Periodic Reporting for period 2 - PICOGAL (Mind the Gap: from Plasma Kinetics to Cosmological Galaxy Formation)
Reporting period: 2024-01-01 to 2025-06-30
Understanding the physical processes leading to the formation of galaxies and galaxy clusters is one of the most exciting puzzles in modern astrophysics. While a basic paradigm exists — the concordance cosmology — many aspects of it are not well understood and appear to conflict with observational data. Most prominently, the observed stellar mass of galaxies, in relation to their total gravitational mass, drops significantly for galaxies both smaller and larger than the Milky Way. This implies that gas is most efficiently converted into stars in galaxies like the Milky Way, with conversion efficiency sharply declining towards smaller and larger systems. The currently favored explanation for regulating star formation in galaxies is baryonic feedback. Cosmological simulations incorporating phenomenological models for stellar feedback in small galaxies and energetic feedback by active galactic nuclei in galaxies larger than the Milky Way have been very successful in reproducing many observed properties, but the underlying physics remains unknown.
Stellar feedback is thought to manifest through galactic winds driven by supernovae, radiation, and relativistic particle populations called cosmic rays, which are accelerated at supernova remnant shocks. While energy and momentum deposition from supernovae and radiation can self-regulate the interstellar medium and launch galactic fountains, they fail to accelerate galactic winds to the observed outflow speeds. This is because photoionization and radiation pressure create channels in the optically thick gas surrounding star-forming regions, allowing radiation to escape without providing significant feedback.
In contrast, cosmic rays are observed to be in pressure equilibrium with the thermal gas in the mid-plane of the Milky Way, suggesting that they play a crucial dynamical role in maintaining the energy balance of the interstellar medium. Cosmic rays can drive powerful galactic winds, significantly altering the circumgalactic medium, as demonstrated in cosmological galaxy formation simulations. These simulations show that small differences in microscopic cosmic ray transport properties have a major impact on feedback strength, thermodynamics, and observable properties of the circumgalactic medium. This motivates us to study cosmic ray transport in detail, from tiny plasma scales to the largest cosmological scales.
Stellar feedback is thought to manifest through galactic winds driven by supernovae, radiation, and relativistic particle populations called cosmic rays, which are accelerated at supernova remnant shocks. While energy and momentum deposition from supernovae and radiation can self-regulate the interstellar medium and launch galactic fountains, they fail to accelerate galactic winds to the observed outflow speeds. This is because photoionization and radiation pressure create channels in the optically thick gas surrounding star-forming regions, allowing radiation to escape without providing significant feedback.
In contrast, cosmic rays are observed to be in pressure equilibrium with the thermal gas in the mid-plane of the Milky Way, suggesting that they play a crucial dynamical role in maintaining the energy balance of the interstellar medium. Cosmic rays can drive powerful galactic winds, significantly altering the circumgalactic medium, as demonstrated in cosmological galaxy formation simulations. These simulations show that small differences in microscopic cosmic ray transport properties have a major impact on feedback strength, thermodynamics, and observable properties of the circumgalactic medium. This motivates us to study cosmic ray transport in detail, from tiny plasma scales to the largest cosmological scales.
New plasma instability sheds light on the nature of cosmic rays
We studied the interaction of cosmic rays with the surrounding plasma using both numerical simulations that follow the trajectories of more than a billion particles and analytic theory. We discovered a new phenomenon that excites electromagnetic waves in the background plasma. These waves exert a force on the cosmic rays, which changes their winding paths. This new phenomenon can be best understood if we consider the cosmic rays not to act as individual particles but instead to support a collective electromagnetic wave. As this wave interacts with the fundamental waves in the background, these are strongly amplified, which slows the cosmic rays collectively down. There are many applications of this newly discovered plasma instability, including a first explanation of how electrons can be accelerated to high energies at supernova remnants or how cosmic rays can shape entire galaxies.
Cosmic rays drive powerful galactic winds
When the fuel supply of stars are depleted, they explode as supernovae and drive shock waves into the interstellar medium, where cosmic ray particles are accelerated. As these are long-lived, they are transported along magnetic field lines and can distribute their energy efficiently within the galaxy. In particular, cosmic rays propagate from supernova remnants into the halo surrounding the galaxies. In doing so, cosmic ray particles drive magnetic Alfvén waves in the background plasma, on which the particles scatter and are slowed down. This leads to a transfer of momentum, which accelerates the plasma away from the disk and generates galactic winds. This demonstrates the need for carefully modelling the transport of cosmic rays which is not described by a simple diffusion process.
Magnetic dynamo and radio emission in galaxies
In a forming galaxy, the gas collapses due to gravity, which compresses and increases the tiny magnetic field present. The cooling and collapsing gas is deflected by dense clumps and becomes turbulent. These turbulent motions stretch, twist and fold the magnetic fields, further amplifying them exponentially. This magnetic dynamo converts kinetic energy from gravitational collapse into magnetic energy. Cosmic ray electrons illuminate these magnetic fields by being forced into orbits around the magnetic fields by the Lorentz force, thus emitting synchrotron radiation in the radio. The more stars are formed in a galaxy, the more high-energy electrons are produced in supernovae, and the more radio radiation such a galaxy emits. Similarly, the stars emit high-energy ultraviolet radiation, which is absorbed by dust particles and emitted again as infrared radiation. Infrared and radio radiation in galaxies are thus correlated and provide both indications of star formation and galactic magnetic fields.
