Atomic Chemistry in Turbulent Astrophysical Media

Abstract

We decribe direct numerical simulations of turbulent astrophysical media exposed to the redshift zero metagalactic background. The simulations assume solar composition and explicitly track ionizations, recombinations, and ion-by-ion radiative cooling for hydrogen, helium, carbon, nitrogen, oxygen, neon, sodium, magnesium, silicon, and iron. Each run reaches a global steady state that not only depends on the ionization parameter, U, and mass-weighted average temperature, T_MW, but also on the the one-dimensional turbulent velocity dispersion, σ_1D.

We carry out runs that span a grid of models with U ranging from 0 to 10⁻² and σ_1D ranging from 12 to 58 km s⁻¹, and we vary the product of the mean density and the driving scale of the turbulence, nL, which determines the average temperature of the medium, from nL =10¹⁶ to nL =10²⁰ cm⁻². The turbulent Mach numbers of our simulations vary from M ≈ 0.5 for the lowest velocity dispersions cases to M ≈ 20 for the largest velocity dispersion cases. When M ≲1, turbulent effects are minimal, and the species abundances are reasonably described as those of a uniform photoionized medium at a fixed temperature. On the other hand, when M ≳ 1, dynamical simulations such as the ones carried out here, are required to accurately predict the species abundances.

We gather our results into a set of tables, to allow future redshift zero studies of the intergalactic medium to account for turbulent effects. They are available at http://zofia.sese.asu.edu/~evan/turbspecies/ and will be updated as we increase our parameter study. These results are explained in more detailed in Gray, Scannapieco, & Kasen (2015), and Gray and Scannapieco (2015)