Early Galaxy Formation: Rapid Magnetic Fields Explained by Collapsing Plasma Clouds

Astronomers are grappling with a persistent puzzle: the unexpectedly early appearance of large-scale magnetic fields in young galaxies. Standard models of galactic magnetism suggest these fields require billions of years to develop, yet observations reveal coherent magnetic structures in galaxies dating back to when the universe was just a few billion years traditional – even as far back as redshift 2.6 and 5.6. This discrepancy has prompted scientists to re-examine the mechanisms driving magnetic field generation in the chaotic environments of early galaxy formation.

The conventional understanding of magnetic field growth relies on “dynamo theory,” which explains how motion of electrically conducting fluids – like plasma in galaxies – can amplify magnetic fields. While dynamo theory successfully explains the amplification of small-scale magnetic fluctuations on timescales of around 10 million years, it struggles to account for the rapid emergence of large-scale, coherent fields. A new study published in Physical Review Letters proposes that the turbulence inherent in a collapsing galaxy cloud may provide a crucial boost to this process.

Almost all visible matter in the universe exists as plasma, an ionized gas that readily interacts with magnetic fields. When this plasma moves in a turbulent manner, magnetic fields can grow through a process known as the turbulent dynamo. However, the new research suggests that the act of a galaxy forming – specifically, the gravitational collapse of a gas cloud – introduces an additional source of turbulence that significantly accelerates this amplification. “When the galaxy is forming, gravity itself can stir the plasma, which can amplify magnetic fields,” explains Muhammed Irshad, a graduate student at the International Centre for Theoretical Sciences (ICTS) and the study’s lead author.

The researchers used analytical calculations and numerical simulations to examine a turbulent cloud undergoing collapse. Their key finding is that magnetic field growth isn’t simply exponential, as predicted by standard dynamo theory. Instead, it becomes “superexponential” – meaning the rate of growth itself increases as the collapse proceeds. This acceleration stems from changes in the turbulence within the collapsing cloud. The plasma contains swirling motions called eddies, and the speed at which these eddies turn over influences magnetic amplification. As the cloud shrinks, the turnover rate increases, leading to faster field growth.

The effect isn’t solely due to the compression of magnetic fields as the gas collapses – a phenomenon known as flux freezing. Even after accounting for flux freezing, the dynamo contribution still accelerates field growth. In a spherically symmetric collapse, a frozen-in field scales with gas density as B ∝ ρ^2/3, while the dynamo-assisted case in this study scales as B ∝ ρ^5/6, demonstrating a more rapid amplification.

To simplify the complex calculations, the team employed a mathematical technique called supercomoving coordinates, commonly used in cosmology to account for the expansion of the universe. “These coordinates essentially develop the equations of a collapsing galaxy the same as a static galaxy, making the calculations very straightforward,” Irshad explains. This allowed them to adapt existing magnetohydrodynamic equations to a collapsing background.

The analytical work was then validated with numerical simulations using the publicly available Dedalus code, run on a periodic cubic box at a resolution of 128³. These simulations tracked both small-scale and large-scale dynamos, confirming that both types of dynamos exhibit faster magnetic amplification in a collapsing turbulent flow compared to a stationary background. The simulations also suggest that the resulting magnetic fields can be stronger than predicted by standard dynamo theory or flux freezing alone.

The timing of magnetic field development has far-reaching implications. Magnetic fields aren’t just relevant for radio observations of galaxies; they can influence stellar mass spectra by suppressing fragmentation and enabling jets and outflows from accretion disks. They also play a role in shaping galaxy evolution and feedback mechanisms through winds and fountains. As Pallavi Bhat, an assistant professor at the International Centre for Theoretical Sciences and a co-author of the study, notes, “There’s still much to learn in this ‘zeroth-order question you ask about the timescale.’”

The study’s findings could also inform computational models of structure formation in the universe. By predicting how quickly magnetic fields develop, scientists can test and refine these models. The framework may extend beyond galaxies to primordial star formation, where strong compression and efficient coupling between gas and magnetic fields could also accelerate field amplification. Understanding these early magnetic fields is crucial for a complete picture of cosmic evolution.

This research provides a new timescale for galaxy and star formation models to incorporate. If collapse genuinely accelerates magnetic growth, simulations may need to account for this effect from the outset. This could refine estimates of when young galaxies first developed observable magnetic structures and help resolve the discrepancy between observations and theoretical predictions. The study’s results suggest that the standard dynamo amplification time could be reduced by a factor of 10 in some cases, as demonstrated by the superexponential growth observed over 15 orders of magnitude in one example.