Description of Physical Models

21cmFAST supports many physical models and effects that can often be toggled on or off interchangeably (though some depend on others). The MatterOptions and AstroOptions classes contain all of the flags that control which models to include in the simulation.

Below we provide a brief explanation on how some of the flags modify the output of 21cmFAST.

Models of the Matter Field

The parameters and flags of MatterOptions are used to control how cosmological matter fields (e.g. densities, velocities and halo properties) are evaluated in the simulation.

Source Model

Note

Set with the SOURCE_MODEL parameter of MatterOptions.

To be filled.

Astrophysical Models

The parameters and flags of AstroOptions are used to control how astrophysical quantities (e.g. star formation rate, UV and ionizing radiation) are evaluated in the simulation.

It is important to stress that the generation of PerturbedField and HaloCatalog objects do not depend on these parameters (nor on AstroParams). Therefore, if the cache contains PerturbedField and HaloCatalog objects that had been previously generated with a different set of AstroOptions, these objects will be loaded from the cache, instead of being re-evaluated. This architecture allows one to quickly simulate different astrophysical models, given a cosmological model.

Spin Temperature Fluctuations

To be filled.

Minihalos

To be filled.

Multiple Scattering of Lyman Alpha Photons

Note

Toggled with the LYA_MULTIPLE_SCATTERING flag of AstroOptions (default: False).

The physical effect that enables the absorption feature in the 21-cm signal during cosmic dawn is the strong coupling between the spin temperature and the gas kinetic temperature. This coupling is obtained through Lyman alpha radiation that comes from the first stars and is absorbed by the IGM. Since the cross section for the interaction between photons near the Lyman alpha resonance frequency and HI atoms in the IGM has a non-negligible width, the Lyman alpha photons that are absorbed by the IGM had not traveled in straight lines, but rather had scattered along their path. This means that the effective Lyman alpha emissivity that the IGM “sees” becomes more local, thereby increasing the contrast in J_alpha (Lyman alpha flux) maps in the simulation, as can be seen below. In the context of the 21-cm signal, this effect becomes negligible at sufficiently low redshifts (normally below \(z<15\)), once the spin temperature completely follows gas kinetic temperature. For more information on this effect in the simulation, see Flitter, Munoz and Mesinger 2026.