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Comparing galaxy formation in semi-analytic models and hydrodynamical simulations.

Mitchell, Peter D. and Lacey, Cedric G. and Lagos, Claudia D. P. and Frenk, Carlos S. and Bower, Richard G. and Cole, Shaun and Helly, John C. and Schaller, Matthieu and Gonzalez-Perez, Violeta and Theuns, Tom (2018) 'Comparing galaxy formation in semi-analytic models and hydrodynamical simulations.', Monthly notices of the Royal Astronomical Society., 474 (1). pp. 492-521.


It is now possible for hydrodynamical simulations to reproduce a representative galaxy population. Accordingly, it is timely to assess critically some of the assumptions of traditional semi-analytic galaxy formation models. We use the EAGLE simulations to assess assumptions built into the GALFORM semi-analytic model, focusing on those relating to baryon cycling, angular momentum and feedback. We show that the assumption in GALFORM that newly formed stars have the same specific angular momentum as the total disc leads to a significant overestimate of the total stellar specific angular momentum of disc galaxies. In EAGLE, stars form preferentially out of low-specific angular momentum gas in the interstellar medium due to the assumed gas density threshold for stars to form, leading to more realistic galaxy sizes. We find that stellar mass assembly is similar between GALFORM and EAGLE but that the evolution of gas properties is different, with various indications that the rate of baryon cycling in EAGLE is slower than is assumed in GALFORM. Finally, by matching individual galaxies between EAGLE and GALFORM, we find that an artificial dependence of active galactic nucleus feedback and gas infall rates on halo mass-doubling events in GALFORM drives most of the scatter in stellar mass between individual objects. Put together our results suggest that the GALFORM semi-analytic model can be significantly improved in light of recent advances.

Item Type:Article
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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2017 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:23 October 2017
Date deposited:01 March 2018
Date of first online publication:25 October 2017
Date first made open access:01 March 2018

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