The observation (top) of the M82 galaxy shows the emission of a particular molecule (PAH = molecules of poly-aromatic hydrocarbon) in the infrared band of JWST's NIRCam. The leftmost plume is enlarged in the left image. The simulation (bottom) also shows the emission map. The inset shows one projection of the emission field, the larger image shows the total emission (the projection of many such slices along the z-axis). The blue arrows indicate the direction of the head-tail gradient in emission from cold gas along the direction of the wind in both simulations and observations. Credit: Observation: Fisher et al 2025; simulation: MPA, A. Dutta
A new study led by Dr. Alankar Dutta at the Max Planck Institute for Astrophysics uncovers why cold gas clouds fail to thrive in powerful winds flowing out of galaxies driven by supernovae. These findings, soon to be published in the Monthly Notices of the Royal Astronomical Society, challenge long standing assumptions about how galaxies exchange matter with their surroundings.
Galactic outflows — giant winds driven by intense star formation — play a key role in shaping the evolution of galaxies. These outflows carry gas, dust, and heavy elements out of the galactic disks and into the surrounding gaseous environment, the circumgalactic medium (CGM). While we know that these winds are multiphase, i.e. containing both hot, ionized plasma and much colder, denser neutral gas, as well as molecular gas. The origin of the hot gas in outflows can be attributed to the highly energetic supernovae explosions that drive them. However, the fate of the cold gas in these outflows and how this survives such a hot and hostile environment has long puzzled astronomers.
In simulations, it has been particularly challenging to resolve and infer the dynamics of the cold component which occur as clumpy parsec/sub-parsec sized clouds in large kilo-parsec scale outflows. This has caused astronomers to turn to idealized wind tunnel simulations to study cloud-wind interactions. Many previous studies used idealized setups where cold clouds face a uniform hot wind from the inner regions of galaxies, but real galactic outflows are not uniform. They expand, and that expansion fundamentally alters the game. To capture this realistic behavior, the group ran high-resolution hydrodynamic simulations of cold gas clouds embedded in expanding galactic winds. A well-known model of starburst-driven outflows formed the basis for the wind’s structure.
The new simulations reveal that as the clouds move outward, they remain in local pressure balance with the background wind. This leads to a steep decline in their density contrast, which means that over time, these clouds become increasingly diffuse and eventually blend into the surrounding hot gas. A marked contrast to earlier simulations with static winds, where the cold gas mass could keep on growing via radiative cooling – whereas in these more realistic simulations, the expanding environment suppresses this growth.
A new study led by Dr. Alankar Dutta at the Max Planck Institute for Astrophysics uncovers why cold gas clouds fail to thrive in powerful winds flowing out of galaxies driven by supernovae. These findings, soon to be published in the Monthly Notices of the Royal Astronomical Society, challenge long standing assumptions about how galaxies exchange matter with their surroundings.
Galactic outflows — giant winds driven by intense star formation — play a key role in shaping the evolution of galaxies. These outflows carry gas, dust, and heavy elements out of the galactic disks and into the surrounding gaseous environment, the circumgalactic medium (CGM). While we know that these winds are multiphase, i.e. containing both hot, ionized plasma and much colder, denser neutral gas, as well as molecular gas. The origin of the hot gas in outflows can be attributed to the highly energetic supernovae explosions that drive them. However, the fate of the cold gas in these outflows and how this survives such a hot and hostile environment has long puzzled astronomers.
In simulations, it has been particularly challenging to resolve and infer the dynamics of the cold component which occur as clumpy parsec/sub-parsec sized clouds in large kilo-parsec scale outflows. This has caused astronomers to turn to idealized wind tunnel simulations to study cloud-wind interactions. Many previous studies used idealized setups where cold clouds face a uniform hot wind from the inner regions of galaxies, but real galactic outflows are not uniform. They expand, and that expansion fundamentally alters the game. To capture this realistic behavior, the group ran high-resolution hydrodynamic simulations of cold gas clouds embedded in expanding galactic winds. A well-known model of starburst-driven outflows formed the basis for the wind’s structure.
