Field Listing

The most fundamental quantities associated with any calculation done in ares are the gas density, species fractions and the gas temperature.

Species Fractions

Our naming convention is to denote ions using their chemical symbol (in lower-case), followed by the ionization state, separated by an underscore. Rather than denoting the ionization state with roman numerals, we simply use integers. For example, neutral hydrogen is h_1 and ionized hydrogen is h_2.

Here is a complete listing:

  • Neutral hydrogen fraction: 'h_1'
  • Ionized hydrogen fraction: 'h_2'
  • Neutral helium fraction: 'he_1'
  • Singly-ionized helium fraction: 'he_2'
  • Doubly-ionized helium fraction: 'he_3'
  • Electron fraction: 'e'
  • Gas density (in \(g \ \text{cm}^{-3}\)): 'rho'

These are the default elements in the history dictionary, which is an attribute of all ares.simulations classes.

We also generally keep track of the ionization and heating rate coefficients:

  • Rate coefficient for photo-ionization, k_ion.
  • Rate coefficient for secondary ionization by photo-electrons, k_ion2.
  • Rate coefficient for photo-heating, k_heat.

Each of these quantities are multi-dimensional because we store the rate coefficients for each absorbing species separately.

Two-Zone IGM Models

For calculations of the reionization history or global 21-cm signal, in which we use a two-zone IGM formalism, all quantities described in the previous sections keep their usual names with one important change: they now also have an igm or cgm prefix to signify which phase of the IGM they belong to. The igm phase is of course short for inter-galactic medium, while the cgm phase stands for the circum-galactic medium (really just meant to indicate gas near galaxies).

  • Kinetic temperature, igm_Tk.
  • HII region volume filling factor, cgm_h_2.
  • Neutral fraction in the bulk IGM, igm_h_1.
  • Heating rate in the IGM, igm_k_heat.
  • Volume-averaged ionization rate, cgm_k_ion.

There are also new (passive) quantities, like the neutral hydrogen excitation (or ``spin’’ temperature), the 21-cm brightness temperature, and the Lyman-\(\alpha\) background intensity:

  • 21-cm brightness temperature: 'igm_dTb'.
  • Spin temperature: 'igm_Ts'.
  • \(J_{\alpha}\): 'igm_Ja'.

Each of these are only associated with the IGM grid patch, since the other phase of the IGM is assumed to be fully ionized and thus dark at 21-cm wavelengths.