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A chronicle of galaxy mass assembly in the EAGLE simulation.

Qu, Y. and Helly, J. C. and Bower, R. G. and Theuns, T. and Crain, R. A. and Frenk, C. S. and Furlong, M. and McAlpine, S. and Schaller, M. and Schaye, J. and White, S. D. M. (2017) 'A chronicle of galaxy mass assembly in the EAGLE simulation.', Monthly notices of the Royal Astronomical Society., 464 (2). pp. 1659-1675.


We analyse the mass assembly of central galaxies in the Evolution and Assembly of Galaxies and their Environments (EAGLE) hydrodynamical simulations. We build merger trees to connect galaxies to their progenitors at different redshifts and characterize their assembly histories by focusing on the time when half of the galaxy stellar mass was assembled into the main progenitor. We show that galaxies with stellar mass M* < 1010.5 M⊙ assemble most of their stellar mass through star formation in the main progenitor (‘in situ’ star formation). This can be understood as a consequence of the steep rise in star formation efficiency with halo mass for these galaxies. For more massive galaxies, however, an increasing fraction of their stellar mass is formed outside the main progenitor and subsequently accreted. Consequently, while for low-mass galaxies, the assembly time is close to the stellar formation time, the stars in high-mass galaxies typically formed long before half of the present-day stellar mass was assembled into a single object, giving rise to the observed antihierarchical downsizing trend. In a typical present-day M* ≥ 1011 M⊙ galaxy, around 20 per cent of the stellar mass has an external origin. This fraction decreases with increasing redshift. Bearing in mind that mergers only make an important contribution to the stellar mass growth of massive galaxies, we find that the dominant contribution comes from mergers with galaxies of mass greater than one-tenth of the main progenitor's mass. The galaxy merger fraction derived from our simulations agrees with recent observational estimates.

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Publisher statement:This article has been published in Monthly Notices of the Royal Astronomical Society ©: 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:26 September 2016
Date deposited:17 March 2017
Date of first online publication:27 September 2016
Date first made open access:17 March 2017

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