Gravitational Collapse Primes Galactic Magnetism
Almost all ordinary (baryonic) matter in the Universe exists as plasma. Wherever plasma stirs, the motions of electrons and ions create magnetic fields. Astronomers call a process that converts kinetic energy into magnetic energy a dynamo. In the case of the Milky Way and other mature galaxies, the principal dynamo is the galactic disk’s rotation and turbulence. That dynamo was theorized to take at least 5 billion years to establish strong fields, so the discovery in 2024 of substantial magnetic fields in galaxies younger than 1 billion years posed a question: What other dynamo operated in those youthful galaxies? Now Muhammed Irshad of the International Centre for Theoretical Sciences in India and his collaborators have proposed an answer: The gravitational collapse of protogalactic clouds rapidly amplifies their nascent magnetic fields [1].
On small scales, any random flow in any plasma can serve as a dynamo, and how quickly the field grows depends on the turnover rate of turbulent eddies. For a flow to act as a dynamo on large scales, it needs helicity or twistedness, and the helicity needs to be large enough and to have opposite sign in the galaxy’s two hemispheres. Meeting both requirements slows the amplification rate—hence the delay before a galactic magnetic field is established.
Irshad and his collaborators realized that the gravitational collapse that begets galaxies could hasten amplification. To test their idea, they used a technique for analyzing astrophysical phenomena within an expanding universe. Equations are recast into so-called supercomoving coordinates, which absorb the expansion or contraction. When Irshad and his collaborators solved the magnetohydrodynamical equations for a collapsing plasma, they discovered that the field grows superexponentially. They also found the cause: The collapse increases the turnover rate of large-scale eddies.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- M. Irshad P. et al., “Turbulent dynamos in a collapsing cloud,” Phys. Rev. Lett. 136, 091201 (2026).