Simulating Gas Inflow at the Disk–Halo Interface

Abstract

The interaction between inflowing gas clouds and galactic outflows at the interface where the galactic disk transitions into the circumgalactic medium is an important process in galaxy fueling, yet remains poorly understood. Using a series of tall-box hydrodynamic Enzo simulations, we have studied the interaction between smooth gas inflow and supernovae-driven outflow at the disk-halo interface with pc-scale resolution. A realistic wind of outflowing material is generated by supernovae explosions in the disk, while inflowing gas is injected at the top boundary of the simulation box with an injection velocity ranging from 10 − 100 km s −1 . We find that cooling and hydrodynamic instabilities drive the injected gas to fragment into cold (∼ 10 3 K) cloud clumps with typical densities of ∼ 1 cm −3 . These clumps initially accelerate before interacting and partially mixing with the outflow and decelerating to velocities in the 50-100 km s −1 range. When the gas clumps hit the disk, 10% − 50% of the injected material is able to accrete (depending on the injection velocity). Clumps originating from gas injected with a higher initial velocity approach the disk with greater ram pressure, allowing them to penetrate through the disk in low density regions. We use (equilibrium) Cloudy photoionization models to generate absorption and emission signatures of gas accretion, finding that our mock HI and Hα observables are prominent and generally consistent with measurements in the Milky Way. We do not predict enhanced emission/absorption for higher ionization states such as OVI.

The Structure of Multiphase Galactic Winds

“…31, respectively, highlights this physical connection between acceleration and metal mixing when other forms of acceleration are subdominant. This is consistent with a wide range of recent studies that have measured the correlation between cold cloud velocity and the degree to which it has been diluted in simulations of galactic winds (Melso et al 2019;Schneider et al 2020) and ram pressure stripped galaxies (Tonnesen & Bryan 2021).…”

Radiative Turbulent Mixing Layers and the Survival of Magellanic Debris

“…Our "2.5D" cloud geometry, motivated by our envisioned geometry of the Leading Arm at swing-out from the Clouds, appears to facilitate a "shielding" effect that prevents shear instabilities from further stripping cloud fragments. This effect is seen as well in Melso et al (2019) in simulations of infalling gas clouds and in Banda- Barragán et al (2020), which models shock-multicloud interactions and reports that most bulk properties are converged even for M = 10 simulations.…”

“…This scale is not resolved in our work or in any published cloud-crushing simulations for overdensities of χ ∼ 100. Nevertheless, with a slight shift from a spherically symmetric cloud to an axisymmetric 2.5D cloud, we find reasonable convergence, likely due to a "shielding" effect, i.e., a more quiescent region in the tail leading to the re-coagulation of the clumps instead of further fragmentation (as seen in McCourt et al 2018;Melso et al 2019, see also the setup of Banda-Barragán et al 2020).…”

Discovery of diffuse optical emission lines from the inner Galaxy: Evidence for LI(N)ER-like gas

“…1 , we show how the estimated boundary of the Fermi Bubble—as seen in gamma-ray and x-ray emission—compares to the structure of the Tilted Disk. If hot gas is intermixed with the Tilted Disk structure, then our gas mass estimates based on Hα emission will be lower limits ( 39 ) to the total amount of ionized gas in the inner Galaxy. Our observations show that the nearest LI(N)ER-like gas to us in the universe is now the inner Milky Way and no longer the inner parts of M31.…”