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

EUGENIA DIAZ-GIMENEZ, ARIEL ZANDIVAREZ, ROBERT PROCTOR, CLAUDIA MENDES DE OLIVEIRA & L. RAUL ABRAMO

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris, Francia; Año: 2010

0004-6361

Defined as X-ray bright galaxy groups (or clusters) with a 2 magnitude dierence between the two brightest galaxies, ’fossil’systems represent an interesting extreme in the population of galaxy agglomerations. However, the physical conditions and processes that result in their formation are still poorly constrained.The purpose of this work is to compare the outskirts of fossil systems with that of normal groups in order to understand ifenvironmental conditions play a significant role in their formation.Using a variety of statistical tools (including the spatial and angular cross-correlation functions and the local densityparameter)  we probe for diferences in the density and structure in the environments of ’normal’ and ’fossil’ systems in theMillennium Simulation. We find that the number density of galaxies surrounding fossil systems evolves from greater than that observed around normal systems at z=0.69, to lower than the normal systems by z=0. Both fossil and normal systems exhibit a ’bump’ in the otherwise radially declining local density measure at distances of order 2.5 rvir from the system centre.We identify this as an increase in the clustering at these radii and are able to show that this is mainly due to the predominance of filamentary structure in the outer regions of these systems. We show that this ’bump’ is stronger in fossil systems than in normal systems and demonstrate that this is linked to the earlier formation epoch of fossil groups. Indeed, within the fossil sample we show the clustering to be a function of assembly time. We also confirm that the features identified in the Millennium Simulation can also be detected in the observations of the SDSS. Our results confirm the commonly held belief that fossil systems assembled earlier than normal systems and also show that they are preferentially located at the junctions of stronger filamentary structures.