Are fossil groups early-forming galaxy systems?

Abstract

Using the Illustris cosmological simulation, we investigate the origin of fossil groups in the $M_{200}=10^{13}-10^{13.5}{M}_{\odot }/h$ mass regime. We examine the formation of the two primary features of fossil groups: the large magnitude gap between their two brightest galaxies, and their exceptionally luminous brightest group galaxy (BGG). For fossils and non-fossils identified at $z=0$, we find no difference in their halo mass assembly at early times, departing from previous studies. However, we do find a significant difference in the recent accretion history of fossil and non-fossil halos; in particular, fossil groups show a lack of recent accretion and have in majority assembled 80\% of their $M_{200}(z=0)$ mass before $z\sim 0.4$. For fossils, massive satellite galaxies accreted during this period have enough time to merge with the BGG by the present day, producing a more massive central galaxy; and, the lack of recent group accretion prevents replenishment of the bright satellite population, allowing for a large magnitude gap to develop within the past few Gyr. We thus find that the origin of the magnitude gap and overmassive BGG of fossils in Illustris depends on the recent accretion history of the groups and merger history of the BGG after their collapse at $z\sim 1$. This indicates that selecting galaxy groups by their magnitude gap does not guarantee obtaining neither early-forming galaxy systems nor undisturbed central galaxies.

12 pages, 10 figures, 3 tables. Accepted for publication in ApJ

References in corpus (14)

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• Introducing the Illustris Project: the evolution of galaxy populations across cosmic time
• On Halo Formation Times and Assembly Bias
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• A fossil galaxy cluster; an X-ray and optical study of RX J1416.4+2315
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• On the Nature of Fossil Galaxy Groups: Are they really fossils ?
• Ultimate age-dating method for galaxy groups; clues from the Millennium Simulations
• Effects of large-scale environment on the assembly history of central galaxies
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• Optically selected fossil groups; X-ray observations and galaxy properties
• The isolated fossil group RXJ1119.7+2126

Cited by in corpus (3)

• On Mass Concentrations & Magnitude Gaps of Galaxy Systems in the CS82 Survey
• Fossil group origins: VIII RXJ075243.6+455653 a transitionary fossil group
• Cosmic Pathways for Compact Groups in the Milli-Millennium Simulation