MHD Simulations of Strongly Magnetized HII Region Evolution: Evidence for Ionized Gas Filamentation

Abstract

Recent JWST observations of HII regions in the Central Molecular Zone have shown a highly filamentary morphology distinct from HII regions in other parts of the galaxy. We present magnetohydrodynamic (MHD) simulations of strongly magnetized (plasma-$β$ << 1) HII region evolution that investigate and describe the formation of these ionized gas filaments. HII region evolution has been simulated in a 30 pc$^3$ box, in distinct models with pre-placed overdensities in the ambient medium and overdensities that have been generated with driven turbulence. We find that when these overdensities are seeded in the ambient medium before the birth of the ionizing source, the photoionized plasma stripped off of these dense blobs is funneled into long filaments along the magnetic field lines. The length and emission measure of these ionized gas filaments are similar to the filaments observed in the Central Molecular Zone. Given that these filaments are effectively magnetically-confined flows of photoionized gas, their density and curvature are influenced by the density of the blob and the geometry of the configuration.

Figures (3)

• Figure 1: Select cutaways from the simulation run with preset blobs. Panel (a) shows an $x-z$ slice (along $y=0$) of the number density at $t=0$, i.e. the initial condition before activation of the ionizing source. Panels (b) and (c) show the same at $t=0.3$ and $t=1.3$ Myr, respectively. Panel (d) shows the emission measure of ionized gas along the line-of-sight. Panel (e) shows the z-velocity, $v_z$, along an $x-z$ slice ($y=0$) with velocity vectors shown in black and scaled to the magnitude of the velocity. Panel (f) shows an $x-z$ slice (along $y=0$) of the plasma $\beta$, or thermal pressure divided by magnetic pressure. The location of the ionizing source is marked with a black cross in all panels.
• Figure 2: Gas density along the $x-z$ plane at the time of activation of the magnetic field and ionizing source ($\mathrm{t_0}$) in the simulation run with driven turbulence. The left panel shows a slice of the gas density in the $y=0$ plane, and the right panel shows the integrated gas density along the line-of-sight. The ionizing source is marked with a black cross in both panels.
• Figure 3: Select cutaways from the simulation run with driven turbulence. Panels (a) and (b) show $x-z$ slices (along $y=0$) of the total number density of gas at $\mathrm{t-t_0}$=0.5 and 2 Myr, respectively. Panels (c) and (d) show the same but for the ionized gas number density. Panels (e) and (f) show the emission measure of ionized gas along the line of sight. The location of the ionizing source is marked with a black cross in all panels.