franco_vazzaNov 3

#paperday:

"Can galactic magnetic fields diffuse into the voids?"
(answer: NO!)

https://arxiv.org/abs/2510.26918 by O. Ghosh et al., (including me) on #astroph today .

It is a necessary rebuttal to a recent paper (https://arxiv.org/abs/2505.14774) who claimed the possible magnetisation of voids by propagating a dipole solution for the magnetic fields of galaxies, assuming cosmic voids are textbook "vacua"
Not good at all!

#astronomy #astrophysics #science #physics

Show threadfranco_vazza

In the paper we discuss at great extent what is the range of plausible diffusion rate of magnetic fields from galaxies to voids, considering the MHD (magneto-hydro-dynamics) description of plasma in voids, given the realistic physical parameters we know of these scales.

Magnetic fields from galaxies can only diffuse at a super-slow rate, and cannot magnetise voids at any significant level (surely not above the 1e-15G level suggested by gamma-ray observations)

Nov 3 at 1:37pmWeb
Show threadfranco_vazzaNov 3
By simulating with the Pencil code the somewhat unrealistic condition of a magnetic field produced by dynamo only (i.e. no outflows or jets) in a cosmic void, we showed that MHD imposes a fast power-law decrease of the B-field followed by a sharp exponential cutoff at a few scale radii from the galaxy, i.e. 100kpc or so. No magnetic field can realistically diffuse into voids in this way!
Show threadfranco_vazzaNov 3
This paper only addresses a scenario recently proposed in the literature, showing it cannot work, while of course other mechanisms to transport magnetic fields from galaxies to filaments and voids are possibly in place, like AGN jets and star formation winds. This is exactly the kind of phenomena which I daily try to simulate in large cosmological simulations (which has so far consistently shown that also jets and winds cannot magnetise voids).
Show threadfranco_vazzaNov 3

For a longer thread explaining why do we think gamma ray observations detected magnetic fields in voids (and radio observations detected it in filaments) you can check this longer contribution:

https://mastodon.social/@franco_vazza/110995561776303266