The new simulations reveal that as the clouds move outward, they remain in local pressure balance with the background wind. This leads to a steep decline in their density contrast, which means that over time, these clouds become increasingly diffuse and eventually blend into the surrounding hot gas. A marked contrast to earlier simulations with static winds, where the cold gas mass could keep on growing via radiative cooling – whereas in these more realistic simulations, the expanding environment suppresses this growth.
The animation shows a slice through the mid-plane of one of our 3D cloud-crushing simulations in an expanding outflow. It shows how the initial cloud material mixes and moves in the wind.
A schematic demonstration of our novel cloud tracking scheme that enables the computational box to follow the cloud. This saves a lot of compute time and data processing required for these simulations and allows us to simulate cloud-wind interaction within the capabilities of our computational resources. © MPA, A. Dutta
Cloud expansion and pressure equilibrium are the key factors that regulate cold gas evolution. Even if initially a cloud would grow, it fades as it travels downstream, losing its ability to stand out from the background.
Moreover, the study finds that cold gas tails – common features seen in both simulations and observations – develop strong head-to-tail gradients in both density and brightness. This offers a natural explanation for recent high-resolution observations from the James Webb Space Telescope of intense star-forming galaxies like M82.
The key new feature to these simulations was a novel ‘cloud-tracking’ algorithm developed by the researchers, that allowed them to follow the cold gas for a long time/distance in an expanding wind without prohibitively expensive computational requirements. It is the first time, that such spatially expanding backgrounds have been self-consistently incorporated into cloud-crushing simulations – a crucial step towards bridging idealized theory and realistic galactic environments.
Looking ahead, the team plans to expand the simulations to include magnetic fields, thermal conduction, and more complex wind structures, like those relevant to active galactic nuclei (AGN) and cluster environments. This work is not just relevant for idealized simulations but has the potential to serve as the basis for building robust models of multiphase gas and their mixing on various scales, which is typically unresolved in cosmological simulations.
This work lays the foundation for a more complete theory of how galaxies lose – or retain – their fuel for star formation and that’s essential for understanding how galaxies live, grow, and die.
Cloud expansion and pressure equilibrium are the key factors that regulate cold gas evolution. Even if initially a cloud would grow, it fades as it travels downstream, losing its ability to stand out from the background.
Moreover, the study finds that cold gas tails – common features seen in both simulations and observations – develop strong head-to-tail gradients in both density and brightness. This offers a natural explanation for recent high-resolution observations from the James Webb Space Telescope of intense star-forming galaxies like M82.
The key new feature to these simulations was a novel ‘cloud-tracking’ algorithm developed by the researchers, that allowed them to follow the cold gas for a long time/distance in an expanding wind without prohibitively expensive computational requirements. It is the first time, that such spatially expanding backgrounds have been self-consistently incorporated into cloud-crushing simulations – a crucial step towards bridging idealized theory and realistic galactic environments.
Looking ahead, the team plans to expand the simulations to include magnetic fields, thermal conduction, and more complex wind structures, like those relevant to active galactic nuclei (AGN) and cluster environments. This work is not just relevant for idealized simulations but has the potential to serve as the basis for building robust models of multiphase gas and their mixing on various scales, which is typically unresolved in cosmological simulations.
This work lays the foundation for a more complete theory of how galaxies lose – or retain – their fuel for star formation and that’s essential for understanding how galaxies live, grow, and die.
Contact:
Alankar Dutta
Tel: 2254
alankard@mpa-garching.mpg.de
Original publication
Alankar Dutta, Prateek Sharma, Max Gronke
Fading in the Flow: Suppression of cold gas growth in expanding galactic outflows
submitted to MNRAS
Source
More Information
Playlist of videos from the simulations
Codebase developed in this work on Github
