Far out cosmology: role of magnetic fields (Introduction)

by David Turell , Monday, July 01, 2024, 19:15 (68 days ago) @ David Turell

Poorly understood:

https://www.quantamagazine.org/tracing-the-hidden-hand-of-magnetism-in-the-galaxy-20240...

Amid the roilings of the Milky Way, immense pockets of gas coalesce into clouds where stars are born. In this process, there is a hidden hand at play: magnetism.

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"Exactly how this fundamental force helps sculpt our galaxy has long eluded scientists, largely because measuring the galactic magnetic field is a considerable challenge.

"While much remains unknown, new tools and methods are bringing us closer to perceiving the influence of magnetism on the evolution of stars and galaxies, and Clark is one of the scientists spearheading this effort. As the leader of the Cosmic Magnetism and Interstellar Physics group at Stanford, she uses a combination of novel observational techniques, simulations and theory to unravel the puzzles of galactic magnetism.

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"... We really want to understand what role the magnetic field is playing in all the different physical processes that shape the diffuse gas between stars, called the interstellar medium, or ISM. We know that the magnetic field is playing some sort of still unclear role in the evolution of the gas to form very dense, cold clumps of material called molecular clouds that are the birthplaces of the stars. And then it plays a role in how that molecular cloud fragments and forms stars. We really don’t understand the details yet at all. But you see these long tendrils or filaments of gas in areas of the ISM with low density that are very well aligned with the magnetic field. We want to understand what that means for the transition of gas between phases in the ISM, and for turbulence in the ISM.

"The gas in the galaxy spans a mind-bending range of physical states. You have these very dense, cold molecular clouds. And then you have, at the other extreme end, very hot plasma. And you have a range of states in between, and we know that the story of star formation and the evolution of galaxies in general involves gas flowing between these different physical states, and we want to understand this whole life cycle of how gas gets converted into stars and spewed back out into the interstellar medium to seed future generations of star formation.

"One thing we’ve discovered in recent years is that as you move to denser structures in the interstellar medium, the filamentary molecular clouds actually prefer to orient orthogonally with respect to the local magnetic field. This is very tantalizing for the idea that the structure of the magnetic field might be important for where and how you sculpt these long filamentary knots of molecular material that are eventually the things that fragment and form stars. There are so many open questions about that, and what we want to do ultimately is put together that whole evolutionary picture of how gas and the magnetic field interact to regulate this process of star formation.

"The ISM has a lot of dust in it, chunks of material that are micron-size or smaller, and these little dust grains have some amorphous shape; they’re not perfect spheres. And they align with a preferred orientation with respect to the local magnetic field that they’re sitting in. One consequence is that if you look at optical or near-infrared light coming off of a background star, the light is filtering through all of these magnetically aligned dust grains, and they absorb the light that’s polarized parallel to their long axis. So you can measure the polarization of the remaining starlight, and what you see is an imprint of the magnetic field in the dust in between your telescope and the star.

"And then those dust grains are also radiating light, and that radiation has a polarization angle set by the orientation of the local magnetic field. So those are just two of the tools that we use.

"We want to understand the ultimate origin of magnetism in the universe. But the really pertinent tricky question is: How do we have these large-scale, coherent, organized magnetic fields? How is it that, probably in the process of forming the galaxy, the motions of the gas were able to amplify and distribute the magnetic field to have the structure that we observe today? And one of the things that has been really helpful for trying to answer this question is to have observations of the magnetic field structure in other galaxies as well.

"One of the very cool observations we published recently is of the polarization of far-infrared light from the Antennae galaxies. These are a pair of merging galaxies, and we observed that in this interaction region between them there was a very coherent magnetic field structure inferred from dust polarization, which is fascinating for all sorts of reasons. We are really at the beginning of understanding these things.

"We do want to know what’s responsible for the “seed” magnetic field that got amplified during the formation of galaxies. It’s possible that that field was primordial, meaning it originated during the universe’s birth. But it seems to me that our measurements of the modern-day magnetic fields in galaxies on their own are not going to tell us where the initial seed field came from. It tells you there was some weak seed field, but that’s all."

Comment: an interview with Susan Clark from Stanford U. Magnetic fields were discussed here earlier: Thursday, July 27, 2023, 18:11. In a universe filled with electrical forces, magnetism is not a surprising component.