Galactic interstellar medium [@sch09], galaxies [@hea09; @gov10; @wol10], galaxy clusters [@db05; @br11; @piz11], and active galactic nuclei [@osu12]. For example, [@hea09] studied FDFs of the WSRT-SINGS galaxies and found multiple nuclear components that are offset to both positive and negative RM by $100-200$ ${\rm rad/m^{-2}}$. [@db05] and [@br11] studied the Perseus cluster field with the WSRT and tried to separate components of the cluster and the Milky Way in RM between $-50$ and $+100$ ${\rm rad/m^{-2}}$. In future, the Square Kilometre Array (SKA) and its pathfinders will enable ultra-wideband observations and the FDF will become a key observable quantity for the study of cosmic magnetism in the next decades.

Galactic polarized radio emissions are mostly contributed from synchrotron radiation by cosmic-ray electrons, so that the FDF reflects distributions of cosmic-ray electrons as well as thermal electrons and magnetic fields which determine RM. Thus, FDFs of galaxies are expected to be a new probe that contains rich information about internal structures of galaxies. An uncertainty of the probe is complexity of the FDF. As reported in the Milky Way, a small volume filling factor of electrons suggests their clumpy distribution [@ber06; @hil08; @gmcm08]. Highly disturbed distributions of polarization angle and RM 