We use idealized three-dimensional hydrodynamic simulations to study the dynamics and thermal structure of the circumgalactic medium (CGM). Our simulations quantify the role of cooling, stellar feedback driven galactic winds, and cosmological gas accretion in setting the properties of the CGM in dark matter haloes ranging from 10 11 − 10 12 M ⊙ . Our simulations support a conceptual picture in which the properties of the CGM, and the key physics governing it, change markedly near a critical halo mass of M crit ≈ 10 11.5 M ⊙ . As in calculations without stellar feedback, above M crit halo gas is supported by thermal pressure created in the virial shock. The thermal properties at small radii are regulated by feedback triggered when t cool /t ff 10 in the hot gas. Below M crit , however, there is no thermally supported halo and selfregulation at t cool /t ff ∼ 10 does not apply. Instead, the gas is out of hydrostatic equilibrium and largely supported against gravity by bulk flows (turbulence and coherent inflow/outflow) arising from the interaction between cosmological gas inflow and outflowing galactic winds. In these lower mass halos the phase structure depends sensitively on the outflows' energy per unit mass and mass-loading, which may allow measurements of the CGM thermal state to constrain the nature of galactic winds. Our simulations account for some of the properties of the multiphase halo gas inferred from quasar absorption line observations, including the presence of significant mass at a wide range of temperatures, and the characteristic O vi and C iv column densities and kinematics. However, we under-predict the neutral hydrogen content of the z ∼ 0 CGM.

A common situation in galactic and intergalactic gas involves cold dense gas in motion relative to hot diffuse gas. Kelvin–Helmholtz instability creates a turbulent mixing layer and populates the intermediate-temperature phase, which often cools rapidly. The energy lost to cooling is balanced by the advection of hot high enthalpy gas into the mixing layer, resulting in growth and acceleration of the cold phase. This process may play a major role in determining the interstellar medium and circumgalactic medium phase structure, and accelerating cold gas in galactic winds and cosmic filaments. Cooling in these mixing layers occurs in a thin corrugated sheet, which we argue has an area with fractal dimension D = 5/2 and a thickness that adjusts to match the hot phase mixing time to the cooling time. These cooling sheet properties form the basis of a new model for how the cooling rate and hot gas inflow velocity depend on the size L, cooling time of the mixed phase , relative velocity , and density contrast of the system. Entrainment is expected to be enhanced in environments with short , large , and large . Using a large suite of three-dimensional hydrodynamic simulations, we demonstrate that this fractal cooling layer model accurately captures the energetics and evolution of turbulent interfaces and can therefore be used as a foundation for understanding multiphase mixing with strong radiative cooling.

We use three-dimensional hydrodynamic simulations of vertically stratified patches of galactic discs to study how the spatio-temporal clustering of supernovae (SNe) enhances the power of galactic winds. SNe that are randomly distributed throughout a galactic disc drive inefficient galactic winds because most supernova remnants lose their energy radiatively before breaking out of the disc. Accounting for the fact that most star formation is clustered alleviates this problem. Super-bubbles driven by the combined effects of clustered SNe propagate rapidly enough to break out of galactic discs well before the clusters' SNe stop going off. The radiative losses postbreakout are reduced dramatically and a large fraction ( 0.2) of the energy released by SNe vents into the halo powering a strong galactic wind. These energetic winds are capable of providing strong preventative feedback and eject substantial mass from the galaxy with outflow rates on the order of the star formation rate. The momentum flux in the wind is only of order that injected by the SNe, because the hot gas vents before doing significant work on the surroundings. We show that our conclusions hold for a range of galaxy properties, both in the local Universe (e.g., M82) and at high redshift (e.g., z ∼ 2 star forming galaxies). We further show that if the efficiency of forming star clusters increases with increasing gas surface density, as suggested by theoretical arguments, the condition for star cluster-driven super-bubbles to break out of galactic discs corresponds to a threshold star formation rate surface density for the onset of galactic winds ∼ 0.03 M yr −1 kpc −2 , of order that observed.

Large scale outflows in star-forming galaxies are observed to be ubiquitous, and are a key aspect of theoretical modeling of galactic evolution in a cosmological context, the focus of the SMAUG (Simulating Multiscale Astrophysics to Understand Galaxies) project. Gas blown out from galactic disks, similar to gas within galaxies, consists of multiple phases with large contrasts of density, temperature, and other properties. To study multiphase outflows as emergent phenomena, we run a suite of ∼ pc-resolution local galactic disk simulations using the TIGRESS framework. Explicit modeling of the interstellar medium (ISM), including star formation and self-consistent radiative heating plus supernova feedback, regulates ISM properties and drives the outflow. We investigate the scaling of outflow mass, momentum, energy, and metal loading factors with galactic disk properties, including star formation rate (SFR) surface density (Σ SFR ∼ 10 −4 − 1 M kpc −2 yr −1 ), gas surface density (∼ 1 − 100 M pc −2 ), and total midplane pressure (or weight) (∼ 10 3 − 10 6 k B cm −3 K). The main components of outflowing gas are mass-delivering cool gas (T ∼ 10 4 K) and energy/metal-delivering hot gas (T ∼ 10 6 K). Cool mass outflow rates measured at outflow launch points (one or two scale heights) are 1-100 times the SFR (decreasing with Σ SFR ), although in massive galaxies most mass falls back due to insufficient outflow velocity. The hot galactic outflow carries mass comparable to 10% of the SFR, together with 10-20% of the energy and 30-60% of the metal mass injected by SN feedback. The characteristic outflow velocities of both phases scale very weakly with SFR, as v out ∝ Σ 0.1−0.2 SFR , consistent with observations. Importantly, our analysis demonstrates that in any physically-motivated cosmological wind model, it is crucial to include at least two distinct thermal wind components.

We use local Cartesian simulations with a vertical gravitational potential to study how supernova (SN) feedback in stratified galactic discs drives turbulence and launches galactic winds. Our analysis includes three disc models with gas surface densities ranging from Milky Way-like galaxies to gas-rich ultra-luminous infrared galaxies (ULIRGs), and two different SN driving schemes (random and correlated with local gas density). In order to isolate the physics of SN feedback, we do not include additional feedback processes. We find that, in these local box calculations, SN feedback excites relatively low mass-weighted gas turbulent velocity dispersions ≈ 3 − 7 km s −1 and low wind mass loading factors η 1 in all the cases we study. The low turbulent velocities and wind mass loading factors predicted by our local box calculations are significantly below those suggested by observations of gas-rich and rapidly star-forming galaxies; they are also in tension with global simulations of disc galaxies regulated by stellar feedback. Using a combination of numerical tests and analytic arguments, we argue that local Cartesian boxes cannot predict the properties of galactic winds because they do not capture the correct global geometry and gravitational potential of galaxies. The wind mass loading factors are in fact not well-defined in local simulations because they decline significantly with increasing box height. More physically realistic calculations (e.g., including a global galactic potential and disc rotation) will likely be needed to fully understand disc turbulence and galactic outflows, even for the idealized case of feedback by SNe alone.