Magnetic Field Alignment in Galaxy Simulations

Abstract

Spiral galaxies host elongated filamentary structures of dense gas and dust, known as feathers, which branch off the main spiral arms into the interarm regions and are associated with star formation. The physical mechanisms governing their formation remain debated, with magnetic fields proposed as one of the contributing factors. This thesis investigates the role of magnetic fields in feather formation by analysing isolated galaxy simulations of self-gravitating, isothermal, magnetised discs. Two simulation runs are compared, differing only in the strength of the initial toroidal magnetic field: a strongly magnetised case at equipartition (β = 1) and a weakly magnetised case at sub-equipartition (β = 10). The geometric relationship between the gas structures and the local magnetic field is quantified using the Histogram of Relative Orientations (HRO), which gives a statistical measure of the alignment between the gas structures and the magnetic field as a function of log-normalised projected surface density.

The simulations reveal that in the strongly magnetised regime, the transition from parallel to perpendicular alignment between the gas structures and the magnetic field begins already at lower densities, well before the feather formation threshold is reached. In the weakly magnetised regime, this transition occurs much later, close to the feather formation threshold itself. Beyond this threshold, both regimes converge to predominantly perpendicular alignment, with the dense feather structures forming preferentially across the field lines rather than along them. However, the strongly magnetised case reaches a more negative value of the alignment parameter, indicating that this configuration is more dominant when the magnetic field is dynamically important. These results suggest that the density at which the alignment transition occurs may serve as an observational indicator of the dynamical importance of the magnetic field in galactic discs.

Popular Abstract

When you look at a photograph of a spiral galaxy, your eyes are immediately drawn to the bright, swirling arms made of billions of glowing stars. However, what we can see is only half the story. The space between these stars is not empty, it is filled with a vast ocean of gas and dust. If you look closely with a powerful telescope, you will see smaller, delicate wisps of gas branching off the spiral arms and reaching into the regions between them. Astronomers call these structures `feathers'. These galactic feathers are far more than just cosmic decorations. They are among the cold, dense lanes of gas in a galaxy, and they are exactly the kind of place where gravity can pull matter together tightly enough to give birth to new stars.

But how do these feathers form in the first place? While astrophysicists are still debating the exact causes, one of the prime suspects is something completely invisible to the human eye: magnetic fields. Just as the Earth has a magnetic field that causes compass needles to point north, entire galaxies have their own enormous magnetic fields threading through them. You can think of galactic magnetic fields as giant, invisible elastic bands weaving through the swirling gas. In my thesis, I set out to understand the relationship between these invisible bands and the dense gas feathers where stars are born.

Because it takes tens of millions of years for a real galaxy to evolve, we cannot simply watch one through a telescope to see what happens. Instead, astrophysicists use powerful computers to build a galaxy in a box. By programming the laws of physics into a simulation, they can hit fast-forward and watch how the gas, gravity, and magnetic fields interact over time. For my thesis, I worked with two such simulations that differed only in one aspect: the strength of the magnetic field threading through the gas. My job was to analyse them and figure out how magnetic fields change the way feathers are formed.

What the simulations reveal is a clear shift in how the gas and magnetic fields are arranged at different densities. In the less crowded parts of the galaxy, the gas structures tend to lie along the magnetic field. As gravity gathers more and more gas into one place, this neat parallel arrangement starts to break down. The reason has to do with the shape of the structures that gravity builds. Feathers are long and narrow, stretching outward from the galactic center, while the magnetic field winds around the galaxy in circles. The two directions naturally cross, so the feathers end up lying across the field rather than along it. This crossing pattern appears whether the magnetic field is strong or weak, but it is sharper and more pronounced when the field is strong.

This crossing pattern is useful for astronomers: it tells us something about the role magnetic fields play in shaping the feathers. By studying this interplay between gravity and magnetic fields, we can better understand how these giant feathers in the sky formed, and potentially where the next generation of stars will be born.

• author: Bizunova, Mariya
• supervisor: Raghav Arora
• organization: Department of Physics; Astrophysics
• course: FYSK04 20261
• year: 2026
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Galactic magnetic fields, Interstellar medium, Spiral galaxies, Feathers, Magnetohydrodynamics (MHD) simulations, Histogram of Relative Orientations (HRO), Magnetic field alignment
• report number: 2026-EXA256
• other publication id: 2026-EXA256
• language: English
• id: 9245737
• date added to LUP: 2026-07-01 20:41:50
• date last changed: 2026-07-01 20:41:50