| Description: | So far, numerical (magnetized and unmagnetized) 3D simulations of the interstellar medium (ISM) (as well as those including the Local Bubble evolution) have assumed collisional ionization equilibrium (CIE) conditions for the ISM. However, the thermal evolution of the ISM is determined by heating and cooling processes, which in general are not synchronized with ionization and recombination processes, respectively (see, e.g., Kafatos 1973; Shapiro & Moore 1976; Sutherland & Dopita 1993). Thus, below 106 K, deviations from CIE conditions occur, thereby affecting the ionization structure of the interstellar gas, and thus the local electron density and distribution. In particular, if delayed recombination plays a role, the number of free electrons may be severely underestimated. The latest developments in the field comprise the joint dynamical and thermal evolution of the gas featuring a detailed determination of the ionization structure and cooling functions on the spot in a time-dependent fashion (Avillez & Breitschwerdt 2012; Avillez et al. 2012). A discussion is made on these developments, simulations results, and comparisons with observations for the electron distribution in the Galactic disk.
Figure: Total (left) and electron (right) density distributions (in log scale) in the Galactic midplane at 400 Myr of evolution. These images are taken from the highest resolution simulation to date of the ISM with effective grids comprising 2000^3 cells per kpc^3 cube. The nest adaptive mesh re nement resolution is 0.5 pc, while the coarse grid has a resolution of 8 pc. Red regions in the left panel represent high density material, with molecular clouds being represented by black. Electron densities smaller than log(n_e) = -4 (white regions; right panel) are located in both high (atomic and molecular clouds) and lower (bubbles) density regions. (From Avillez et al. 2012, MNRAS) |
