Dark-ages Reionization and Galaxy Formation Simulation - VIII.

Suppressed growth of dark matter haloes during the Epoch of Reionization
Yuxiang Qin1, Alan R. Duffy2, Simon J. Mutch1, 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
We investigate how the hydrostatic suppression of baryonic accretion affects the growth rate of dark matter haloes during the Epoch of Reionization. By comparing halo properties in a simplistic hydrodynamic simulation in which gas only cools adiabatically, with its collisionless equivalent, we find that halo growth is slowed as hydrostatic forces prevent gas from collapsing. In our simulations, at the high redshifts relevant for reionization (between ∼6 and ∼11), haloes that host dwarf galaxies (≤ 109 M) can be reduced by up to a factor of 2 in mass due to the hydrostatic pressure of baryons. Consequently, the inclusion of baryonic effects reduces the amplitude of the low mass tail of the halo mass function by factors of 2-4. In addition, we find that the fraction of baryons in dark matter haloes hosting dwarf galaxies at high redshift never exceeds ∼90 per cent of the cosmic baryon fraction. When implementing baryonic processes, including cooling, star formation, supernova feedback and reionization, the suppression effects become more significant with further reductions of ∼30−60 per cent. Although convergence tests suggest that the suppression may become weaker in higher resolution simulations, this suppressed growth will be important for semi-analytic models of galaxy formation, in which the halo mass inherited from an underlying N-body simulation directly determines galaxy properties. Based on the adiabatic simulation, we provide tables to account for these effects in N-body simulations, and present a modification of the halo mass function along with explanatory analytic calculations.
Key words: methods: analytical — methods: numerical — galaxies: haloes — cosmology: theory — early Universe.


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