Tutorial: Reionization Calculations

Galacticus can self-consistently solve for the evolution of the as it becomes photoionized by light emitted by stars and AGN. To activate this calculation, include the following in your parameters file:

      <!-- IGM evolver -->
      <intergalacticMediumStateMethod value="internal"/>
      <universeOperatorMethod value="intergalacticMediumStateEvolve">
        <timeCountPerDecade value=" 10"/>
        <redshiftMaximum    value="150"/>
      </universeOperatorMethod>    

      <!-- Background radiation -->
      <radiationFieldIntergalacticBackgroundMethod value="intergalacticBackgroundInternal">
        <wavelengthCountPerDecade value="50"/>
        <timeCountPerDecade       value="10"/>
      </radiationFieldIntergalacticBackgroundMethod>

      <!-- Halo accretion options -->
      <accretionHaloMethod value="naozBarkana2007"/>

The first block of parameters switches Galacticus to using an internal calculation for the state of the IGM, instructs it to solve for properties as a function of time, and specifies that properties should be updated 10 times per decade of cosmic time. Specifically, at each of these time intervals, solving of galaxy evolution is halted and the IGM evolved up to this time using the currently computed photoionizing background spectrum.

The second block of parameters activates an internal calculation of cosmic background radiation, in which the background is computed from the emissivities of model galaxies and AGN. The number of points at which to tabulate the background per decade of wavelength and cosmic time are specified.

Finally, the third block of parameters tells Galacticus to use the Naoz & Barkana (2007; naozBarkana2007) prescription for computing gas accretion into halos from the IGM. This prescription uses the filtering mass to determine accretion rates, and will take the filtering mass from the internal evolution calculation.

With these three sets of configurations, Galacticus will perform a self-consistent evolution of the IGM—in the sense that the is ionized by photons emitted by model galaxies and AGN, while galaxy evolution is affected by the computed state of the model IGM. Note that, when run in this way, Galacticus needs to keep all merger trees in memory simultaneously (as they are run synchronously to allow the properties to evolved alongside galaxy properties).

Once completed, data on the and background radiation are written to the output file in the igmProperties and backgroundRadiation groups respectively.