A typical absorption profile produced by a cloud embedded in a turbulent wind along a quasar sightline. Including turbulence in the wind (purple curve) leads to multiple kinematic components and a larger equivalent width compared to its laminar-wind counterpart (yellow curve). © MPA
Visualising the difference in the evolution of the projected column densities of clouds in laminar (top) and turbulent winds (bottom). Turbulent winds result in shorter, clumpier, and much more laterally extended clouds, in contrast to the filamentary clouds found in laminar winds. Video here
Turbulent outflows can strip away cold gas from galactic disks – yet, some cold clouds survive and travel tens of kiloparsecs, challenging the notion that turbulence inevitably destroys them. Using three-dimensional hydrodynamic simulations, MPA researchers reveal that turbulence can actually enhance cloud survival by increasing the surface area for radiative cooling, enabling hot gas to condense onto the cold phase and boost its mass by up to an order of magnitude. This also leads to clumpier, laterally extended structures instead of long comet-like tails. These findings reshape our understanding of cloud evolution in galactic outflows, not only reshaping and sustaining cold gas but also enriching spectral signatures with multiple kinematic components.
Galactic winds are powerful, turbulent outflows of hot gas that sweep cooler, denser clouds away from galactic disks. Despite being immersed in these harsh environments, some cold clouds manage to survive and travel vast distances of several kiloparsecs — that is to say, further than the thickness of a typical galaxy disk. This raises the interesting question of how cold clouds survive their journey through a hot, fast-moving galactic wind.
Simulations to explain this observation have so far been limited to idealised 'wind-tunnel' scenarios, where the surrounding wind is treated as a smooth, laminar flow. However, in reality, galactic winds are turbulent and are driven by processes such as stellar feedback and AGN activity. In a recent study, researchers from the Max Planck Institute for Astrophysics in Garching and the Indian Institute of Science in Bangalore explored this question using three-dimensional hydrodynamic simulations. They examined how a cold cloud evolves when moving through a hot wind in which turbulence is continuously driven.
The results are surprising. Turbulence does not necessarily destroy the cold cloud; in regimes where radiative cooling is efficient, turbulence can actually help the cloud to grow. Turbulent motions stretch and deform the cold gas, dramatically increasing the surface area of interaction between the cold and hot phases. This creates more areas where radiative cooling can be efficient, allowing more hot gas to condense onto the cold phase. Consequently, the mass of the cold gas can increase by up to an order of magnitude compared to a laminar wind..
Turbulence also alters the motion of the cloud. As newly cooled gas from the wind joins the cold phase, it transfers momentum to the cloud, enabling it to become entrained in the wind much more rapidly. In other words, the turbulence that reshapes the cloud also helps to accelerate it..
The visual difference is striking, too. Instead of the long, narrow, comet-like tails produced by laminar winds, turbulent winds produce shorter, clumpier, and much more laterally extended clouds. The cold gas is stretched and dispersed over a much larger area, which could potentially change how we interpret observations of cold atomic and molecular phases in galactic outflows. From different viewing angles, this more complex, spatially extended gas could produce richer spectral features, with multiple kinematic components in quasar spectra indicating gas moving at different velocities..
Together, these results demonstrate that turbulence is not merely a force that tears cold clouds apart. It can also reshape them, help them grow and carry them along with the wind. Therefore, the fate of a cloud depends on a delicate balance between turbulent driving, hydrodynamic mixing, and radiative cooling..
The team is now looking to simulate the cooler molecular phase and dust, as such observations are becoming increasingly abundant with JWST. This will involve understanding the influence of additional physical processes, particularly magnetic fields and thermal conduction, on the evolution of cold gas in these turbulent galactic outflows. Exploring these effects will help to build a more complete picture of the broader role played by galactic outflows in shaping their host galaxies.
Galactic winds are powerful, turbulent outflows of hot gas that sweep cooler, denser clouds away from galactic disks. Despite being immersed in these harsh environments, some cold clouds manage to survive and travel vast distances of several kiloparsecs — that is to say, further than the thickness of a typical galaxy disk. This raises the interesting question of how cold clouds survive their journey through a hot, fast-moving galactic wind.
Simulations to explain this observation have so far been limited to idealised 'wind-tunnel' scenarios, where the surrounding wind is treated as a smooth, laminar flow. However, in reality, galactic winds are turbulent and are driven by processes such as stellar feedback and AGN activity. In a recent study, researchers from the Max Planck Institute for Astrophysics in Garching and the Indian Institute of Science in Bangalore explored this question using three-dimensional hydrodynamic simulations. They examined how a cold cloud evolves when moving through a hot wind in which turbulence is continuously driven.
The results are surprising. Turbulence does not necessarily destroy the cold cloud; in regimes where radiative cooling is efficient, turbulence can actually help the cloud to grow. Turbulent motions stretch and deform the cold gas, dramatically increasing the surface area of interaction between the cold and hot phases. This creates more areas where radiative cooling can be efficient, allowing more hot gas to condense onto the cold phase. Consequently, the mass of the cold gas can increase by up to an order of magnitude compared to a laminar wind..
Turbulence also alters the motion of the cloud. As newly cooled gas from the wind joins the cold phase, it transfers momentum to the cloud, enabling it to become entrained in the wind much more rapidly. In other words, the turbulence that reshapes the cloud also helps to accelerate it..
The visual difference is striking, too. Instead of the long, narrow, comet-like tails produced by laminar winds, turbulent winds produce shorter, clumpier, and much more laterally extended clouds. The cold gas is stretched and dispersed over a much larger area, which could potentially change how we interpret observations of cold atomic and molecular phases in galactic outflows. From different viewing angles, this more complex, spatially extended gas could produce richer spectral features, with multiple kinematic components in quasar spectra indicating gas moving at different velocities..
Together, these results demonstrate that turbulence is not merely a force that tears cold clouds apart. It can also reshape them, help them grow and carry them along with the wind. Therefore, the fate of a cloud depends on a delicate balance between turbulent driving, hydrodynamic mixing, and radiative cooling..
The team is now looking to simulate the cooler molecular phase and dust, as such observations are becoming increasingly abundant with JWST. This will involve understanding the influence of additional physical processes, particularly magnetic fields and thermal conduction, on the evolution of cold gas in these turbulent galactic outflows. Exploring these effects will help to build a more complete picture of the broader role played by galactic outflows in shaping their host galaxies.
Author:
Alankar Dutta
Tel: 2254
alankard@mpa-garching.mpg.de
Original publication
Ritali Ghosh, Max Gronke, Prateek Sharma, Alankar Dutta
Woven by the whirls: the growth and entrainment of cold clouds in turbulent hot winds
Monthly Notices of the Royal Astronomical Society, Volume 550, Issue 1, July 2026
DOI
