AGORA model

AGORA’s model corresponds to the subgrid model adopted by the AGORA Project (High-resolution Galaxy Simulations Comparison Project). Details of the model is described in Kim & al. 2014 and Kim & al. 2016.

This model can be selected with the configuration option --with-subgrid=AGORA and run with the option --agora. A few examples exist and can be found in examples/AGORA.

Gas cooling/heating: Grackle

The AGORA model uses the Grackle cooling library which is also used by the GEAR model.

Star formation

The AGORA model uses the GEAR model scheme, however with the GEARStarFormation:star_formation_mode parameter set to agora. Instead of requiring the gas density to reach the pressure floor, we simply require it to be denser than a density threshold defined by GEARStarFormation:density_threshold_Hpcm3.

Recommended parameters for the AGORA model should be:

GEARStarFormation:
  star_formation_mode: agora
  star_formation_efficiency: 0.01
  maximal_temperature_K:  1e10
  n_stars_per_particle: 1
  min_mass_frac: 0.5
  density_threshold_Hpcm3:   10

Stellar Feedback and Chemistry

In the current implementation, only two elements, iron (Fe) and the sum of all elements heavier than helium (metallicity) are considered. Only stellar feeback from core collapse supernovae (CCSNe) is considered. The model assumes that a number AGORAFeedback:ccsne_per_solar_mass of CCSNe per solar mass will form out of each stellar particle. Those supernovae will all explode after a time AGORAFeedback:supernovae_explosion_time_myr after the birth of the stellar particle. At this time, each stellar particle will expel:

  • AGORAFeedback:ejected_mass_in_solar_mass_per_CCSN amount of gas (in solar mass), per CCSN formed

  • AGORAFeedback:ejected_Fe_mass_in_solar_mass_per_CCSN amount of iron (in solar mass), per CCSN formed

  • AGORAFeedback:ejected_metal_mass_in_solar_mass_per_CCSN amount of metals (in solar mass), per CCSN formed

In addition stellar particles will release energy:

  • AGORAFeedback:energy_in_erg_per_CCSN erg per CCSN formed

The energy released effectively into surrounding gas particles can be mitigated with the parameter AGORAFeedback:supernovae_efficiency, used as a simple factor to AGORAFeedback:energy_in_erg_per_CCSN.

Both energy and mass are ejected into the surrounding gas according to the SPH kernel.

The mass fraction of elements received by gas particles will be smoothed (smoothed metallicity scheme) by using the SPH kernel. Note that this scheme does not exchange any material between particles. Snapshots can store both the smoothed metallicity (SmoothedMetalMassFractions) and/or the non-smoothed one (MetalMassFractions), i.e., the mass fraction of elements effectively received by the gas particles.

The initial metallicity of the gas can be defined by the parameter AGORAChemistry:initial_metallicity. A value less than 0 forces the code to take the gas metallicity from the initial condition file (snapshot). Instead, if AGORAChemistry:scale_initial_metallicity is different than 0, the initial mass fraction of elements will be set to:

  • AGORAChemistry:initial_metallicity time AGORAChemistry:solar_abundance_Metals for the iron

  • AGORAChemistry:initial_metallicity time AGORAChemistry:solar_abundance_Metals for the metals

Instead, they will be set to:

  • AGORAChemistry:initial_metallicity for the iron

  • AGORAChemistry:initial_metallicity for the metals

Recommended parameters for the AGORA model should be:

AGORAChemistry:
 initial_metallicity: 1
 scale_initial_metallicity: 1
 solar_abundance_Fe: 0.001771
 solar_abundance_Metals: 0.02

AGORAFeedback:
 energy_in_erg_per_CCSN: 1e51
 supernovae_efficiency: 1
 supernovae_explosion_time_myr: 5
 ccsne_per_solar_mass : 0.010989
 ejected_mass_in_solar_mass_per_CCSN : 14.8
 ejected_Fe_mass_in_solar_mass_per_CCSN : 2.63
 ejected_metal_mass_in_solar_mass_per_CCSN : 2.63

Pressure Floor

The AGORA model uses precisely the same pressure floor than the GEAR model.

Initial Conditions

Note that if in the initial conditions, the time of formation of a stellar particle is given (BirthTime) and set to a negative value, the stellar particle will provide no feedback. A similar behavior will be obtained if the parameter overwrite_birth_time is set to 1 and birth_time to -1.