Context

Cosmic Rays are high-energy charged particles, mostly protons and electrons, accelerated to relativistic speeds in supernova remnants. When a star explodes, it generates a powerful shock wave trapping charged particles at the shock front, where a process called the Fermi acceleration repeatedly accelerates them as they move back and forth between the downstream and upstream region. These particles eventually escape the supernova vicinity at velocities close to the speed of light, and propagate throughout the galaxy.

In disk galaxies, cosmic rays gradually build a pressure gradient pointing towards the disk. This is important as cosmic rays are dynamically coupled to the ambient gas, transferring energy and momentum to it. This pressure gradient therefore pushes gas away from the disk, creating powerful outflows called cosmic ray driven winds.

My work

Using the grid-MHD PIERNIK code, I ran numerical simulations of spiral galaxies including N-body particles for stars and dark matter, as well as gas, and cosmic rays injected in supernova feedback. These galaxies receive an inflow of gas from the inter-galactic medium, which triggers a burst of star formation, generating powerful winds.

In a first paper, I investigated purely cosmic-ray driven winds, finding that cosmic rays can generate powerful outflows escaping the disk permanently. By varying the inflow velocity, I controlled the angular momentum supplied to the galaxy and showed that low-AM accretion leads to stronger winds. I also found that the gas escaping the galaxy predominantly carries low angular momentum, while high angular momentum gas remains in the vicinity of the disc.

In a second paper, I added thermal stellar feedback to model thermally driven winds alongside cosmic-ray-driven outflows. I found that the combined effects of thermal and cosmic-ray feedback suppress star formation and produce two distinct phases in the outflows: hot, low-angular-momentum gas that escapes the disk and reaches high altitudes, and a cooler, higher-angular-momentum phase that remains in the disc's vicinity. By comparing a simulation with purely thermally driven winds, to a simulation with both thermal and cosmic ray feedback, I showed that cosmic rays lead to stronger outflows, made of colder material. Videos of the simulations can be found here.

This work was pursued with M. Hanasz of the Nicolaus Copernicus University in Torun, Poland, in collaboration with T. Naab at the Max Planck Institute for Astrophysics (MPA) in Garching, Germany.