摘要：Context.It is thought that fossil systems are relics of structure formation in the primitive Universe. They are galaxy aggregations that have assembled their mass at high redshift with few or no subsequent accretion. Observationally these systems are selected by large magnitude gaps between their 1st and 2nd ranked galaxies (Δm12). Nevertheless, there is still debate over whether or not this observational criterium selects dynamically evolved ancient systems.Aims.We have studied the properties of the nearby fossil group RX J075243.6+455653 in order to understand the mass assembly of this system.Methods.Deep spectroscopic observations allow us to construct the galaxy luminosity function (LF) of RX J075243.6+455653 down toMr*+6. The analysis of the faint-end of the LF in groups and clusters provides valuable information about the mass assembly of the system. In addition, we have analyzed the nearby large-scale structure around this group.Results.We identified 26 group members withinr200~ 0.96Mpc. These galaxies are located atVc= 15551 ± 65km s-1and have a velocity dispersion ofσc= 333 ± 46km s-1. The X-ray luminosity of the group isLX = 2.2 × 1043 h70-2erg s-1, resulting in a mass ofM= 4.2 × 1013 h70-1within 0.5r200. The group hasΔm12= 2.1within 0.5r200, confirming the fossil nature of this system. RX J075243.6+455653 has a central brightest group galaxy (BGG) withMr= −22.67, one of the faintest BGGs observed in fossil systems. The LF of the group shows a flat faint-end slope (α= −1.08 ± 0.33). This low density of dwarf galaxies is confirmed by the low value of the dwarf-to-giant ratio (DGR= 0.99 ± 0.49) for this system. Both the lack of dwarf galaxies and the low luminosity of the BGG suggests that RX J075243.6+455653 still has to accrete mass from its nearby environment. This mass accretion will be achieved because it is the dominant structure of a rich environment formed by several groups of galaxies (15) within~ 7Mpc from the group center and with± 1000km s-1.Conclusions.RX J075243.6+455653 is a group of galaxies that has not yet completed the process of its mass assembly. This new mass accretion will change the fossil state of the group. This group is an example of a galaxy aggregation selected by a large magnitude gap but still in the process of the accretion of its mass.
