Implementing Feedback in Simulations of Galaxy Formation: A Survey of Methods

Abstract

We present a detailed investigation of a number of different approaches to modelling feedback in simulations of galaxy formation. Gas-dynamic forces are evaluated using Smoothed Particle Hydrodynamics (SPH). Star formation and supernova feedback are included using a three parameter model which determines the star formation rate (SFR) normalization, feedback energy and lifetime of feedback regions. The star formation rate is calculated for all gas particles which fall within prescribed temperature, density and convergent flow criteria, and for cosmological simulations we also include a self-gravity criterion for gas particles to prevent star formation at high redshifts. A Lagrangian Schmidt law is used to calculate the star formation rate from the SPH density. Conversion of gas to stars is performed when the star mass for a gas particle exceeds a certain limit, typically half that of the gas particle. Feedback is incorporated by returning a precalculated amount of energy to the ISM as thermal heating. 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 so that the decay of energy occurs over a set half-life, or by turning off cooling completely and allowing feedback regions a brief period of adiabatic expansion. We test the models on the formation of galaxies from cosmological initial conditions and also on isolated disk galaxies. For isolated prototypes of the Milky Way and the dwarf galaxy NGC 6503 we find feedback has a significant effect, with some algorithms being capable of unbinding gas from the dark matter halo ('blow-away'). As expected feedback has a stronger effect on the dwarf galaxy, producing significant disk evaporation and also larger feedback 'bubbles' for the same parameters. In the critical-density CDM cosmological simulations, evolved to a redshift z = 1, we find the reverse to be true. Further, feedback only manages to produce a disk with a specific angular momentum value approximately twice that of the run with no feedback, the disk thus has an specific angular momentum value that is characteristic of observed elliptical galaxies. We argue that this is a result of the extreme central concentration of the dark halos in the standard CDM model and the pervasiveness of the core-halo angular momentum transport mechanism (even in light of feedback). A simulation with extremely violent feedback, relative to our fiducial models, leads to a disk that resembles the other simulations at z = 1 and has a specific angular momentum value that is more typical of observed disk galaxies. At later times, z = 0.5, a large amount of halo gas which does not suffer an angular momentum deficit is present, however the cooling time is too long to accrete on to the disk. We further point out 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...

Cluster scaling relations from cosmological hydrodynamic simulations in a dark-energy dominated universe

“…The SPH implementation follows that used by Thacker & Couchman (2000) and conserves both energy and entropy. All simulations analysed below include a model for radiative cooling using the method described in Thomas & Couchman (1992) and based on the cooling tables of Sutherland & Dopita (1993).…”

Galactic outflow and diffuse gas properties at z ≥ 1 using different baryonic feedback models

“…Depositing the SN energy in the kinetic form is a more popular implementation in the literature, which has been shown to have significant feedback effects (e.g., Navarro & White 1993;Cen & Ostriker 2000;Kawata 2001;Dubois & Teyssier 2008;Oppenheimer et al 2012). Some other approaches of numerical SN feedback are: consider that a part of the neighbouring gas undergoes adiabatic evolution by turning off radiative cooling temporarily (e.g., Mori et al 1997;Thacker & Couchman 2000;Brook et al 2005;Stinson et al 2006;Piontek & Steinmetz 2011); distribute SN energy to hot and cold gas phases separately (e.g., Marri & White 2003;Scannapieco et al 2006). …”

Probing the faint end of the Galaxy luminosity function at $\vec{z}=$ 3 with Lyαemission

“…Efstathiou 2000;Thacker & Couchman 2000;Poli et al 2001) and the fraction of the background of hydrogen ionising photons produced by high mass stars at high redshift Haehnelt et al 2001).…”