Implementing Feedback in Simulations of Galaxy Formation: A Survey of Methods
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We present a detailed investigation of different approaches to modeling
feedback in simulations of galaxy formation. Gas-dynamic forces are evaluated
using Smoothed Particle Hydrodynamics (SPH) while star formation and supernova
feedback are included using a three parameter model which determines the star
formation rate normalization, feedback energy and lifetime of feedback regions.
The star formation rate is calculated using a Lagrangian Schmidt Law for all
gas particles which satisfy temperature, density and convergent flow criteria.
Feedback is incorporated as thermal heating of the ISM. We compare the effects
of distributing this energy over the smoothing scale or depositing it on a
single particle. Radiative losses are prevented from heated particles by
adjusting the density used in radiative cooling. We test the models on the
formation of galaxies from cosmological initial conditions and also on isolated
Milky Way and dwarf galaxies. Extremely violent feedback is necessary to
produce a gas disk with angular momentum remotely close to that of observed
disk galaxies. This is a result of the extreme central concentration of the
dark halos in the sCDM model, and the pervasiveness of the core-halo angular
momentum transport mechanism. We emphasize that the disks formed in
hierarchical simulations are partially a numerical artifact produced by the
minimum mass scale of the simulation acting as a highly efficient `support'
mechanism. Disk formation is strongly affected by the treatment of dense
regions in SPH, which along with the difficulty of representing the
hierarchical formation process, means that realistic simulations of galaxy
formation require far higher resolution than currently used.
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