  We show that the motion of spin-polarized impurity (ferron) in ultracold
atomic gas is characterized by a certain critical velocity which can be traced
back to the amount of spin imbalance inside the impurity. We have calculated
the effective mass of ferron in 2D. We show that the effective mass scales with
the surface of the ferron and in general it scales as $M_\mathrm{eff} \propto
R^{D-1}$, where $D$ is the dimensionality of the system. We discuss the impact
of these findings, in particular we demonstrate that ferrons become unstable in
the vicinity of a vortex.
