Dark-ages reionization and galaxy formation simulation VII:

Comparison between hydrodynamic simulations and semi-analytic models
Yuxiang Qin1, Simon J. Mutch1, Alan R. Duffy2, Gregory B. Poole1, Paul W. Angel1, Paul M. Geil1, Andrei Mesinger3 and J. Stuart B. Wyithe1

1School of Physics, University of Melbourne, Parkville, VIC 3010, Australia
2Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218, Hawthorn, VIC 3122, Australia
3Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy
As part of the Dark-ages Reionization And Galaxy formation Observables Numerical Simulation (or DRAGONS) project, we investigate the effect of different physical processes including reionization on galaxy formation using both hydrodynamic simulations and semi-analytic models (SAMs). Comparing these two numerical methods provides an improved understanding of the essential physics of early galaxy formation, and also helps us comprehend and improve the physics implemented in SAMs. In this work, we run a set of hydrodynamic simulations and SAMs, which contain recipes to model the critical physics including gas cooling, star formation, supernovae feedback, metal enrichment, merger and reionization. Starting from identical initial conditions, we match the resulting galaxies between hydrodynamic simulations and SAMs. We then compare the properties and evolutions of these matched galaxies at high redshift (z≥5). After tuning, we find a good agreement between two models and observations, suggesting most of the principal physics schemes are effective to describe the galaxy formation at early times in both models. However, there is scatter from the essential differences between two modelling techniques. For instance, in the SAM, the mechanism of forming stars is more bursty and hence has difficulty in matching the corresponding hydrodynamic star formation rate history and stellar mass evolution. Based on these comparisons, we propose modifications to the merger trees and SAM by introducing a redshift dependent star formation efficiency which provide a more accurate description of the build up of stellar mass at high-z.
Key words: cosmology: reionization — galaxies: formation — method: hydrodynamic simulation — method: semi-analytic model


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