Fossil Groups in the Millennium Simulation: Their environment and its evolution

DIAZ GIMENEZ, EUGENIA; ZANDIVAREZ, ARIEL; PROCTOR, ROBERT; MENDES DE OLIVEIRA, CLAUDIA; ABRAMO, RAUL

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2011 vol. 527 p. 129 - 139

0004-6361

Context. Fossil systems are defined to be X-ray bright galaxy groups (or clusters) with a two-magnitude difference between their twobrightest galaxies within half the projected virial radius, and represent an interesting extreme of the population of galaxy agglomerations.However, the physical conditions and processes leading to their formation are still poorly constrained.Aims. We compare the outskirts of fossil systems with that of normal groups to understand whether environmental conditions play asignificant role in their formation. We study the groups of galaxies in both, numerical simulations and observations.Methods. We use a variety of statistical tools including the spatial cross-correlation function and the local density parameter Δ5 toprobe differences in the density and structure of the environments of ?normal? and ?fossil? systems in the Millennium simulation.Results. We find that the number density of galaxies surrounding fossil systems evolves from greater than that observed aroundnormal systems at z = 0.69, to lower than the normal systems by z = 0. Both fossil and normal systems exhibit an increment intheir otherwise radially declining local density measure (Δ5) at distances of order 2.5 rvir from the system centre. We show that thisincrement is more noticeable for fossil systems than normal systems and demonstrate that this difference is linked to the earlier formationepoch of fossil groups. Despite the importance of the assembly time, we show that the environment is different for fossil andnon-fossil systems with similar masses and formation times along their evolution. We also confirm that the physical characteristicsidentified in the Millennium simulation can also be detected in SDSS observations.Conclusions. Our results confirm the commonly held belief that fossil systems assembled earlier than normal systems but also showthat the surroundings of fossil groups could be responsible for the formation of their large magnitude gap.