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

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.