Intergalactic Medium and Reionization

The thermal and ionization history of the IGM enters galaxy formation in two places: it sets the ultraviolet background that photo-ionizes and heats gas, and it raises the Jeans mass so that low-mass halos accrete less than the cosmic baryon fraction. The IGM state class provides the neutral and ionized fractions of hydrogen and helium, the temperature, and the electron-scattering optical depth as functions of time, either from RecFast, from a file, from an instantaneous reionization prescription, or evolved internally by the intergalacticMediumStateEvolve universe operator from the ionizing emissivity of the model’s own galaxies.

        flowchart LR
   State[<a href='https://galacticus.readthedocs.io/en/latest/physics/intergalacticMediumState.html' style='text-decoration: none'>IGM State</a>]
   Radiation[<a href='https://galacticus.readthedocs.io/en/latest/physics/radiationField.html' style='text-decoration: none'>Radiation Field</a>]
   Filtering[<a href='https://galacticus.readthedocs.io/en/latest/physics/intergalacticMediumFilteringMass.html' style='text-decoration: none'>Filtering Mass</a>]
   Accretion[<a href='https://galacticus.readthedocs.io/en/latest/physics/accretionHalo.html' style='text-decoration: none'>Halo Accretion</a>]
   Cooling[<a href='https://galacticus.readthedocs.io/en/latest/physics/coolingFunction.html' style='text-decoration: none'>Cooling Function</a>]
   Environment[<a href='https://galacticus.readthedocs.io/en/latest/physics/haloEnvironment.html' style='text-decoration: none'>Environment</a>]
   Galaxy([Galaxy])
   State --> Filtering
   State --> Radiation
   Filtering --> Accretion
   Radiation --> Cooling
   Environment --> Accretion
   Accretion --> Galaxy
   Cooling --> Galaxy
    

The filtering mass of Gnedin (2000) converts the IGM temperature history into the halo mass below which accretion is suppressed, and the halo accretion classes apply that suppression, optionally with a dependence on the large-scale environment that reionized earlier or later. Radiation fields (the cosmic microwave background, tabulated or internally computed ultraviolet backgrounds, and stellar fields) are integrated against atomic cross-sections to give photo-ionization and photo-heating rates for the cooling and chemistry of circumgalactic gas.