盖了广泛的主题，其中包括晕圈全局缩放关系，动态子结构，恒星种群和星系光度函数，这些研究中的大多数似乎都对这种情况提出了挑战。通过回顾所有观察工作可获得的总体图景是化石状态可能是过渡性的。的确，在化石系统中观察到的大幅度间隙的形成可能与内部过程有关，而不是与旧的形成有关。