Measuring the Velocity Dispersion of the Warm Neutral Medium of the Milky Way at Galactic Scale using the Code FLASH
Fig. 1 shows a movie of the projected density of one of the runs (A disk with a spiral perturbation with no self gravity) over one orbital time.
Author: Wajdee Chayeb
We observe a velocity dispersion in atomic hydrogen of ~9 km/s in the solar neighbourhood. Such dispersion is a critical factor in understanding star formation and the evolution of the interstellar medium (ISM), as it might be linked to turbulence. The origin of this dispersion at different scales and environments is still unclear. Therefore, we model the ISM of the Milky Way and investigate the roles of supernova remnants, shear, and spiral arms at the galactic scale (~1 kpc).
Our model accounts for self-gravity; gravitational forces from a background stellar and dark matter halo; supernovae; a chemical network of five species (H0, H+, H2, CO, and C+); a background interstellar radiation field; and a cosmic-ray ionisation rate that varies with galactocentric radius.
Four different simulations are run using the finite-volume scheme code FLASH: a gaseous disk with and without self-gravity, and a spiral disk with and without self-gravity. The physical properties in all runs are matched to observational data of the warm neutral medium (WNM) . In every run, the disk is initially set in hydrostatic equilibrium, after which a chemical network is applied and supernova events are injected to form the ISM. To generate spiral arms in the gas, a spiral perturbation with a fixed pattern speed of 20 km/s/kpc is added to the background galactic potential of the stellar population, with a strength of 1%. After ~100 Myr, the disk reaches an equilibrium temperature similar to that of the WNM (~3000–6000 K), and a maximum rotation curve of ~240 km/s with a shift of +10 km/s in the spiral arms.
When analysing the velocity dispersion in radial annuli of 1 kpc size and azimuthal width of 45 degrees, averaged over one orbital time (~200 Myr) and over the azimuthal bins, we find that gaseous spiral arms in the ISM of the Milky Way drive a dispersion of up to ~9 km/s, which decreases with galactocentric radius and is relatively higher in the inter arms than inside the spiral arms, as shown in Figure 2. Shear, supernova remnants, and thermal instabilities do not appear to play a significant role in driving velocity dispersion at such scales.
