Origin of Magnetic Field in the Intracluster Medium: Primordial or Astrophysical?

TL;DR

This paper investigates whether primordial or astrophysical processes are responsible for the magnetic fields observed in galaxy clusters by analyzing constraints from turbulence dynamo models and observational limits.

Contribution

It provides new constraints on seed magnetic field strengths, favoring astrophysical origins over primordial ones based on turbulence dynamo modeling and observational data.

Findings

Primordial seed fields likely weaker than 10^{-11} G.

Astrophysical seed fields can be stronger than 10^{-9} G.

Primordial magnetic fields are less likely the direct source of current ICM fields.

Abstract

The origin of magnetic fields in clusters of galaxies is still an unsolved problem, which is largely due to our poor understanding of initial seed magnetic fields. If the seed magnetic fields have primordial origins, it is likely that large-scale pervasive magnetic fields were present before the formation of the large-scale structure. On the other hand, if they were ejected from astrophysical bodies, they were highly localized in space at the time of injection. In this paper, using turbulence dynamo models for high magnetic Prandtl number fluids, we find constraints on the seed magnetic fields. The hydrodynamic Reynolds number based on the Spitzer viscosity in the intracluster medium (ICM) is believed to be less than $O(10^2)$, while the magnetic Reynolds number can be much larger than that. In this case, if the seed magnetic fields have primordial origins, they should be stronger than $O(10^{-11})$G, which is very close to the upper limit of $O(10^{-9})$G set by the cosmic microwave background (CMB) observations. On the other hand, if the seed magnetic fields were ejected from astrophysical bodies, any seed magnetic fields stronger than $O(10^{-9})$G can safely magnetize the intracluster medium. Therefore, it is less likely that primordial magnetic fields are the direct origin of present-day magnetic fields in the ICM.