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The mass assembly of galaxy groups and the evolution of the magnitude gap
Dariush, Ali A.; Raychaudhury, Somak; Ponman, Trevor J.; Khosroshahi, Habib G.; Benson, Andrew J.; Bower, Richard G.; Pearce, Frazer
AA(School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT; School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA), AB(School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT), AC(School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT), AD(School of Astronomy, Institute for Research in Fundamental Sciences (IPM), PO Box 19395-5746, Tehran, Iran), AE(California Institute of Technology, Pasadena, CA 91125, USA), AF(Department of Physics, University of Durham, South Road, Durham DH1 3LE), AG(School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD)
Monthly Notices of the Royal Astronomical Society, Volume 405, Issue 3, pp. 1873-1887. (MNRAS Homepage)
Publication Date:
Astronomy Keywords:
hydrodynamics, methods: numerical, galaxies: formation, galaxies: kinematics and dynamics, cosmology: theory
Abstract Copyright:
(c) Journal compilation © 2010 RAS
Bibliographic Code:


We investigate the assembly of groups and clusters of galaxies using the Millennium dark matter simulation and the associated Millennium gas simulations, and semi-analytic catalogues of galaxies. In particular, in order to find an observable quantity that could be used to identify early-formed groups, we study the development of the difference in magnitude between their brightest galaxies to assess the use of magnitude gaps as possible indicators. We select galaxy groups and clusters at redshift z = 1 with dark matter halo mass M(R200) >= 1013 h-1 Msolar, and trace their properties until the present time (z = 0). We consider only the systems with X-ray luminosity LX,bol >= 0.25 × 1042h-2ergs-1 at redshift z = 0. While it is true that a large magnitude gap between the two brightest galaxies of a particular group often indicates that a large fraction of its mass was assembled at an early epoch, it is not a necessary condition. More than 90per cent of fossil groups defined on the basis of their magnitude gaps (at any epoch between 0 < z < 1) cease to be fossils within 4 Gyr, mostly because other massive galaxies are assembled within their cores, even though most of the mass in their haloes might have been assembled at early times. We show that compared to the conventional definition of fossil galaxy groups based on the magnitude gap Delta m12 >= 2 (in the R-band, within 0.5 R200 of the centre of the group), an alternative criterion Delta m14 >= 2.5 (within the same radius) finds 50per cent more early-formed systems, and those that on average retain their fossil phase longer. However, the conventional criterion performs marginally better at finding early-formed groups at the high-mass end of groups. Nevertheless, both criteria fail to identify a majority of the early-formed systems.
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