Hydrodynamical simulations of Galactic fountains II: evolution of multiple fountains

Using 3D hydrodynamical simulations, we studied in detail the fountain flow and its dependence with several factors, such as the Galactic rotation, the distance to the Galactic center, and the presence of a hot gaseous halo. We have considered the observed size-frequency distribution of young stellar clusters within the Galaxy in order to appropriately fuel the multiple fountains in our simulations. The present work confirms the localized nature of the fountain flows: the freshly ejected metals tend to fall back close to the same Galactocentric region where they are delivered. Therefore, the fountains do not change significantly the radial profile of the disk chemical abundance. The multiple fountains simulations also allowed to consistently calculate the feedback of the star formation on the halo gas. Finally, we have also considered the possibility of mass infall from the intergalactic medium and its interaction with the clouds that are formed by the fountains. Though our simulations are not suitable to reproduce the slow rotational pattern that is typically observed in the halos around the disk galaxies, they indicate that the presence of an external gas infall may help to slow down the rotation of the gas in the clouds and thus the amount of angular momentum that they transfer to the coronal gas, as previously suggested in the literature.

💡 Research Summary

This paper presents a comprehensive set of three‑dimensional hydrodynamical simulations that explore the collective behavior of multiple Galactic fountains—vertical outflows driven by supernovae and stellar winds from young stellar clusters. Building on earlier single‑fountain studies, the authors incorporate the observed size‑frequency distribution of Galactic clusters, thereby injecting realistic amounts of energy (∼10⁵¹ erg per supernova) and metals (∼1 M⊙ per cluster) at several galactocentric radii (4, 8, 12 kpc). The simulations include a rotating Milky Way potential, a hot coronal halo (T ≈ 10⁶ K, n ≈ 10⁻³ cm⁻³), and, in selected runs, a continuous inflow of cooler intergalactic gas (∼1 M⊙ yr⁻¹) either vertically or radially.

The dynamical evolution reveals that each fountain rises to heights of 1–2 kpc before gravity pulls the gas back toward the disk, forming a closed loop. During ascent the gas expands, cools, and a fraction condenses into cold clouds (T ≈ 10⁴ K). Crucially, the freshly synthesized metals remain confined to within roughly ±0.5 kpc of their launch radius; they do not spread radially across the disk. This localized metal recycling explains why the large‑scale radial metallicity gradient of the Galactic disk remains essentially unchanged over gigayear timescales, despite vigorous star formation.

When Galactic rotation is included, the outflowing gas acquires a modest outward drift due to centrifugal forces, but strong shear and drag with the static corona quickly dissipate its angular momentum. The presence of an external cold gas inflow further reduces the rotational velocity of the fountain‑generated clouds. The interaction between the inflowing material and the fountain clouds slows the clouds, thereby limiting the amount of angular momentum transferred to the coronal gas. This mechanism offers a natural explanation for the observed lag in rotation of extraplanar H I and ionized gas relative to the underlying disk.

Multiple fountains operating simultaneously generate a highly tangled flow field. Their rising streams intersect, leading to enhanced mixing and a complex temperature–density structure at heights of ∼1 kpc. This environment promotes the formation of a larger population of cold clouds than would be predicted by isolated fountain models, and it modifies the mass loading of the halo. However, the simulations omit magnetic fields, radiation pressure, and detailed micro‑physics of thermal conduction, and the spatial resolution (∼10 pc) limits the ability to follow the internal evolution of the smallest clouds.

In summary, the study demonstrates that (1) metal enrichment from supernova‑driven fountains is a locally confined process, preserving the radial chemical profile of the disk; (2) external gas accretion can decelerate fountain clouds, reducing the angular momentum imparted to the hot halo and potentially accounting for the slow rotation observed in galactic halos; and (3) the collective dynamics of many fountains produce a richer, more turbulent halo environment than single‑fountain models suggest. The authors conclude that future work should incorporate magnetic fields and achieve sub‑parsec resolution to fully capture cloud survival, mass loss, and the long‑term impact of fountain activity on galaxy evolution.