Estimating ionization fractions in SILCC simulations

Author: Lennart Buhlmann

The interstellar medium (ISM) consists of gas, dust, and radiation that fill the space between stars in a galaxy. The gaseous component is commonly described as a multiphase medium, characterized by different densities, temperatures, and ionization states. These phases are typically classified as the Cold Neutral Medium (CNM), Warm Neutral Medium (WNM), Warm Ionized Medium (WIM), and Hot Ionized Medium (HIM).

In this work, we focus on the ionization state of the ISM. Ionization describes the process by which atoms lose electrons and become charged. In the ISM, this can happen in two main ways: through collisions between particles or through the absorption of radiation (photoionization), for example from massive stars. Collisional ionization depends strongly on temperature, while photoionization mainly depends on the strength of the radiation field. As a result, photoionization dominates in colder gas, whereas collisional ionization becomes increasingly important at higher temperatures and dominates above a few 10⁴ K.

To study ionization we analyze simulations from the SILCC project and post-process them using the CHIMES chemistry code. From the simulations, we obtain quantities such as the hydrogen number density, temperature, radiation field, and shielding. Based on this information, CHIMES calculates the ionization fractions of 11 elements (H, He, C, N, O, Ne, Si, Mg, S, Ca and Fe). This allows us to extend the chemical information from the SILCC simulations by adding further species and their respective ionization states.

Based on these calculations, we investigate whether the gas is in ionization equilibrium or in a non-equilibrium ionization (NEI) state. In ionization equilibrium, ionization and recombination processes balance each other, allowing reliable predictions of cooling rates, line ratios, and gas temperatures. In contrast, NEI conditions arise when the gas is rapidly heated or cooled, driving it out of equilibrium and resulting in under- or over-ionized states. In such cases, cooling rates and spectral line ratios can deviate significantly from equilibrium predictions, making equilibrium-based models unreliable.

Studying the ionization states of individual elements enhances our understanding of underlying physical processes, e.g. cooling and heating of the gas, and with this helps to understand the NEI evolution of the ISM.

Furthermore, we can gain a detailed view of the ionization structure inside a molecular cloud. In Figure 1 the left panel shows slices of temperature an hydrogen number density from the SILCC simulation. On the right side we present slices of a sub-region around the centre of the SILCC simulation. Each column corresponds to a different element, and each row represents its ionization state. The ionization level increases from top to bottom, ranging from neutral (non-ionized) gas at the top to fully or highly ionized gas at the bottom. For better visibility, the bottom row combines the higher ionization states, beginning with the nine-times ionized state.

Within molecular clouds, characterized by low temperatures (< a few 10³ K) and high densities (n > 1 cm⁻³), most elements are expected to be neutral (H, He, C, N, O, Ne) or in low ionization states (Mg, Si, S, Ca, Fe). Moving outward, the density decreases and the temperature increases, leading to higher ionization states. In low-density (n < 10⁻¹ cm⁻³), high-temperature (> 10⁴ K) regions, elements are expected to be fully ionized (H, He, C, N, O, Ne) or occupy high ionization states (Mg, Si, S, Ca, Fe).