Whistler suppression of thermal conduction in galaxy clusters
In the hot and dilute intracluster plasma, electrons do not often collide with ions or other electrons so that they gyrate largely unaffected around the magnetic field and conduct heat alongside. Because the temperature declines from intermediate radii to the cluster outskirts, there is an outwards heat flux carried by the electrons that drives turbulence as they rise buoyantly in the cluster potential, carrying the magnetic fields alongside. Because these electrons are slowed down after scattering at whistler waves, so is thermal conduction and hence, the buoyantly driven turbulence dies. However, external turbulence from accreting gas or cluster mergers revives thermal conduction by growing magnetic fields via a dynamo.
Supermassive black holes in galaxy clusters
In the dense centres of galaxy clusters, the gas is cold enough to form many stars in a short time. However, this is not observed and instead active supermassive black holes can be found. Magneto-hydrodynamic simulations show that these black holes accrete gas and eject fast jets. This process heats and mixes the cooling central gas with hotter gas further out, leading to a stable self-regulation process and slowing down star formation. According to the simulations, magnetic fields are important to couple the two phases effectively. Moreover, only light jets can be deflected by the cooling gas filaments, leading to an extended distribution of filaments and twisted jets, consistent with observations.
We studied the interaction of cosmic rays with the surrounding plasma using both numerical simulations that follow the trajectories of more than a billion particles and analytic theory. We discovered a new phenomenon that excites electromagnetic waves in the background plasma. These waves exert a force on the cosmic rays, which changes their winding paths. This new phenomenon can be best understood if we consider the cosmic rays not to act as individual particles but instead to support a collective electromagnetic wave. As this wave interacts with the fundamental waves in the background, these are strongly amplified, which slows the cosmic rays collectively down. There are many applications of this newly discovered plasma instability, including a first explanation of how electrons can be accelerated to high energies at supernova remnants or how cosmic rays can shape entire galaxies.
Cosmic rays drive powerful galactic winds
When the fuel supply of stars are depleted, they explode as supernovae and drive shock waves into the interstellar medium, where cosmic ray particles are accelerated. As these are long-lived, they are transported along magnetic field lines and can distribute their energy efficiently within the galaxy. In particular, cosmic rays propagate from supernova remnants into the halo surrounding the galaxies. In doing so, cosmic ray particles drive magnetic Alfvén waves in the background plasma, on which the particles scatter and are slowed down. This leads to a transfer of momentum, which accelerates the plasma away from the disk and generates galactic winds. This demonstrates the need for carefully modelling the transport of cosmic rays which is not described by a simple diffusion process.
Magnetic dynamo and radio emission in galaxies
In a forming galaxy, the gas collapses due to gravity, which compresses and increases the tiny magnetic field present. The cooling and collapsing gas is deflected by dense clumps and becomes turbulent. These turbulent motions stretch, twist and fold the magnetic fields, further amplifying them exponentially. This magnetic dynamo converts kinetic energy from gravitational collapse into magnetic energy. Cosmic ray electrons illuminate these magnetic fields by being forced into orbits around the magnetic fields by the Lorentz force, thus emitting synchrotron radiation in the radio. The more stars are formed in a galaxy, the more high-energy electrons are produced in supernovae, and the more radio radiation such a galaxy emits. Similarly, the stars emit high-energy ultraviolet radiation, which is absorbed by dust particles and emitted again as infrared radiation. Infrared and radio radiation in galaxies are thus correlated and provide both indications of star formation and galactic magnetic fields.
Whistler suppression of thermal conduction in galaxy clusters
In the hot and dilute intracluster plasma, electrons do not often collide with ions or other electrons so that they gyrate largely unaffected around the magnetic field and conduct heat alongside. Because the temperature declines from intermediate radii to the cluster outskirts, there is an outwards heat flux carried by the electrons that drives turbulence as they rise buoyantly in the cluster potential, carrying the magnetic fields alongside. Because these electrons are slowed down after scattering at whistler waves, so is thermal conduction and hence, the buoyantly driven turbulence dies. However, external turbulence from accreting gas or cluster mergers revives thermal conduction by growing magnetic fields via a dynamo.
Supermassive black holes in galaxy clusters
In the dense centres of galaxy clusters, the gas is cold enough to form many stars in a short time. However, this is not observed and instead active supermassive black holes can be found. Magneto-hydrodynamic simulations show that these black holes accrete gas and eject fast jets. This process heats and mixes the cooling central gas with hotter gas further out, leading to a stable self-regulation process and slowing down star formation. According to the simulations, magnetic fields are important to couple the two phases effectively. Moreover, only light jets can be deflected by the cooling gas filaments, leading to an extended distribution of filaments and twisted jets, consistent with observations.
This action started successfully and already produced results beyond the state of the art:
1. The team conducts kinetic plasma simulations of cosmic ray acceleration and transport in tandem with large-scale hydrodynamic calculations within a single research group. This already enabled tremendous progress and promises even more transformational developments in the near future, potentially giving rise to a new influential approach to computational astrophysics in the context of galaxy and cluster formation.
2. The team compares novel three-dimensional magneto-hydrodynamics simulations, which incorporate an increasing level of plasma-kinetic physics, with observations. This process either validates the simulations and enhances understanding of the underlying physics or identifies weaknesses and failures in the models, thereby helping to pinpoint essential missing physics.
1. The team conducts kinetic plasma simulations of cosmic ray acceleration and transport in tandem with large-scale hydrodynamic calculations within a single research group. This already enabled tremendous progress and promises even more transformational developments in the near future, potentially giving rise to a new influential approach to computational astrophysics in the context of galaxy and cluster formation.
2. The team compares novel three-dimensional magneto-hydrodynamics simulations, which incorporate an increasing level of plasma-kinetic physics, with observations. This process either validates the simulations and enhances understanding of the underlying physics or identifies weaknesses and failures in the models, thereby helping to pinpoint essential missing physics.