The Launching of Galactic Winds from a Multiphase ISM

TL;DR

The paper investigates how cold gas survives entrainment in hot galactic winds by modeling a multiphase ISM with controlled parameters $(f_v, L_{ m ISM}, r_{ m cl})$ in high-resolution 3D wind–ISM simulations. It derives a universal survival criterion, showing cold gas grows when the effective ISM depth satisfies $f_v L_{ m ISM} \,\gtrsim\ r_{ m crit}$ (with a possible tilting factor $\\alpha$), extending single-cloud results to fractal-like ISMs, and independent of initial clump size for $r_{ m cl} \ll r_{ m crit}$. The cold phase rapidly organizes into self-similar structures with a Zipf mass spectrum $dN/dm\propto m^{-2}$, while turbulence is driven in both phases with a Kolmogorov-like cascade and an injection scale $\,\sim L_{ m ISM}$; shells or plumes form with areal covering fractions approaching unity, explaining the observed 'misty' appearance of winds. These results connect small-scale cloud–wind interactions to galaxy-scale feedback and provide a robust framework for interpreting multiphase wind observations and informing larger-scale simulations.

Abstract

Galactic outflows are a key agent of galaxy evolution, yet their observed multiphase nature remains difficult to reconcile with theoretical models, which often fail to explain how cold gas survives interactions with hot, fast winds. We present high-resolution 3D hydrodynamic simulations of hot outflows interacting with a multiphase interstellar medium (ISM), parameterised by its cold-gas volume filling fraction $f_v$, depth $L_{\rm ISM}$, and clump size $r_{\rm cl}$. We identify a universal survival criterion $f_v L_{\rm ISM} \gtrsim r_{\rm crit}$ that generalises the classical single-cloud condition ($r_{\rm cl} > r_{\rm crit}$) and correctly predicts cold-gas survival across diverse ISM configurations - including scale-free - down to $r_{\rm cl}/r_{\rm crit} \sim 10^{-2}$. The surviving cold phase rapidly loses memory of the initial ISM structure and evolves toward a self-similar clump mass spectrum following Zipf's law ($\mathrm{d}N/\mathrm{d}m \propto m^{-2}$), implying that turbulent mixing and radiative condensation universally shape multiphase outflows. Cold gas assembles into plumes or confined shells of size $\sim χr_{\mathrm{cl,min}}$, growing as mass is accreted from the hot phase. The interaction of a laminar wind with a clumpy ISM drives turbulence in both phases, with first-order velocity structure functions following a Kolmogorov scaling and an injection scale set by $L_{\rm ISM}$, while velocity dispersions reach $σ\sim c_{\rm s,cold}$. The areal covering fraction of cold gas approaches unity even for $f_v \sim 10^{-3}$, though its volume filling fraction stays low, explaining the "misty" appearance of observed outflows. Together, these results link small-scale cloud-wind interactions to galaxy-scale feedback, and we discuss their implications for interpreting observations and for modelling multiphase galactic winds in larger-scale simulations.

Figures (20)

• Figure 1: Volume rendering of our simulation set-up: a stellar wind with $\mathcal{M}_w = 1.5$ enters from the left, driving turbulent mixing as it moves through 1/4 of the cropped domain box. The rendering highlights regions of varying gas density. Bright yellow denotes gas that is overdense by a factor of 100 relative to the diffuse background phase (shown in light blue). Intermediate overdensities appear in muted colors, with black indicating regions where the overdensity is around 10.
• Figure 2: 2D slices focused on our initial binary multiphase ISM fields where the wind enters from the left boundary (outside the shown region). We vary the volumetric cold gas filling fraction, $f_\mathrm{v}$, along the $x$ axis, and clump size $r_\mathrm{cl}$ along the $y$ axis, expressed as a fraction of the critical lengthscale for single cloud survival, $r_\mathrm{crit}$. Colour-coded in yellow and navy the density values for cold and hot phases, respectively.
• Figure 3: Density projection ($\int \rho(z) \mathrm{d}z / (\ell_z \rho_{\rm hot})$) at the time of wind-ISM interaction (left column), mid-way through entrainment (middle) and at time of entrainment (right) for different simulations. Each panel is divided into two halves by a dashed white line, where the top uses a total integration depth $\ell_z = r_\mathrm{cl}$, and bottom $\ell_z = L_\mathrm{z, box}$. Top (rows 1-3): runs of varying $r_\mathrm{cl}$ and fixed $f_v=0.1, L_\mathrm{ISM} = 300r_\mathrm{cl}$. Middle (rows 4-6): runs of varying $f_v$ and fixed $L_\mathrm{ISM} = 300r_\mathrm{cl}, r_\mathrm{cl} = r_\mathrm{crit}$. Bottom (rows 7-9): 2 runs of varying $L_\mathrm{ISM}$ and fixed $r_\mathrm{cl}=r_\mathrm{crit},f_v = 0.1$. Note that we do not show the entire simulation domain but instead focus on the upstream boundary of the cold gas with the hot wind, and use a length of $L_y \approx 100 r_\mathrm{cl}$ along the $y$ axis.
• Figure 4: Total cold gas mass (top) and shear speed (bottom) evolution as function of their dimensionless temporal variable, $\tau$ (see § \ref{['sec:results/multiplot']}). Left panels display runs for $r_\mathrm{cl}/r_\mathrm{crit} = 10$ (where the classical survival criteria of cold gas is strictly satisfied), and $r/r_\mathrm{crit} = 1$ for the right panels. Linestyles represent the volumetric filling fraction of cold gas in the ISM, and runs are colour-coded by the total depth of the initial density field in units of clump size $r_\mathrm{cl}$.
• Figure 5: Top: mass carried per velocity bin during the wind-ISM interaction ($t \sim 0.1 \, t_\mathrm{ent}$, left), mid-way through entrainment ($t \sim 0.5 \, t_\mathrm{ent}$, middle) and at time of entrainment ($t \sim \,t_\mathrm{ent}$, right). In black, total mass velocity curve; blue, green and red show the mass curves for temperature cuts $T \leq 10^5, 10^5 \leq T \leq 10^{5.8}, T \geq 10^{5.8}$ K, respectively. We perform a least-squares fit (see S\ref{['sec:phase_plot']}) to the mass distribution of cold gas in the middle panel (dashed black). Bottom: phase diagrams for gas temperature (y-axis) and bulk flow speed along the wind direction (x-axis) as a fraction of wind speed $v_w$, weighted by phase bin mass (colourbar). This corresponds to run $r_\mathrm{cl}/r_\mathrm{crit} = 1, f_v = 0.1$ and $L_\mathrm{ISM} = 30r_\mathrm{cl}$ .