Modeling the Milky Way wind: Supernova-driven outflows accelerate HI clouds near the Galactic center

TL;DR

This work develops semi-analytical, multiphase wind models for the Milky Way center, where a hot SN-driven wind accelerates embedded HI clouds through drag and cooling-induced mass exchange. By comparing model outputs to a robust HI cloud dataset via Bayesian inference, the authors constrain the hot-wind mass and energy loading factors, injection radius, cloud kick velocity, and mixing efficiency, finding a consistent picture with $\eta_{\rm M}\sim0.1$ and $\eta_{\rm E}\sim0.4$ near the injection region. The results show HI clouds reaching $v_{\rm long}\sim300$–$400$ km s$^{-1}$ at 1–2 kpc, while sustaining substantial mass loss ($>70\%$ for small clouds) and modest tangential velocities, implying a dominant role for hot wind entrainment and phase mixing. The inferred wind properties align with recent simulations and offer a physically grounded framework to interpret the Milky Way’s central multiphase outflow, with potential implications for the Fermi Bubbles and Galactic feedback processes.

Abstract

Multiwavelength observations, from radio to X-rays, have revealed the presence of multiphase high-velocity gas near the center of the Milky Way likely associated with powerful galactic outflows. This region offers a unique laboratory to study the physics of feedback and the nature of multiphase winds in detail. To this end, we have developed physically motivated semi-analytical models of a multiphase outflow consisting of a hot gas phase ($T \gg 10^6$ K) that embeds colder clouds ($T \sim 5000$ K). Our models include the gravitational potential of the Milky Way; the drag force exerted by the hot phase onto the cold clouds; and the exchange of mass, momentum, and energy between gas phases. Using Bayesian inference, we compared the predictions of our models with observations of a population of HI high-velocity clouds detected up to $\sim$1.5 kpc above the Galactic plane near the Galactic center. We find that a class of supernova-driven winds launched by star formation in the central molecular zone can successfully reproduce the observed velocities, spatial distribution, and masses of the clouds. In our two-phase models, the mass and energy loading factors of both phases are consistent with recent theoretical expectations. The cold clouds are accelerated by the hot wind via ram pressure drag and via accretion of high-velocity material, resulting from the turbulent mixing and subsequent cooling. However, this interaction also leads to gradual cloud disruption, with smaller clouds losing over 70\% of their initial mass by the time they reach $\sim$2 kpc.

Figures (13)

• Figure 1: Population of H i high-velocity clouds from the surveys of cluregriffiths13 and diteodoro18, taken with the ATCA and the GBT. Left: Galactic latitude-longitude map. The data-points show the positions of the clouds and are color-coded by their $v_{\rm{LSR}}$. The white mask shows the region surveyed by the ATCA and the GBT. Right: H i cloud latitudes as a function of their velocities. The kinematic pattern shows a signature of acceleration (see main text and lockman20). The points are color-coded by the cloud mass and the full 1D mass distribution is shown in the inset panel.
• Figure 2: Behavior of the multiphase wind for three different choices of models (see main text): model A (top row), model B (central row), and model C (bottom row). On the left, hot gas velocities (solid curves) and densities (dashed curves) are shown as a function of the distance, $r$. The right panels show instead the cold cloud orbits (solid curves), indicating in particular the effects of drag (dashed curves) and mixing (dash-dotted curves) on the original ballistic orbit (dotted curves). The symbols mark the positions of the modeled cloud at the beginning (circles), after 5 Myr (squares) and after 10 Myr (rhomboids).
• Figure 3: Cloud mass evolution as a function of the distance from the center, for models A, B, and C (see Figure \ref{['fig:meth_orb']}). In all cases, the cloud starts with a mass of $5\times10^2\ M_{\odot}$, but in model A it continuously loses its mass and slowly evaporates into the background medium due to hydrodynamical interactions with the hot wind, while in the other two models it initially increases its mass due to the cooling of the mixing layers.
• Figure 4: Corner plot showing the posterior distributions of the five free parameters of our models. The vertical lines mark the positions of the 2.5, 50 (solid line) and 97.5 percentiles of the one-dimensional posterior distributions.
• Figure 5: Comparison of the outputs of the best-fit models with the observational data. The cloud kinematics is shown as a function of Galactic longitude and latitude, with models on the top and data on the bottom. The one-dimensional distributions of the observed (orange, solid line) and the model (teal, dashed line) longitudes are reported on top.