Wednesday, December 3, 2014

Modeling Reionization

As the result of work by Caltech SURF student, Daniel McAndrew, Galacticus now has a fully self-consistent calculation of reionization (and the ionization and thermal state of the intergalactic medium in general). This model uses the spectrum of photons emitted by model galaxies and AGN to compute the evolution of ionization states of hydrogen and helium in the intergalactic medium (IGM), along with the IGM temperature. The thermal evolution of the IGM is then used to compute the filtering mass, and this affects later galaxy evolution via the gas accretion rate into halos (Naoz & Barkana, 2007).

The result is an internally self-consistent model which can predict when reionization occurs. There are some simplifications (the biggest among them probably being the escape fraction of ionizing photons from galaxies, and the clumping factor in the IGM) but we hope that future work will address some of those.

To activate this calculation, include the following in your parameters file:

  <!-- IGM evolver -->
  <parameter>
    <name>intergalacticMediumStateMethod</name>
    <value>internal</value>
  </parameter>
  <parameter>
    <name>igmPropertiesCompute</name>
    <value>true</value>
  </parameter>
  <parameter>
    <name>igmPropertiesTimeCountPerDecade</name>
    <value>10</value>
  </parameter>
 
  <!-- Background radiation -->
  <parameter>
    <name>backgroundRadiationCompute</name>
    <value>true</value>
  </parameter>
  <parameter>
    <name>radiationIntergalacticBackgroundMethod</name>
    <value>internal</value>
  </parameter>
  <parameter>
    <name>backgroundRadiationWavelengthCountPerDecade</name>
    <value>50</value>
  </parameter>
  <parameter>
    <name>backgroundRadiationTimeCountPerDecade</name>
    <value>10</value>
  </parameter>

  <!-- Halo accretion options -->
  <parameter>
    <name>accretionHaloMethod</name>
    <value>naozBarkana2007</value>
  </parameter>


The first block of parameters switches Galacticus to using an internal calculation for the state of the IGM, instructs it to solve for IGM properties as a function of time, and specifies that IGM 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) 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 IGM evolution calculation.

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

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