  We study concentration phenomena in the vanishing viscosity limit for
second-order stationary Mean-Field Games systems defined in the whole space
$\mathbb{R}^N$ with Riesz-type aggregating nonlocal coupling and external
confining potential. In this setting, every player of the game is attracted
toward congested areas and the external potential discourages agents to be far
away from the origin. Focusing on the mass-subcritical regime
$N-\gamma'<\alpha<N$, we study the behavior of solutions in the vanishing
viscosity limit, namely when the diffusion becomes negligible. First, we
investigate the asymptotic behavior of rescaled solutions as $\varepsilon\to0$,
obtaining existence of classical solutions to potential free MFG systems with
Riesz-type coupling. Secondly, we prove concentration of mass around minima of
the potential.
